2. Cellular Metabolism (HT)

Question 1

What are the two types of therapeutic nutrition?

Answer 1

• Disease prevention
  
  • Deficiency disease
  • Malnutrition
  • Chronic disease
• Disease management
  
  • Chronic diseases
  • Inborn errors (e.g. phenylketonuria (PKO))

Question 2

What is the difference between micronutrients and macronutrients?

Answer 2

• Micronutrients: vitamins, minerals
• Macronutrients: carbohydrate, fat, protein, (fibre?)

Question 3

Give some sources of information about nutrition in the UK.

Answer 3

• National Diet and Nutrition Survey
  
  • Joint initiative between the Food Standards Agency and the Department of Health
  • Data on diets of individuals (interviews, diaries)
• National Food Survey
  
  • Continuous since 1940s
  • Commissioned by the Department for Environment, Food and Rural Affairs (DEFRA)
• Biobank
  
  • Wide range of genetic, anthropometric and physiological data on 500,000 participants
  • 24-hour dietary recall data
  • Data available on request

Question 4

What are the UK Government’s eight Guidelines for a Healthy Diet?

Answer 4

1. Enjoy your food.
2. Eat a variety of different foods.
3. Eat the right amount to be a healthy weight.
4. Eat plenty of foods rich in starch and fibre.
5. Eat plenty of fruit and vegetables.
6. Don’t eat too many foods that contain a lot of fat.
7. Don’t have sugary foods and drinks too often.
8. If you drink alcohol, drink sensibly.

Question 5

Draw the Government’s Eatwell Guide.

Answer 5

Question 6

What are the recommended daily energy intakes for men and women? How do these compare to the reported average intakes?

Answer 6

• Recommended men = 2500kcal
• Recommended women = 2000kcal
• Reported men = 2255kcal
• Reported woemn = 1645kcal

Question 7

What percentage of men and women are overweight or obese?

Answer 7

• Women = 60%
• Men = 70%

Question 8

Is the prevalence of obesity increasing?

Answer 8

Yes

Question 9

What are the average reference intakes (RIs) that are used on packaging for:

• Energy
• Fat
• Saturates
• Carbohydrate
• Total sugars
• Protein
• Salt

Answer 9

• Energy = 8400 kJ/2000 kcal
• Fat = 70 g
• Saturates = 20 g
• Carbohydrate = 260 g
• Total sugars = 90 g
• Protein = 50 g
• Salt = 6 g

Question 10

What are typical daily intakes of carbohydrates (not recommended) and their subsets?

Answer 10

• Carbohydrates = 300g
  
  • Polysaccharides = 66%
  • Disaccharides = 31%
  • Monosaccharides = 3%
• Variable amount of fibre: From 10 to 20g

Question 11

What is the calorie density of carbohydrates?

Answer 11

4kcal/g

Question 12

What are the two main types of carbohydrates?

Answer 12

• Glycaemic: Sugars and starch
• Non-glycaemic: “Fibre”
  
  • Separate fibre further into “soluble” and “non-soluble” fibre

Question 13

What are free sugars?

Answer 13

• All added sugars in any form
• All sugars naturally present in fruit and vegetable juices, purees and pastes and similar products in which the structure has been broken down

Question 14

What are the current recommendations about free sugars?

Answer 14

They should not make up more than 5% of daily caloric intake (approx. 25g/day).

Question 15

Explain the sugar tax and how this affected consumption of free sugars.

Answer 15

• Avg intake of free sugar by UK adults accounted for ~12% of total energy
• Sugar tax decreased intake of free-sugars from sugar-sweetened beverages by ~30%

Question 16

What are some foods that contain high levels of polysaccharides?

Answer 16

• Cereals (wheat, rice)
• Root vegetables (potatoes)
• Legumes (baked beans)

Question 17

Draw a graph to compare the glycaemic response to glucose and white bread.

Answer 17

Question 18

What is glycaemic index (GI)?

Answer 18

• Ranking of carbohydrate based on the rate at which they raise blood glucose levels
• High GI values given to foods that break down quickly thus raise blood glucose quickly

Some evidence manipulation of the glycaemic response may be useful in management of diabetes, aid with weight loss, lower risk of cardiometabolic diseases.

Question 19

What are typical daily intakes of fats (not recommended) and their subtypes?

Answer 19

• Fat = 100g
  
  • Triacylglycerols = 94%
  • Phospholipids = 5%
  • Cholesterol = 1%

Question 20

What is the calorie density of fats?

Answer 20

9kcal/g

Question 21

What are the 3 types of fatty acid and some examples of the foods they may be found in?

Answer 21

Question 22

What are the guidelines for fat intakes for men and women?

Answer 22

• Women = Less than 70g
• Men = Less than 95g

Question 23

What are the different effects of saturated, unsaturated and polyunsaturated fats? [IMPORTANT]

Answer 23

1. Saturated: raises serum cholesterol
2. Monounsaturated: may lower serum cholesterol
3. Polyunsaturated: strongly lowers serum cholesterol (but HDL-cholesterol (“good cholesterol”) may fall)
4. Saturated fat tends to be associated with insulin resistance, polyunsaturated with insulin sensitivity

Question 24

Describe two important types of polyunsaturated fatty acids and their effect.

Answer 24

Question 25

What is the name of a major study that proposed the link between saturated fatty acid consumption and heart disease?

Answer 25

Seven Countries Study

Question 26

Draw the triangle that explains why the relations between saturated fat and heart disease is dubious.

Answer 26

Question 27

State the calorie density of carbohydrate, fat, protein and alcohol.

Answer 27

• Carbohydrate = 4kcal/g
• Fat = 9kcal/g
• Protein = 4kcal/g
• Alcohol = 7kcal/g

Question 28

What are the UK recommendations for alcohol consumption?

Answer 28

No more than 14 units a week for men and women

Question 29

What is the typical daily consumption (not recommended) of protein?

Answer 29

100g

Question 30

What is the recommended intake of protein based on bodyweight?

Answer 30

0.75g/kg/day.

Question 31

Describe how protein intake requirement changes with age.

Answer 31

Young and eldery people need more protein than adults.

Question 32

For these foods, name the amino acid they are lacking in and some foods they may be complemented with:

• Beans
• Grains
• Nuts/Seeds
• Vegetables
• Corn

Answer 32

Question 33

Describe the current UK recommendations about consumption of these foods compared to the mean actual intake:

• Fruit and vegetables
• Red meat
• Oily fish

Answer 33

Question 34

Describe the current UK recommendations about consumption of the macronutrients by the percentage of energy they should provide, along with a comparison to the mean actual intake. [IMPORTANT]

Answer 34

Question 35

Describe the important of micronutrients.

Answer 35

• Enable the body to produce enzymes, hormones and other substances essential for proper growth and development
• The consequences of their absence are severe

Question 36

Describe the reference nutrient intakes (RNI) and lower reference nutrient intakes (LRNI) for these micronutrients:

• Vitamin A
• Folate
• Vitamin C
• Calcium
• Iron
• Iodine

Answer 36

Question 37

For zinc, iodine and vitamin A, state:

• How common deficiency is
• Sources of the micronutrient
• Health consequences of deficiency
• Prevention

Answer 37

Question 38

What measure is used to compare the risk between two groups, where one is exposed to a factor while the other is not?

Answer 38

Relative risk:

• If RR=1 -> Risk in exposed equal to risk in unexposed (no association)
• If RR>1 -> Risk in exposed greater than risk in unexposed (positive association)
• If RR<1 -> Risk in exposed is less than risk in unexposed (negative association)

Question 39

Describe the classic diet-CVD hypothesis.

Answer 39

Question 40

What is metabolism?

Answer 40

• The chemical processes that occur within a living organism in order to maintain life.
• Purely a means of getting from A to B by the most efficient route.

Question 41

What are the two main divisions of metabolic reactions?

Answer 41

Reactions can be either:

• Catabolic -> Breaking things down
• Anabolic -> Building things up

Question 42

Give some examples of catabolic reactins in metabolism.

Answer 42

• Synthesis of energy
  
  • Glycolysis
  • Fatty acid oxidation (β-oxidation)
• Breakdown of glycogen (glycogenolysis)
• Breakdown of ketone bodies (ketolysis)

Question 43

Give some examples of anabolic reactions in metabolism.

Answer 43

• Synthesis of storage molecules
  
  • Glycogen (glycogenesis)
  • Triglycerides (lipogenesis)
• Synthesis of glucose (gluconeogenesis)
• Synthesis of ketone bodies (ketogenesis)

Question 44

Draw the general model of metabolism, including inputs and outputs of the organism.

Answer 44

Question 45

What kinds of molecules are the substrates in metabolism and (briefly) how is energy released from these?

Answer 45

• Relatively reduced compounds of carbon -> Contain high amounts of hydrogen
• Therefore, energy is released by oxidation

Question 46

According to the syllabus, what is the general strategy and logic of human metabolism?

Answer 46

Quoted from the syllabus:

• Partial and complete oxidation to release energy
• Trapping of energy as ATP
• Coupling of ATP hydrolysis to energy-requiring reactions
• CO₂ and water production.

Question 47

Compare oxidation of substrates in vitro and in an organism.

Answer 47

In vitro combustion:

• 1 step
• Complete conversion to CO₂ and H₂O
• All of the energy lost as heat and light

Biological oxidation:

• Controlled process, occuring in steps
• Can be complete of partial oxidation (since stepwise)
• (Some) Energy trapped in chemically useful form (ATP)
• Also yields waste products: CO₂ and H₂O

Question 48

What determines whether a reaction is spontaneous and how can it be made to happen if it is not spontaneous?

Answer 48

• If the Gibbs Free Energy (ΔG) is negative, then the reaction can take place spontaneously
• If it is not, then the reaction can be coupled to ATP hydrolysis to allow it to happen

Question 49

What is the equation for Gibbs Free Energy and what is the significance of each sign?

Answer 49

ΔG = ΔH - TΔS

Where:

• ΔG = Gibbs Free Energy (kJ/mol - CHECK THIS)
• ΔH = Enthalpy change (kJ/mol)
• T = Temperature (K)
• ΔS = Entropy change (J/K/mol)

If ΔG is negative, then the reaction is spontaneous.

Question 50

What are the 3 main dietary metabolic fuels?

Answer 50

• Glucose
• Fatty acids
• Amino acids

Question 51

What are some typical dialy macronutrient intakes?

Answer 51

Question 52

Describe the stores of glucose in the body and the relative amounts stored in each. (Based on a 70kg man)

Answer 52

Question 53

Describe the stores of fatty acids in the body and the relative amounts stored in each. (Based on a 70kg man)

Answer 53

Question 54

Describe the stores of amino acids in the body and the relative amounts stored in each. (Based on a 70kg man)

Answer 54

Question 55

Are amino acids a true storage form?

Answer 55

No, not really.

Question 56

State the energy (in kJ) stored per gram of glycogen, triglycerides and protein in the body. What are the total energy stores (in kJ) of these in the body of a 70kg man?

Answer 56

Question 57

Draw the structure of ATP.

Answer 57

Question 58

Compare the amount of ATP contained in most tissues, the amount the whole body contains and the amount the whole body uses per day.

Answer 58

• Most tissues contain about 6mM ATP
• Whole body contains 75g of ATP
• Whole body uses 75kg per day

Question 59

Draw the process of ATP cycling.

Answer 59

Question 60

Compare and explain the amounts of ATP and ADP in the body.

Answer 60

• ADP is much lower because it is a stimulus for ATP synthesis, so ATP is rapidly resynthesised.
• This is the main signal driving ATP synthesis, not a decrease in ATP!

Question 61

What is the Gibbs Free Energy for ATP hydrolysis?

Answer 61

ΔG = -30.5 kJ/mol (-7.3 kcal/mol)

Question 62

What are the two main methods of synthesising ATP and what are their relative contributions to total ATP synthesis?

Answer 62

• Substrate level phosphorylation
  
  • Oxygen independent
  • e.g. Glycolysis, etc.
  • 5% total ATP
• Oxidative phosphorylation
  
  • Oxygen dependent
  • 95% total ATP

Question 63

What are the 3 common stages of energy extraction in catabolism?

Answer 63

1. Larger macromolecules broken down to smaller ones (e.g. glucose, amino acids)
2. Small molecules degraded to common units (e.g. acetyl-CoA)
3. TCA cycle / Oxidative phosphorylation to complete oxidation to water, carbon dioxide and ATP

Question 64

What molecule is the most common hub for metabolic pathways?

Answer 64

• Acetyl-CoA
• Many pathways utilise or feed into acetyl-CoA, depending on conditions

Question 65

Draw the general pathways to show how acetyl-CoA is connected to glucose, triglycerdies, fatty acids, ketone bodies and amino acids.

Answer 65

Question 66

On this diagram, label where glycolysis, fatty acid oxidation and oxidative phosphorylation are.

Answer 66

Question 67

On this daigram, label where glycogenesis and de novo lipogenesis are.

Answer 67

Question 68

On this diagram, label where glycogenolysis, gluconeogenesis, amino acid oxidation, ketolysis and ketogenesis are.

Answer 68

Question 69

Draw a diagram to show compartmentalisation in metabolism.

Answer 69

Question 70

What are some advantages and disadvantages of compartmentalisation in metabolism?

Answer 70

Question 71

Metabolism can differ in different physiological states. Give some examples of these.

Answer 71

Anabolic vs Catabolic:

• Fed vs. fasted vs. starvation state
• Relaxed vs. fight/flight response
• Rest vs. exercise
• Healthy vs. disease

Question 72

When considering metabolism in a question, what factors is it important to consider?

Answer 72

• Physiological state (e.g. fed vs. fasted)
• Organ (e.g. liver)

Question 73

Give a summary of the tissues in which glucose, fatty acids and amino acids may be used.

Answer 73

Question 74

What factors control metabolism in the short-term? How long do these last?

Answer 74

• Allosteric (millisecs)
  
  • Binding of effector to site away from enzymes active site
  • Usually intracellular effector
• Covalent (secs to mins)
  
  • Addition/removal of molecule attached to the enzyme via a chemical bond that shares electrons
  • Usually in response to extracellular effector
• Translocation (secs to mins)
  
  • Movement from one cell compartment to another

Question 75

What factors control metabolism in the long-term? How long do these last?

Answer 75

• Transcription/translation (hrs to days)
  
  • Enzyme induction or suppression
  • Multiple enzymes targeted together

Question 76

Describe the principle that control of metabolism is not just at the target organ. [IMPORTANT]

Answer 76

Control of metabolism includes 2 main points of regulation outside the target organ:

• Delivery of substrate
  
  • Substrate must be supplied from somewhere
  • This is done via the circulation
• Transmembrane movement
  
  • Substrate must be taken up selectively
  • Control of membrane transporters, as well as enzyme regulation

Question 77

State an example of a tissue cycle.

Answer 77

Cori cycle

Question 78

Describe the Cori cycle (briefly because it will be covered later in more depth).

Answer 78

Question 79

Give some examples of what happens when metabolism goes wrong.

Answer 79

Question 80

What is the origin of the word “glycolysis”?

Answer 80

Greek:

• Glycos = sugar (sweet)
• Lysis = split

Question 81

Describe the relative use of glucose by these tissues: brain, skeletal muscle, RBCs and renal medulla.

Answer 81

• Glucose is the primary fuel for the brain, RBCs & the renal medulla
• Skeletal muscle also uses glucose majorly

Question 82

Describe glycolysis is terms of these key points:

• Whether it is anaerobic or aerobic
• Number of main stages
• Where it occurs
• The fate of the products
• Control
• How conserved the pathway is

Answer 82

• Anaerobic process – allows us to make ATP in the absence of oxygen
• Occurs in 3 stages (energy investment / 6C splitting / energy harvest)
• Occurs in the cytosol
• The fate of the end products depends on the conditions of the cell
• Control is mediated by “supply & demand”
  
  • Supply in terms of selecting the “best” fuel
  • Demand in terms of the energy needs of the cell
• Glycolysis is a highly conserved pathway that occurs in virtually all cells

Question 83

Why is the renal medulla so highly reliant on glyolysis for energy?

Answer 83

It is very hypoxic.

Question 84

What two parts are there to glucose uptake (prior to glycolysis)?

Answer 84

• Transport
• Phosphorylation

Question 85

What are the two types of transporter involved in the uptake of glucose (prior to glycolysis) and what process does each use? Where is each found?

Answer 85

• GLUT 1-4 transporters (GLUcose Transporters)
  
  • Facilitated diffusion (Na⁺-independent)
  • Tissue-specific; all tissues
  • Specialised functions
• SGLT transporters (Sodium/GLucose-co Transporter)
  
  • Secondary active transport -> Co-transport with sodium
  • Energy requiring, against concentration gradient
  • Epithelial cells of intestine, renal tubule, choroid plexus

Question 86

On what tissues are GLUT transporters found?

Answer 86

All tissues.

Question 87

On what tissues are SGLT transporters found?

Answer 87

• Epithelial cells of intestine
• Renal tubule
• Choroid plexus

Question 88

Draw a 2x2 table to show how glucose uptake in different tissues is insulin-sensitive or insulin-insensitive and active or passive.

Answer 88

Question 89

Draw a diagram to show how GLUT transporters work.

Answer 89

Question 90

Draw a table to summarise the different types of glucose transporter, including their tissue distribution, K_m and important features.

Answer 90

Question 91

What are the two most important types of GLUT transporter that you need to know about?

Answer 91

• GLUT2
  
  • Insulin-independent
  • Found in liver, kidney, intestine and pancreatic ß-cell
• GLUT4
  
  • Insulin-dependent
  • Found in muscle and adipose tissue

Question 92

What are the two different types of glucose uptake that occur in tissues? [IMPORTANT]

Answer 92

• Uptake dependent on plasma glucose concentration
  
  • In liver and endocrine pancreas
  • Uses insulin-independent GLUT2 transporters
• Uptake dependent on energy needs of tissue and regulated in tissues that can also use non-glucose energy substrate
  
  • In peripheral tissues
  • Uses insulin-dependent GLUT4 transporters

Question 93

In GLUT transporter kinetics, how do changes in [S] affect the rate of uptake if:

• Km << physiological [S]
• Km ≥ physiological [S]

Answer 93

• If Km << physiological [S] – uptake is independent of [S]
• If Km ≥ physiological [S] – uptake is highly dependent on [S]

Question 94

What is the glucose transporter found on liver cells and how does this relate to its role?

Answer 94

• GLUT2 (insulin-indepedent)
• The liver plays a key role in buffering blood glucose concentrations
• The presence of GLUT2 – with a high Km (7-20mM) ensures that:
  
  • If blood glucose concentration is high – it is taken up into the liver for “storage”
  • If blood glucose concentration is low – the liver doesn’t take it up – sparing it for organs that need it (i.e. brain, RBC, renal medulla)

Question 95

What is the glucose transporter found on pancreas cells and how does this relate to its role?

Answer 95

• GLUT2 (insulin-independent)
• The pancreas plays a key role in regulation of blood glucose concentrations through the production of insulin / glucagon
• The presence of GLUT2 – with a high Km (7-20mM) ensures that the pancreas can release insulin when blood glucose is high

Question 96

Draw a diagram to show how GLUT2 transporters on pancreas cells enable a response to low glucose.

Answer 96

Question 97

Draw a diagram to show how GLUT2 transporters on pancreas cells enable a response to high glucose.

Answer 97

Question 98

What is the glucose transporter found in peripheral tissues (muscle and adipose tissue) and how does this relate to its function?

Answer 98

• GLUT4
• This is the ‘insulin-regulatable’ glucose transporter, so it allows glucose uptake to be dependent on insulin levels

Question 99

Draw the mechanism by which GLUT4 transporters respond to insulin.

Answer 99

Question 100

Draw a summary of glycolysis, split into the 2 or 3 main stages.

Answer 100

Question 101

What are the 3 parts of glycolysis?

Answer 101

1. Glucose priming
2. Splitting of phosphorylated intermediate
3. Oxidoreduction-phosphorylation

Question 102

Draw the full pathway for glycolysis.

Answer 102

Question 103

What is the point of glucose phosphorylation after uptake into the cell?

Answer 103

• Locks glucose inside cell
• Activates glucose
• Maintains concentration gradient

Question 104

What are the two enzymes used to phosphorylate glucose after uptake and where is each found?

Answer 104

• Hexokinase -> Most tissues
• Glucokinase -> Liver, Pancreatic islets (glucose sensor)

Question 105

Draw the phosphorylation of glucose.

Answer 105

Question 106

Compare the properties and inhibition of glucokinase and hexokinase, and how this relates to their function. [IMPORTANT]

Answer 106

Glucokinase (liver, pancreatic islets):

• Co-operative kinetics
• High K_m and V_max
• This enables sensitivity to glucose concentration (‘glucose sensor’). At low glucose concentrations during fasting, the high Km means that glucose is not unnecessarily converted to glycogen, thereby engendering hypoglycaemia. The high Vmax means that the liver is able to rapidly convert glucose into glycogen after a meal, performing its role as an excess glucose store.
• Glucokinase is induced by insulin and inhibited by cortisol, epinephrine, glucagon and growth hormone, helping the liver respond to blood glucose more efficiently.

Hexokinase (most tissues):

• Michaelis Menten kinetics
• Low K_m and V_max
• Means that glucose conversion occurs at a relatively high and constant rate, even at low blood glucose concentrations, meaning that glucose uptake into cells can occur due to maintenance of concentration gradients.
• Regulation of glucose conversion (and therefore uptake) occurs by allosteric inhibition of hexokinase by G6P, which is a form of negative feedback, so that G6P levels remain constant in skeletal muscle cells.

Question 107

What is hexokinase inhibited by?

Answer 107

Glucose-6-phosphate (this is a form of negative feedback).

Question 108

Draw the entire glycolysis pathway, including enzymes.

Answer 108

Question 109

Fructose 1,6-bisphosphate is a 6C molecule that is split into two 3C molecules in glycolysis. What are these 3C molecules and what enzyme catalyses this?

Answer 109

• Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
• Enzyme: Triose phosphate isomerase (TPI)
  
  • Catalytically perfect enzyme
  • At equilibrium, 96% of triose phosphate is DHAP
  • TPI deficiency is a rare autosomal recessive disorder -> Leads to haemolytic anaemia and cardiomyopathy

Question 110

Considering this pathway, describe the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate and how this process helps generate ATP.

Answer 110

Glyceraldehyde-3-P + NAD⁺ + P_i → 1,3-bisphosphoglycerate + NADH + H⁺

Of the original energy of glucose:
• Energy in 1,3-bisPGA used directly for synthesis of ATP (later)
• Energy in NADH used indirectly for synthesis of ATP (also later)

Question 111

What is the name for the ATP-generating stages of glycolysis? [IMPORTANT]

Answer 111

Substrate-level phosphorylation

Question 112

Define substrate-level phosphorylation.

Answer 112

• Substrate-level phosphorylation refers to the formation of ATP from ADP and a phosphorylated intermediate, rather than from ADP and inorganic phosphate, as is done in oxidative phosphorylation.
• It is one of the two ways that respiration generates

Question 113

How much ATP does glycolysis produce?

Answer 113

• 2 ATP per glucose molecule
• This is because 2 ATP is invested near the start, then 2 ATP is produced per 3C molecules generated

NOTE: The pyruvate can then continue further through respiration to produce more ATP.

Question 114

What are the products of glycolysis?

Answer 114

Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules.

Question 115

At what points does glycolysis generate ATP?

Answer 115

Question 116

What are the fates of the products of glycolysis?

Answer 116

• Pyruvate -> It depends on the metabolic state of the cell (aerobic or anaerobic)
• NADH -> It needs to get inside the mitochondria to participate in the ETC

Question 117

Compare the fate of pyruvate (after glycolysis) in aerobic and anaerobic conditions.

Answer 117

Question 118

What are two metabolic cycles that involve pyruvate?

Answer 118

• Cori cycle
• Cahill (or glucose-alanine) cycle

Question 119

Draw the Cori cycle.

Answer 119

Question 120

Draw the Cahill cycle.

Answer 120

Question 121

Remember to add notes about the malate-aspartate shuttle.

Answer 121

Do it.

Question 122

In one sentence, describe how glycolysis is regulated.

Answer 122

Glycolysis is regulated by the energy needs of the cell.

Question 123

Draw a diagram to show the different points where glycolysis can be regulated.

Answer 123

Question 124

Describe how control of glycolysis occurs at hexokinase (most tissues).

Answer 124

Hexokinase is inhibited by its product, G-6-P.

Question 125

Describe how control of glycolysis occurs at glucokinase (liver and pancreas).

Answer 125

Glucokinase is induced by insulin and inhibited by cortisol, epinephrine, glucagon and growth hormone, helping the liver respond to blood glucose more efficiently.

Question 126

What enzyme is the key point of regulation of glycolysis? What reaction does it catalyse? Why is it this such an important regulatory step?

Answer 126

• Phosphofructokinase-1 (PFK)
• It catalyses the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate
• This is such an important step because it is:
  
  • Irreversible
  • It consumes energy
  • It is a committed step (Glycogen synthesis / PPP are other fates for G-6-P)

Question 127

Describe how control of glycolysis occurs at phosphofructokinase.

Answer 127

• Inhibited by:
  
  • ATP [IMPORTANT]
  • Citrate
  • H⁺
• Activated by:
  
  • AMP
  • Fructose-2,6-bisphosphate [IMPORTANT]

Question 128

Describe the importance of fructose-2,6-bisphosphate in control of glycolysis. What type of regulation is this? [IMPORTANT]

Answer 128

Question 129

What type of regulation of pyruvate kinase in muscle do you need to know about?

Answer 129

Feed-forward activation by fructose-1,6-bisphosphate.

Question 130

Describe how control of glycolysis occurs at pyruvate kinase.

Answer 130

Question 131

Remember to add flashcards on the different isoenzymes of glycolytic enzymes.

Answer 131

Do it.

Question 132

Aside from glucose, what are some different sugars that can feed into glycolysis?

Answer 132

• Fructose
• Sucrose
• Lactose
• Galactose

Question 133

Describe how fructose can enter the glycolysis pathway.

Answer 133

Question 134

Describe how galactose can enter the glycolysis pathway.

Answer 134

Question 135

Describe the Warburg effect.

Answer 135

• Most cancer cells produce energy from a high rate of glycolysis followed by the conversion of pyruvate to lactate, rather than oxidative phosphorylation in the mitochondria, even in the presence of oxygen
• Many different reasons have been proposed for this effect but there are still many questions to be answered

Question 136

What are some other names for the Krebs cycle?

Answer 136

• TCA cycle (Tricarboxylic acid cycle)
• Citric acid cycle

Question 137

Is the Krebs cycle aerobic? Why?

Answer 137

• Yes, because oxygen is required in the electron transport chain FURTHER DOWN the pathway (oxidative phosphorylation) in order to regenerate NAD⁺ from NADH.
• Without oxygen, NADH accumulates and the cycle cannot continue as it needs NAD⁺ to run. Also, glycolysis produces lactic acid instead of pyruvate, which is necessary in the Krebs cycle.

Question 138

Name the 3 stages of cellular respiration and name which ones are aerobic and anaerobic.

Answer 138

• Glycolysis - Anaerobic
• Krebs cycle - Aerobic
• Electron transport chain - Aerobic

Question 139

In which part of the cell does the Krebs cycle occur?

Answer 139

Mitochondrial matrix

Question 140

Is the Krebs cycle only involved in cellular respiration?

Answer 140

No, it also participates in several important synthetic reactions:

• Glucose
• Amino acids
• Haem

Question 141

What molecule is seen as the “starting point” of the Krebs cycle?

Answer 141

Acetyl-CoA is seen as the main susbstrate. Other intermediates can also feed into the cycle though.

Question 142

Describe how the Krebs cycle fits into other metabolic pathways.

Answer 142

• Different pathways of substrate oxidation converge on a small number of common molecules -> Acetyl-CoA (which feeds into the Krebs cycle) or one of the Krebs cycle intermediates
• This means that not only glucose metabolism feeds into the Krebs cycle (via glycolysis), but also fat oxidationand protein oxidation

Question 143

At which point in glucose oxidation does the pathway enter the mitochondria?

Answer 143

After glycolysis, pyruvate is transported into the mitochondria, where it is converted to acetyl-CoA, then enters the Krebs cycle.

Question 144

Draw a summary of cellular respiration, including the place in the cell where each stage take place.

Answer 144

Question 145

Draw a diagram to show how glucose, fatty acid, amino acid and ketone body oxidation fits into the Krebs cycle.

Answer 145

Question 146

What are some of the functions of the Krebs cycle?

Answer 146

• Common intermediates can be completely oxidised to CO₂ and H₂O (allowing full oxidation of fuels)
• Common intermediates can be used as starting materials for biosynthetic pathways
• Produces NADH (for oxidation and energy yield)
• Produces FADH₂ (for oxidation and energy yield)
• Produces energy in the form of GTP (-> ATP)

Question 147

Summarise the function of the Krebs cycle in a simple diagram.

Answer 147

Question 148

In cellular metabolism, name the 3 main types of molecule involved in oxidation-reduction reactions.

Answer 148

• NAD⁺/NADH
• FAD/FADH₂
• NADP⁺/NADPH

Question 149

Out of NADH, NADPH and FADH₂, which are involved in the Krebs cycle and electron transport chain?

Answer 149

NADH and FADH₂

Question 150

What structure is this?

Answer 150

NADH

Question 151

What structure is this?

Answer 151

ATP

Question 152

What structure is this?

Question 153

NAD⁺ and FAD are … agents.

Answer 153

Oxidising

i.e. They accept electrons.

Question 154

What are pyridine dehydrogenases and flavin dehydrogenases?

Answer 154

Enzymes that use NAD⁺ and FAD as electron acceptors, respectively.

Question 155

Write the equation for the use of NAD⁺ as an oxidising agent. What is the standard redox potential for this?

Answer 155

E’_O = - 0.32 V

Question 156

Write the equation for the use of FAD as an oxidising agent. What is the standard redox potential for this?

Answer 156

E’_o = -0.18V

Question 157

What type of molecule are NAD⁺ and FAD?

Answer 157

Coenzymes/Cofactors

(NOTE: Cofactors are molecules that assist the function of an eznyme. Coenzymes are a type of cofactor that are organic. CHECK THIS!)

In the lecture, NAD⁺ is named a coenzyme, while FAD is a cofactor.

Question 158

Give the equation for how NAD⁺ is used in reactions.

Answer 158

XH₂ + NAD⁺ -> X + NADH + H⁺

This is reversible.

Question 159

GIve the equation for how FAD is used in reactions.

Answer 159

YH₂ + FAD -> Y + FADH₂

This is reversible.

Question 160

Compare how NAD⁺ and FAD may be considered as cofactors.

Answer 160

• NAD+ and NADH should really be considered as substrate/product, rather than a cofactor -> They are free to diffuse away from the enzyme
• FAD and FADH₂ should be considered as a cofactor -> They are covalently bound to the enzyme and are not free to diffuse to the inner membrane. The enzymes must be physically associated with the membrane for the ETC.

Question 161

Which has a higher reducing potential: NAD⁺ or FAD?

Answer 161

NAD⁺

Question 162

Draw the Krebs cycle, showing the intermediates and products.

Answer 162

Question 163

How many carbon atoms is an acetyl-CoA molecule?

Answer 163

2

Question 164

When asked to name the products of the Krebs cycle, what is it important to remember?

Answer 164

Acetyl-CoA enters the Krebs cycle and goes round it once. However, two acetyl-CoA molecules are produced per molecule of glucose, so if asked to name the products of the Krebs cycle per molecule of glucose, you must double the numbers.

Question 165

What are the products of one turn of the Krebs cycle?

Answer 165

• 3 NADH
• 1 FADH₂
• 1 GTP (equivalent of 1 ATP)
• 2 CO₂

NOTE: This must be doubled for a molecule of glucose because each glucose molecule produces two acetyl-CoA molecules that enter the Krebs cycle.

Question 166

Calculate how much ATP is produced per turn of the Kreb cycle.

Answer 166

NOTE: This must be doubled for a molecule of glucose because each glucose molecule produces two acetyl-CoA molecules that enter the Krebs cycle.

Question 167

What macromolecules does the acetyl-CoA feeding into the Krebs cycle come from? By what processes are these molecules converted to acetyl-CoA?

Answer 167

• Fatty acids -> β-oxidation
• Ketone bodies -> Ketone body oxidation
• Amino acids -> Amino acid degradation
• Sugar carbohydrates -> Glycolysis to pyruvate, then link reaction

Question 168

Does glycolysis feed directly into the Krebs cycle?

Answer 168

No, the pyruvate produced by glycolysis must go through the link reaction to produce acetyl-CoA, which feeds into the Krebs cycle.

Question 169

What is another name for the link reaction?

Answer 169

The pyruvate dehydrogenase reaction.

Question 170

What enzyme catalyses the link reaction?

Answer 170

Pyruvate dehydrogenase

Question 171

What is the importance of pyruvate dehydrogenase?

Answer 171

• It catalyses the conversion of pyruvate (from glycolysis) to acetyl-CoA (which can enter the Krebs cycle)
• This is an important step because it is irreversible and therefore commits the glucose to respiration

Question 172

How is pyruvate transported into the mitochondrion (prior to the Krebs cycle)?

Answer 172

By a specific H⁺ symport.

Question 173

Is pyruvate dehydrogenase a single enzyme?

Answer 173

No, it is really a complex of 3 enzymes that perform 3 functions:

• E₁ - Pyruvate decarboxylase / Pyruvate dehydrogenase
• E₂ - Dihydrolipoyl transacetylase
• E₃ - Dihydrolipoyl dehydrogenase

Question 174

What do points of metabolic control have in common?

Answer 174

• Irreversible / energy costly
• Committed steps – “points of no return”
• Energy sensing

Question 175

What are the two forms of pyruvate dehydrogenase and how are they interchanged?

Answer 175

• There is an inactive and active form
• PDH is inactivated by PDH kinase
• PDH is activated by PDH phosphatase

Question 176

Describe the control of the activity of pyruvate dehydrogenase. [IMPORTANT]

Answer 176

• PDH is directly inhibited by NADH and acetyl-CoA
• PDH kinase phosphorylates PDH to make it inactive
  
  • Stimulated by: ATP, NADH and acetyl-CoA
  • Inhibited by: ADP, pyruvate, CoA-SH and NAD⁺
  • Levels upregulated by fasting, high-fat feeding and diabetes
• PDH phosphatase dephosphorylates PDH to make it active
  
  • Inhibited by: Mg²⁺, Ca²⁺ and insulin

Question 177

Draw the Krebs cycle in detail.

Answer 177

Question 178

What is the importance of GTP in the Krebs cycle?

Answer 178

• Phosphoryl donor in protein synthesis, gluconeogenesis
• Signal transduction
• Translocation of proteins into the mitochondrial matrix
• Conversion to ATP

Question 179

Describe how GTP is essentially converted to ATP.

Answer 179

Question 180

Succinate dehydrogenase catalyses the conversion of succinate to fumarate in the Krebs cycle. Describe the properties of this enzyme and what makes it special.

Answer 180

• Embedded in inner mitochondrial membrane
• Directly linked to the electron transport chain
• Only enzyme common to both TCA cycle and ETC

Question 181

In the Krebs cycle, the conversion of succinate to fumarate by succinate dehydrogenase produces FADH₂ instead of NADH. How does this happen and why?

Answer 181

• FAD is hydrogen acceptor because free energy is insufficient to reduce NAD+
• Two electrons from FADH₂ are transferred directly to enzyme Fe-S clusters and then on to ubiquinone (QH₂)

Question 182

What is the significance of the Krebs cycle being a cycle rather than a linear pathway?

Answer 182

• Catalytically small amounts of cycle intermediates are required to oxidise large amounts of acetyl-CoA
• For example, if the cycle is blocked between succinate and oxaloacetate, lots of oxaloacetate is required to react with the acetyl-CoA since it is not regenerated

Question 183

Describe the regulation of the Krebs cycle.

Answer 183

Question 184

Describe how the Krebs cycle may be activated under an conditions that involve high energy demand.

Answer 184

Question 185

Show the routes be which some amino acids and odd-chain fatty acids can enter the Krebs cycle.

Answer 185

Question 186

Show how the Krebs cycle can be a starting point for biosynthesis of different molecules.

Answer 186

Question 187

Is the control of the Krebs cycle dependent on substrate availability?

Answer 187

No, it is related to demand for ATP, not substrate availability.

Question 188

What are anaplerotic reactions and what is their significance in the Krebs cycle? [IMPORTANT]

Answer 188

• Reactions that are used to resynthesise the intermediates of the Krebs cycle
• They are important becausethe intermediates of the Krebs cycle are used in biosynthesis, so their anaplerotic reactions help maintain their concentrations

Question 189

Give an example of an anaplerotic reaction in the Krebs cycle and how this relates to biosynthesis pathways.

Answer 189

Question 190

Draw all of the anaplerotic reactions in the Krebs cycle.

Answer 190

Question 191

What reaction does pyruvate carboxylase catalyse and what is the importance of this?

Answer 191

• It catalyses the conversion of pyruvate directly to oxaloacetate
• This is an anaplerotic reaction that is used to replenish oxaloacetate when it is depleted (the reaction is triggered by a build-up of acetyl-CoA)
• It allows the Krebs cycle to keep happening

Question 192

Describe the regulation of pyruvate decarboxylase.

Answer 192

It is activated by acetyl-CoA, which builds up when there is a lack of oxaloacetate (so it is a marker for this).

Question 193

Does the intracellular ATP concentration fluctuate much?

Answer 193

No, it is near constant at 6mM.

Question 194

What enzyme catalyses the production of a molecule of AMP and ATP from two molecules of ADP?

Answer 194

Adenylate kinase

Question 195

Compare the relative concentrations of ATP, ADP and AMP. [IMPORTANT]

Answer 195

• ATP = 6mM
• ADP = 10μM
• AMP = 5nM

Question 196

What does the presence of AMP signal?

Answer 196

It signals energy distress, resulting in activation of AMP-activated protein kinase (AMPK), which upregulates energy generating pathways and suppresses energy consuming processes within the cell.

Question 197

What do ADP and AMP signal? [IMPORTANT]

Answer 197

• ADP -> Signals ATP utilisation to mitochondria
• AMP -> Cytoplasmic signal to upregulate glycolysis

Question 198

Compare the permeability of the two mitochondrial membranes.

Answer 198

• Outer membrane = Relatively permeable
• Inner membrane = Highly impermeable

Question 199

For which pathways does mitochondria contain the enzymes?

Answer 199

• Electron transport chain
• Krebs cycle and PDH
• β-oxidation
• Ketone body metabolism
• Urea cycle

Question 200

State the percentage of ATP synthesis by susbtrate level phosphorylation and by oxidative phosphorylation.

Answer 200

• Substrate-level phosphorylation = 5% total ATP
• Oxidative phosphorylation = 95% total ATP

Question 201

Define oxidative phosphorylation.

Answer 201

The process of ATP synthesis resulting from the transfer of electrons from NADH and FADH₂ to oxygen by a series of electron carriers.

Question 202

Describe how the rest of metabolism relates to oxidative phosphorylation.

Answer 202

Metabolism of carbohydrates, fatty acids and amino acids (mostly in the Krebs cycle) produces reduced co-factors (NADH and FADH₂), which can they been oxidised in oxidative phosphorylation to produce ATP.

Question 203

What is the location of oxidative phosphorylation?

Answer 203

Electron transport chain -> A series of electron transporters embedded in the inner mitochondrial membrane .

Question 204

What are the two coupled processes in oxidative phosphorylation?

Answer 204

1) Generation of a proton gradient

• Respiration: Oxidises hydrogen carriers, transports electrons, consumes oxygen and produces water
• This generates a proton gradient across the mitochondrial inner membrane

2) ATP synthesis

• Uses proton gradient across inner mitochondrial membrane
• Phosphorylates ADP to ATP

Question 205

What is the chemiosmotic theory?

Answer 205

In ATP synthesis, electron transport and ADP phosphorylation are indirectly linked:

1. Movement of electrons drives proton pumping from the matrix to the intermembrane space
2. This creates an electrical and pH gradient across the inner memrbane (proton motive force)
3. Protons then move down their gradient through the phosphorylation apparatus to drive ATP synthesis

Question 206

Is the inner mitochondrial membrane permeable to H⁺ ions? What is the result of this?

Answer 206

No, it is impermeable, which means that extrusion of H⁺ creates a pH and electric potential gradient across the membrane.

Question 207

How is oxidative phosphorylation defined in the syllabus?

Answer 207

Indirect coupling of energy release from oxidation of energy substrates to the synthesis of ATP.

Question 208

What are the parts of the electron transport chain?

Answer 208

It can be thought of as large protein complexes linked by smaller, mobile intermediates.

4 large complexes, each consisting of tens of proteins:

• Complex I
• Complex II
• Complex III
• Complex IV

Linked by 2 small mobile electron carriers:

• Ubiquinone (Q) links Complexes I and II onto Complex III
• Cytochrome c links Complex III to IV.

Question 209

Draw the structure of the electron transport chain.

Answer 209

Question 210

What are the names of the two intermediates that link the complexes in the electron transport chain?

Answer 210

• Ubiquinone (symbolised by Q)
• Cytochrome (symbolised by Cyt c)

Question 211

Describe the reactions and how the redox potential changes along the electron transport chain.

Answer 211

• The ETC features multiple redox centres allowing sequential redox reactions
• The redox potential of these reactions increases along the ETC
• This allows the transfer of electrons from NADH/FADH₂ to O₂ via the ETC

Question 212

What is the starting electron donor and final electron acceptor in the electron transfer chain?

Answer 212

• Starting donor = NADH or FADH₂
• Final acceptor = O₂

Question 213

What is the overall redox potential of the electron transport chain and what is the significance of this?

Answer 213

• E’_o = +1.14V
• This is a large potential difference, which is important because it maintains movement of electrons along the ETC and provides lots of energy for H⁺ pumping

Question 214

Draw the sequence of redox reactions in the electron transport chain.

Answer 214

Question 215

What’s the overall reaction for the electron transport chain?

Answer 215

1/2O₂ + NADH + H⁺ -> H₂O

Question 217

What are oxidation/reduction centres in the ETC and what is their importance?

Answer 217

• They are groups within complexes that allow electron movement through oxidation/reduction reactions along the ETC
• Different complexes have different oxidation/reduction centres
• Each oxidation/reduction centre has a different affinity for the electrons, so movement continues

Question 218

What factors affect the redox potential of an oxidation/reduction centre?

Answer 218

• Type of oxidation/reduction centre (e.g. haem)
• Protein environment of the complex it is in

Question 219

What are some of the oxidation/reduction centres you need to know about?

Answer 219

• Haem
• Iron-sulphur centre
• Ubiquinone
• Copper

Question 220

Describe where the different oxidation/reduction centres in the electron transport chain are found.

Answer 220

• Iron-sulphur clusters -> In complexes I, II and III
• Haem -> In complexes III and IV, and in cytochrome c
• Copper -> In complex IV
• Ubiquinone

Question 221

What reaction occurs at the iron-sulphur cluster oxidation/reduction centre?

Answer 221

Fe²⁺ ⇌ Fe³⁺ + e^-

Question 222

What reaction occurs at the haem oxidation/reduction centre?

Answer 222

Fe²⁺ ⇌ Fe³⁺ + e^-

Question 223

What reaction occurs at the copper oxidation/reduction centre?

Answer 223

Cu⁺ ⇌ Cu²⁺ + e^-

Question 224

Describe what the substrates are for complex I in the ETC, what the mechanism is, and what the products are.

Answer 224

Question 225

Where in the ETC are NADH and FADH₂ substrates?

Answer 225

NADH -> Complex I

FADH₂ -> Complex II

Question 226

Is there any communication between complexes I and II in the ETC?

Answer 226

No, they both feed into complex III though.

Question 227

Describe what the substrates are for complex II in the ETC, what the mechanism is, and what the products are.

Answer 227

Question 228

Does complex II contribute to the proton gradient in the ETC?

Answer 228

No, complex II does not fully span the membrane, so no proton pumping occurs at complex II.

Question 229

What is another name for ubiquinone?

Answer 229

Co-enzyme Q10 (hence the symbol Q)

Question 230

In the ETC, what does the symbol Q symbolise?

Answer 230

Ubiquinone

Question 231

How is ubiquinone retained into the intermembranal space?

Answer 231

It is highly hydrophobic.

Question 232

What is the role of ubiquinone in the ETC?

Answer 232

Note: Ubiquinol is then passed to complex III.

Question 233

Describe what the substrates are for complex III in the ETC, what the mechanism is, and what the products are.

Answer 233

Question 234

What is the role of cytochrome c in the ETC?

Answer 234

Transports 1 electron from complex III to IV.

Question 235

What is the important prosthetic group on cytochrome c?

Answer 235

Haem

Question 236

Describe what the substrates are for complex IV in the ETC, what the mechanism is, and what the products are.

Answer 236

Question 237

How many cytochromes c are used and how many protons are moved at complex IV?

Answer 237

• The process itself requires 4 cytochrome c molecules (i.e. 4 e^-), which enable 4 protons to be moved and one O₂ molecule to be reduced
• However, each NADH or FADH₂ donates 2 electrons, so 2 NADH or FADH₂ molecules are required to reduce an O₂ molecule
• To get round the doubling up situation you often see it referred to as 2 cytochrome c, ½ O₂ and 2 H⁺ pumped

Question 238

Compare the number of H⁺ ions pumped by NADH and FADH₂ in the electron transport chain and show this in a diagram of the ETC.

Answer 238

Question 239

Draw the organisation of the phosphorylation apparatus of the ETC.

Answer 239

Question 240

What is the enzyme that uses the proton gradient (generated by the ETC) to phosphorylate ADP to ATP?

Answer 240

ATP synthase

Question 241

What are the two subunits of ATP synthase? [IMPORTANT]

Answer 241

• F₀
• F₁

Question 242

Draw the structure of ATP synthase and briefly describe how it works.

Answer 242

Question 243

Describe how the F₀ subunit of ATP synthase works.

Answer 243

Question 244

Describe how the F₁ subunit of ATP synthase works.

Answer 244

Question 245

What happens to ATP synthase if there is no H⁺ gradient?

Answer 245

It reverses.

Question 246

Describe the transport of ADP and ATP across the inner mitochondrial membrane and how this affects the electrochemical gradient across it.

Answer 246

• Adenine nucleotide translocase (ANT)
• This moves ATP out into the intermembranal space, while moving ADP into the matrix
• This discharges the electrochemical gradient (i.e. the proton gradient created by the ETC)

Question 247

Aside from the ANT (exchanging ADP and ATP), what is an example of a different transporter in the ETC that discharges the electrochemical gradient across the inner mitochondrial membrane?

Answer 247

The phosphate pump.

Question 248

Describe how discharge of the proton gradient in oxidative phosphorylation affects the electron transport chain. What is the term for this?

Answer 248

• Discharge of the proton gradient stimulates the electron transport chain
• This in turn means that there is more need for susbtrate oxidation (e.g. use of glucose)
• This is called “respiratory control” coupling

Question 249

What is thermogenesis and where is it important?

Answer 249

• Generation of heat
• It is important mainly in brown adipose tissue in newborns

Question 250

Describe how thermogenesis in brown adipose tissue works.

Answer 250

• Cells express uncoupling protein 1 (UCP1), also known as thermogenin, which is a membrane protein
• It acts in parallel to ATP synthase and uses the same H⁺ gradient, except it does not produce ATP
• Therefore, it dissipates the gradient without producing ATP, instead resulting in non-shivering heat generation while using up the fuel to maintain the proton gradient

Question 251

What are un-couplers in cellular respiration?

Answer 251

Drugs that uncouple substrate oxidation and ATP generation, leading to less efficient respiration overall.

Question 252

Name a drug that can uncouple substrate oxidation from ATP generation.

Answer 252

2,4-dinitrophenol (a.k.a. DNP)

Question 253

Describe briefly how hormones are involved in control of metabolism.

Answer 253

Hormones signal to metabolism in distant tissues:

• The whole body’s nutritional status (fed vs fasted state)
• The whole body’s energy needs (fight or flight requires more ATP)

Metabolism in distant tissues responds by:

• Taking substrate from blood into tissue
• Returning substrate from tissue to blood
• Diverting substrate within tissue into energy generating pathways

Question 254

What are the 3 main hormones influencing metabolism?

Answer 254

• Insulin
• Glucagon
• Adrenaline

Question 255

For insulin, state:

• Where it is released
• What stimulates its release
• What tissues it acts on
• What is signals + the effect

Answer 255

• Released by β-cells of the pancreas
• Stimulated by high blood glucose, certain amino acids and certain fatty acids
• Acts on muscle, adipose and other tissues
• Signals fed state and tells tissue to store fuel and breakdown glucose

Question 256

For glucagon, state:

• Where it is released
• What stimulates its release
• What tissues it acts on
• What is signals + the effect

Answer 256

• Released by α-cells of the pancreas
• Stimulated by low blood glucose
• Acts only on liver
• Signals fasted state, so that glucose is released from the liver into the blood

Question 257

For adrenaline, state:

• Where it is released
• What stimulates its release
• What tissues it acts on
• What is signals + the effect

Answer 257

• Released by the adrenal gland
• Stimulated by sympathetic innervation
• Acts on nearly all tissues, although effect varies between tissues
• Signals the fight or flight response + Tells tissues to divert substrates towards making ATP

Question 258

What are lipids? Name some examples.

Answer 258

A diverse group of molecules:

• Non-esterified fatty acids (a.k.a. free fatty acids)
• Triglycerides (TAG)
• Sterols
• Phospholipids

Question 259

What is the shorthand for triglycerides and free fatty acids? Why?

Answer 259

• Triglycerides = TAG (this stands for triacylglycerides)
• Free fatty acid = NEFA (this stands for non-esterified fatty acid)

Question 260

What are the 3 main functions of lipids? Which lipid types carry out each role?

Answer 260

1. Source of energy -> NEFA (free fatty acids) and triglycerides
2. Structural roles -> Phospholipids and cholesterol
3. Signalling role -> Phospholipids, prostoglandins and steroid hormones

Question 261

What is the approximate average amount of fat stores in the body?

Answer 261

11kg

Question 262

Name two types of tissue that cannot use fat as fuel.

Answer 262

• Brain
• Red blood cells

Question 263

What are some of the advantages and disadvantages of fats as metabolic fuels?

Answer 263

Advantages:

• More energy dense per molecule than glucose
• Rapidly mobilised and stored
• Ideal for tissues with high energy demand

Disadvantages:

• Can’t be used by all tissues (brain, red blood cells)
• Requires more oxygen to extract the energy than glucose

Question 264

Draw a summary of the processes including fats that you need to know about.

Answer 264

Question 265

Describe generally how fats are taken from the diet into tissues that use them.

Answer 265

• They are digested in the lumen of the small intestine
• When they have been packed into micelles, they can be taken into intestinal cells
• Components are then packed into chylomicrons, which are taken via lymphatics to tissues (including muscle and adipose tissues)

Question 266

Describe the digestion and absorption of dietary fat in the small intestine.

Answer 266

• Bile salts in the small intestine break down large triglyceride droplets into smaller droplets
• Lipases from the pancreas can now attack these smaller droplets (they require a large surface area to function)
• Lipases break down the triglycerides (TAG) into free fatty acids (NEFA) and glycerol -> This forms micelles
• Micelles can now be taken up into intestinal cells

Question 267

What is a brush border membrane?

Answer 267

The microvilli-covered surface of simple cuboidal and simple columnar epithelium found in different parts of the body (e.g. the small intestine).

Question 268

What are chylomicrons and what is their function?

Answer 268

• Small fat globule composed of protein and lipid.
• Chylomicrons are found in the blood and lymphatic fluid where they serve to transport fat from the intestine to the peripheral tissues.

Question 269

Describe the packing of chylomicrons.

Answer 269

• Micelles containing free fatty acids and glycerol cross the brush border membrane of intestinal cells to release the contents inside
• The glycerol and fatty acids are reassembled:
  
  • First, they form 2-monoacylglycerol
  • Then they form 1,2-diacylglycerol
  • Finally, they the triacylglycerol
• In other words, the triglycerides are broken down in the lumen of the intestine, taken across the membrane, and then reassembled in intestinal cells
• Triglycerides, cholesterol and phospholipids are then packed into chylomicrons

Question 270

Describe how fats are transported to peripheral tissues after absorption in the small intestine.

Answer 270

Chylomicrons are transported in the lymphatic system to peripheral tissues.

Question 271

Once chylomicrons containing triglycerides reach peripheral tissues, how are they taken up by those tissues? How is this controlled by hormones?

Answer 271

• The endothelial cells of the lymphatic system have enzymes on the surface called lipoprotein lipase (LPL)
• LPL hydrolyses triglycerides into free fatty acids and glycerol, enabling their uptake into adipose tissue and muscle
• LPL is induced and upregulated by insulin

Question 272

What happens to the free fatty acids that are taken up into adipose tissue?

Answer 272

They are recombined with glycerol to form triglycerides for storage.

Question 273

What enzyme enables the breakdown of triglycerides in adipose tissue?

Answer 273

Hormone-sensitive lipase (HSL)

Question 274

In fat metabolism, what do LPL and HSL stand for?

Answer 274

• LPL = Lipoprotein lipase
• HSL = Hormone-sensitive lipase

Question 275

How are free fatty acids transported in the blood?

Answer 275

Bound to albumin protein, as NEFA isn’t water soluble.

Question 276

Name three lipases that break down triglycerides into free fatty acids and glycerol.

Answer 276

• Adipose triglyceride lipase
• Hormone sensitive lipase (HSL)
• Monoacylglycerol lipase

Question 277

Describe how lipolysis in adipose tissues can be regulated by hormones.

Answer 277

• The enzyme responsible for lipolysis is hormone-sensitive lipase (HSL).
• Stimulated by adrenaline:
  
  • Adrenaline increases cAMP
  • cAMP stimulates PKA
  • PKA phosphorylates HSL, activating it
• Also stimulated by fasting glucagon and thyroxine (thyroid hormone)
• Inhibited by insulin:
  
  • Insulin stimulates protein phosphatase
  • Protein phosphatase dephosphorylates HSL, inactivating it

Question 278

Describe plasma NEFA levels under different metabolic conditions:

• Fed state
• Prolonged exercise
• Stress/trauma
• Fasting
• Under influence of thyroxine

Answer 278

• Fed state (insulin) -> 0.3 - 0.6 mmol/L
• Prolonged exercise -> 2.0 mmol/L
• Stress/trauma -> 0.8 - 1.8 mmol/L
• Fasting -> 0.5 - 2.0 mmol/L
• Under influence of thyroxine -> 0.6 - 0.8 mmol/L

Question 279

Describe the amount of fats stored in non-adipose tissue and why this is the case.

Answer 279

• Small stores of TAG in cells provide a source of energy if need to suddenly increase oxidative phosphorylation.
• TAG stores must be kept low to avoid compromising normal cell function.
• If you put too much TAG into non-adipose tissue it can lead to steatosis. i.e. non-alcoholic fatty liver disease and cardiac lipotoxicity in type 2 diabetes.
• Example stores in: Heart, renal cortex and skeletal muscle

Question 280

Describe how the heart uses fatty acids. How are they supplied to the heart?

Answer 280

• It has a very high metabolic demand and it is one of the organs that can use non-glucose substrates for fuel
• Therefore, 60-70% of ATP in the heart comes from oxidation of fatty acids
• These are supplied as NEFA-albumin, CM-TAG or VLDL-TAG (very low density lipoprotein)
• If external (exogenous) fatty acid supply is insufficient, the heart will metabolise its intracellular (endogenous) TAG reserves in addition

Question 281

Describe how the renal cortex and medulla use fatty acids in metabolism and what fraction of renal cortex metabolism comes from different fuel sources.

Answer 281

Renal cortex:

• Able to use various metabolic fuels:
  
  • Plasma NEFAs = >50%
  • TAG (endogenous) = 40-50%
  • Amino acids = 5-10%
  • Glucose = <1%

Renal medulla:

• Has poor oxygen supply so there is limited mitochondrial respiration and a more anaerobic metabolic phenotype

Question 282

What are the names of the three muscle types?

Answer 282

• Type I - Slow twitch
• Type IIa - Slower fast twitch
• Type IIb - Faster fast twitch

Question 283

Which of the muscle fibre types is most oxidative and glycolytic?

Answer 283

• Most oxidative (aerobic) = Type I, Slow twitch
• Most glyolytic (anaerobic) = Type IIb, Faster fast twitch

Question 284

Compare the different muscle fibre types in terms of:

• Rate of contraction
• Blood flow
• Glycolysis
• Glycogen stores
• Mitochondrial number
• Fatty acid oxidation
• Triglyceride stores

Answer 284

Question 285

Describe the percentage fuel contribution of NEFA, glucose and glycogen:

• At rest
• During exercise <40 mins
• During exercise >4 hours

Answer 285

Question 286

In order for NEFA that are taken up into cells to be metabolised (e.g. in the Krebs cycle), what must they be converted into?

Answer 286

Acetyl-CoA (and reduced co-factors are also produced)

Question 287

What are the 4 steps that are required to convert NEFAs (that have been taken up from the blood into) into acetyl-CoA so that they can be metabolised in the Krebs cycle?

Answer 287

1. Uptake -> NEFAs cross the cell membrane from the blood into the cell
2. Activation -> NEFAs converted to fatty acyl CoA
3. Carnitine shuttle
4. β-oxidation

Question 288

Describe the uptake of NEFAs across cell membranes.

Answer 288

Two mechanisms for fatty acid uptake:

• Diffusion across the membrane via a flip-flop mechanism
• Transporter-mediated uptake -> Allows for control

Question 289

What is the transporter involved in the uptake of NEFAs from the blood into oxidative tissues?

Answer 289

Fatty acid translocase (FAT/CD36) -> It is a primary transporter

Question 290

How are NEFAs made to be soluble in the cytosol of oxidative tissues?

Answer 290

• Intracellular fatty acids are bound to cytoplasmic Fatty Acid Binding Protein (cFABP).
• This protein performs a similar role to albumin.

Question 291

How are NEFAs actiavted when they are uptaken into oxidative tissues and what is the significance of this?

Answer 291

• Activated by adding a co-enzyme A molecule to their structure, which requires ATP.
• This produces fatty acyl-CoA
• This is catalysed by acyl-CoA synthetase (ACS).
• Significance: This traps the fatty acid in the cell and commits it to a metabolic pathway (just like phosphorylation of glucose)

Question 292

What structure is this?

Answer 292

Co-enzyme A

Question 293

After NEFAs are converted to fatty acyl CoA, what is the difficulty with getting them into the mitochondria (prior to the Krebs cycle, for example)? What is the solution to this?

Answer 293

Problems:

• Needs to get across highly impermeable mitochondrial inner membrane to be oxidised, but there are no specific transporters for fatty acyl CoA

Solution:

• Carnitine shuttle
  
  • Turn the fatty acyl CoA into a molecule that can be transported
  • Carnitine (an amino acid derivative) can be transported, and can be conjugated to fatty acids
  • This is achieved by two enzymes (1 to add the carnitine on and 1 to take it back off) and one transporter (to get it across the inner membrane)

Question 294

Across which membrane does the carnitine shuttle work?

Answer 294

Inner mitochondrial membrane

Question 295

What are the names and roles of the two enzymes and one transporter involved in the carnitine shuttle?

Answer 295

• Carnitine Palmitoyl Transferase (CPT) 1 -> Catalyses the conjugation of fatty acyl CoA to carnitine
• Carnitine Acyl Translocase (CAT) -> Inner membrane transporter
• Carnitine Palmitoyl Transferase (CPT) 2 -> Catalyses the reverse reaction of CPT1.

Question 296

Draw the carnitine shuttle.

Answer 296

Question 297

Explain the concept of β-oxidation of NEFAs.

Answer 297

• Once NEFAs (in the form of fatty acyl CoA) have been taken up into mitochondria via the carnitine shuttle, in order to be used in the Krebs cycle and other pathways, they must be converted to acetyl-CoA.
• Acetyl-CoA is a 2 carbon molecule, so β-oxidation is a cycle in which 2 carbons are lost from the fatty acid with each loop.
• Reduced cofactors are also produced.

Question 298

Draw the pathway of β-oxidation.

Answer 298

Question 299

Why is β-oxidation called that?

Answer 299

All of the oxidation occurs at the beta carbon.

Question 300

How is β-oxidation adapted to different lengths of fatty acid and the fact that they get shorter with each cycle?

Answer 300

• There are 4 different acyl CoA dehydrogenases (the first enzyme in the cycle)
• Each one has a peak activity at a different fatty acid chain length

Question 301

What are the products of β-oxidation?

Answer 301

1. Several acetyl-CoA molecules
2. Several NADH + H⁺
3. Several FADH₂

Question 302

Using palmitate (16C) as an example, what are the products of β-oxidation?

Answer 302

• 8 acetyl CoA units
• 7 NADH + H⁺
• 7 FADH₂

Question 303

Where do the products of β-oxidation go?

Answer 303

• Acetyl-CoA goes to the Krebs cycle
• Reduced co-factors go to the electron transport chain

Question 304

Compare the ATP yield from a molecule of glucose and a molecule of palmitate. [EXTRA]

Answer 304

The numbers are somewhat disputed. For example, glucose is sometimes quoted as between 36-38 as opposed to 30-32, but this is dependent on the assumed energy yield per redued co-factor.

Question 305

Describe fatty acid nomenclature.

Answer 305

Question 306

Give the names and structures of the only 2 essential fatty acids.

Answer 306

Question 307

Describe the oxidation of other fatty acids, including unsaturated FAs, FAs with odd number of carbons, short chains FAs, very long chain FAs and branched chain FAs.

Answer 307

Question 308

What are the two ways of regulating fatty acid oxidation?

Answer 308

1. Hormone sensitive lipase:

• This changes delivery of fatty acids to peripheral tissues from adipose
• Regulation is from an extracellular signal

2. The Carnitine Shuttle

• Regulates entry of the fatty acyl CoA into the mitochondria for oxidation
• Regulation is intracellular

Question 309

Describe the control of the carnitine shuttle.

Answer 309

• Carnitine Palmitoyl Transferase 1 (CPT1) is allosterically inhibited by malonyl CoA
• Malonyl CoA is an intermediate in synthesing new fats -> Through this inhibition it also prevents the breakdown of pre-existing fat and prevents futile cycling

Question 310

What are some defects of fatty acid oxidation? [EXTRA]

Answer 310

• Systemic carnitine deficiency
• Jamaican Vomiting Sickness
• Medium Chain Acyl CoA Dehydrogenase deficiency

Question 311

What is the role of glycogen in the liver and muscle?

Answer 311

• Liver -> Used as a buffer of blood glucose
• Muscle -> Provides a local store of energy for contraction

Question 312

What are the names fo glycogen synthesis and glycogen breakdown respectively?

Answer 312

Glycogenesis and glycogenolysis

Question 313

Where in the cell is glycogen found?

Answer 313

As granules in the cytoplasm.

Question 314

What is the size of glycogen granules in the cytoplasm?

Answer 314

10 to 40nm

Question 315

Glycogen is a polymer of what?

Answer 315

α glucose

Question 316

Describe the structure of glycogen. What are the bonds?

Answer 316

• Polymer of α glucose
• Joined by mostly α-1,4 glycosidic bonds
• Every 8-10 residues contain an α-1,6 bond, which allows for branching

Question 317

Explain the concept of reducing and non-reducing ends of glycogen. Which ends is the glycogen broken down from?

Answer 317

• The molecule has one reducing end (start of the chain) and many non-reducing ends (ends of the branches)
• The non-reducing ends are the locations of all glucose additions or removals.

Question 318

Why are glycogen stores limited?

Answer 318

It is not a very efficient storage form.

Question 319

In what tissues is glycogen stored?

Answer 319

• Liver
• Muscle
• Most tissues store small amounts of their own use

Question 320

What percentage of the mass of the liver and muscle is glycogen? What is the total mass of glycogen stored in each?

Answer 320

• Liver -> 10% of mass is glycogen
• Muscle -> 2% of mass is glycogen

However, the total muscle mass is much greater, so muscle stores 200g while liver stores 70g.

Question 321

What is the function of glycogen breakdown in the liver and in the muscle?

Answer 321

In the liver:

• Glycogen is converted into glucose-6-phosphate
• G6P is converted to glucose, which can then be released to increase blood glucose

In the muscle:

• Glycogen is converted to glucose-6-phosphate
• G6P enters glycolysis to generate ATP for muscle contraction

Question 322

Why do we use glycogen as a storage form instead of storing that energy as fat?

Answer 322

• We need to be able to maintain a constant blood glucose concentration for the brain, RBC and renal medulla
• We need to be able to produce ATP rapidly in the absence of O₂

Question 323

Why does glycogen have a branched structure?

Answer 323

• Offers up multiple end points for rapid degradation
• Increases the solubility meaning that it is easier to store close to the site of utilization

Question 324

Draw the process of glycogenolysis, including the main enzymes.

Answer 324

Note: The debranching enzyme is also involved.

Question 325

Describe the process of glycogenolysis in the liver and in muscle.

Answer 325

• The major enzyme involved in breaking down glycogen is glycogen phosphorylase, which catalyses a phosphorolysis reaction that releases glucose-1-phosphate molecules from the non-reducing ends of glycogen following the attack by a phosphate molecule.
• Glycogen phosphorylase is a homodimer enzyme with active sites in clefts, which allows water to be kept from the active site, but also means that the enzyme is subject to steric hinderances at branch points.
• As a result, the glycogen must first be “debranched” by a debranching enzyme. This has dual function, with it having both transferase activity (that moves 3 residues from the branch to other non-reducing ends) and glucosidase activity (that hydrolyses alpha-1,6 bonds to remove the final molecule of glucose from the branch).
• Glucose-1-phopshate molecules released from the glycogen molecule are then typically converted to glucose-6-phosphate by the aforementioned phosphoglucomutase enzyme.
• In muscle, often this is the end of glycogenolysis, because G6P can enter other metabolic pathways, namely glycolysis, which is required in ATP production for contraction.
• In the liver, the G6P may be converted back to glucose by glucose phosphatase, which occurs because the glucose phosphatase enzyme is translated at a much higher rate in the liver.

Question 326

What does glycogen phosphorylase do and what is the name for this reaction?

Answer 326

• Involved in glycogenolysis
• Phosphorylates glucose monomers on the non-reducing ends of glycogen, releasing glucose 1-phosphate (G1P)
• Name: Phosphorolysis

Question 327

What is the significance of glycogen phosphatase catalysing a phosphorolysis reaction and how is this overcome?

Answer 327

• Water must be excluded from the active site
• Glycogen phosphorylase is a homodimer enzyme with active sites in clefts, which allows water to be kept out of the cleft
• However, the effect of this is that the enzyme is subject to steric hinderances at branch points.
• As a result, the glycogen must first be “debranched” by a debranching enzyme.

Question 328

What does phosphoglucomutase do IN GLYCOGENOLYSIS and why is this important in glycogenolysis?

Answer 328

• It converts glucose 1-phosphate (G1P) that has been released from glycogen into glucose 6-phosphate (G6P)
• In muscle cells, the G6P can directly enter the glycolytic pathway -> 50% increase in glycolytic ATP production
• In the liver, the glucose 6-phosphate is converted to glucose inside the smooth endoplasmic reticulum, ready for release into the blood

NOTE: This is a reversible reaction, with the opposite reaction occuring in glycogenesis.

Question 329

What does glucose 6-phosphatase do in glycogenolysis?

Answer 329

In the liver, glucose phosphatase converts G6P to glucose, ready for release back into the blood.

Question 330

What makes the debranching enzyme unusual?

Answer 330

It is a bifunctional enzyme.

Question 331

What are the two functions of the debranching enzyme in glycogenolysis?

Answer 331

• Transferase -> Moves 3 residues from the branch to other non-reducing end
• Glucosidase -> Hydrolyses alpha-1,6 bonds to remove the final molecule of glucose from the branch

Question 332

Draw the process of glycogenesis, including the main enzymes.

Answer 332

Question 333

Describe the process of glycogenesis in the liver and in muscle.

Answer 333

• First is the conversion of glucose into glucose-6-phophate, which follows the uptake of glucose into liver or muscle cells. This traps the molecule within the cell. This phosphorylation requires ATP hydrolysis and either glucokinase (in the liver and pancreas) or hexokinase (in all cells, including skeletal muscle).
• G6P is a metabolite involved in multiple metabolic pathways and therefore a glucose molecule converted to G6P is not yet committed to glycogenesis. The G6P is then converted to glucose-1 phosphate by phosphoglucomutase, which is a reversible reaction.
• The first irreversible reaction is the reaction of this G1P with uridine triphosphate (a nucleoside triphosphate) to form UDP-glucose. This is catalysed by UTP-glucose phosphorylase. Pyrophosphate is the other product of this reaction and is later hydrolysed by pyrophosphatase into two phosphate molecules, which drives the reversible reaction preceding this.
• UDP-glucose monomers are combined into glycogen initially by glycogenin, which acts as a primer, but after around 8 glucose molecules are in the chain, it is extended by glycogen synthase.
• Glycogen synthase forms alpha-1,4 bonds to join the next monomer onto the chain. The process is energy-requiring, in the form of UTP being converted to UDP.
• Branching enzyme enables branching by forming alpha-1,6 bonds at intervals of at least 4 residues. This is done by transferring 6 or 7 molecules from one of the non-reducing ends of the glycogen to a point along the chain that was synthesised earlier, as long as this attaches on a branch having at least 11 residues.

Question 334

What does phosphoglucomutase do IN GLYCOGENESIS and what about the enzyme is ususual?

Answer 334

• It converts glucose 6-phosphate (G6P) that has been released from glycogen into glucose 1-phosphate (G1P).
• This is a reversible reaction (the reverse reaction occurs in glycogenolysis)
• This forwards reaction is driven by the cleavage of PPi (pyrophosphatase) by pyrophosphatase (PPi is produced as a by product of the next reaction in glycogenesis)

Question 335

What does UDP-glucose phosphorylase do and what is the equation for this reaction? Why is it important?

Answer 335

• It catalyses the reaction of G1P with uridine triphosphate (a nucleoside triphosphate) to form UDP-glucose.
• It is the first irreversible reaction in glucogenesis.
• Pyrophosphate is the other product of the reaction catalysed by UTPG phosphorylase and is later hydrolysed by pyrophosphatase into two phosphate molecules, which drives the reversible reaction preceding this.

Question 336

What is glycogenin and what does it do?

Answer 336

• It is the primer and catalyst that starts the process of forming glycogen from glucose residues in glycogenesis
• It is a glycosyltransferase
• It is formed of two identical subunits that add glucose units (from UDP-glucose) to one another
• Once the chain is 8 residues long, glycogen synthase takes over

Question 337

What is glycogen synthase and what are the details of the reaction it catalyses?

Answer 337

• Glycogen synthase forms the α-1,4 linkages to grow the glycogen molecule
• It takes over from glycogenin after 8 residues are in the chain
• The cost of each limkage is 1 molecule of ATP, in the form of UTP->UDP

Question 338

What are the different energy-requiring steps of glycogenesis and in what form is this energy?

Answer 338

• 1 ATP is used by hexokinase/glucokinase to phosphorylate glucose upon entering the cell (although this is not strictly part of glycogenesis)
• 1 UTP (the equivalent of 1 ATP) is used when forming UDP-glucose. The UDP from this is released when joining UDP-glucose residues by glycogen synthase.

Question 339

What is the branching enzyme and what does it do?

Answer 339

• In glycogenesis, the branching enzyme that forms alpha-1,6 bonds at intervals of at least 4 residues.
• This is done by transferring 6 or 7 molecules from one of the non-reducing ends of the glycogen to a point along the chain that was synthesised earlier, as long as the branch has more than 11 residues.

Question 340

What are the main points of regulation of glycogenesis and glycogenolysis? How are these related?

Answer 340

• Glycogenesis -> Glycogen synthase
• Glycogenolysis -> Glycogen phosphorylase

These two enzymes are related because they have reciprocal regulation, which means that the same hormones have opposing effects on them.

Question 341

Describe the effect of phosphorylating glycogen sythase and glycogen phosphorylase. How does this enable reciprocal control?

Answer 341

• Glycogen synthase is in the inactive state when phosphorylated and in the active state when it is not phosphorylated.
• The opposite is true for glycogen phosphorylase.
• This means that hormones that lead to phosphorylation can affect both of these enzymes in opposing manners, so that the response to physiological changes can be rapid.

Question 342

What is the name for the active and inactive state of glycogen synthase and glycogen phosphorylase?

Answer 342

• a = Active
• b = Inactive

Question 343

Describe the regulation of glycogen synthase. [IMPORTANT]

Answer 343

Occurs through regulation of glycogen synthase, which is in the inactive state when phosphorylated and in the active state when it is not phosphorylated:

• Covalent control:
  
  • Glucagon + Adrenaline
    
    • Increase intracellular cAMP, which stimulates PKA to phosphorylate glycogen synthase (inhibiting it)
  • Insulin
    
    • Activates protein phosphatase-1, which dephosphorylates glycogen synthase (activating it)
    • Inhibits glycogen synthase kinase (GSK) which will reduce the phosphorylation of glycogen synthase (activating it)
• Allosteric control
  
  • G6P -> Activator of glycogen synthase

Question 344

What are the 4 states of glycogen phosphorylase and how does regulation affect these?

Answer 344

• Glycogen phosphorylase can be in the hosphorylated “active” state (a) or dephosphorylated “inactive” state (b)
• Each of these states can be “relaxed” (R) or “tense” (T)
• Phosphorylase a prefers the r state, while phosphorylase b prefers the t state

Covalent regulation (i.e. due to hormones) tends to switch between active and inactive state, while allosteric regulation tends to switch between relaxed and tense state.

Question 345

Describe the regulation of glycogen phosphorylase in liver and skeletal muscle. [IMPORTANT]

Answer 345

Occurs through regulation of glycogen phosphorylase, which is in the active state when phosphorylated and in the inactive state when it is not phosphorylated:

• Covalent control:
  
  • Glucagon (in liver) + Adrenaline (in muscle)
    
    • Increase intracellular cAMP, which stimulates PKA to phosphorylate phosphorylase kinase, which will phosphorylate glycogen phosphorylase (activating it)
  • Insulin
    
    • Activates protein phosphatase-1, which dephosphorylates glycogen phosphorylase (inhibiting it)
• Allosteric control:
  
  • In liver
    
    • Glucose -> Transitions phosphorylase a in the relaxed (R) state to the tense (T) state
    • Insensitive to AMP/ATP
  • In muscle
    
    • AMP -> Transitions phosphorylase b in the tense (T) state to the active (R) state
    • ATP and G6P -> Inhibit this transition from T to R state

Question 346

What is phosphorylase kinase and how can it be regulated?

Answer 346

• It is an enzyme that phosphorylases glycogen phosphorylase, causing it to be activated and increasing glucose production
• Hormonally, it it is stimulated by glucagon (in liver) and adrenaline (in muscle) -> These increase intracellular cAMP, which stimulates PKA to phosphorylate phosphorylase kinase
• Allosterically, it is activated by calcium:
  
  • Calcium binds to the d-subunit – “Calmodulin”
  • In muscle, the calcium is released in response to muscular contraction
  • In the liver it is released in response to adrenaline

Question 347

Draw a simple diagram to show the principle of reciprocal hormonal regulation of glycogen synthase and glycogen phosphorylase.

Answer 347

Question 348

Draw a more complex diagram to show the principle of reciprocal hormonal regulation of glycogen synthase and glycogen phosphorylase.

Answer 348

Question 349

What is the pentose phosphate pathway and what is the purpose?

Answer 349

• A branch from the glycolytic pathway
• The purpose is to generate:
  
  • NADPH -> To use in reducing reactions, such as the synthesis of fatty acids
  • Various different sugars, especially pentoses (5C) for nucleic acid synthesis

Question 350

What are the 2 phases of the pentose phosphate pathway?

Answer 350

• Oxidative phase
• Non-oxidative phase

Question 351

What are some alternative names for the pentose phosphate pathway?

Answer 351

• Hexose monophosphate shunt
• 6-phosphogluconate pathway

Question 352

Where in the cell does the pentose phopshate pathway occur?

Answer 352

In the cytosol

Question 353

Does the pentose phosphate pathway use or produce ATP?

Answer 353

No ATP is used or produced.

Question 354

Are the oxidative and non-oxidative parts of the pentose phosphate pathway reversible?

Answer 354

• Oxidative -> Irreversible
• Non-oxidative -> Reversible

Question 355

Describe in detail the functions of the pentose phosphate pathway.

Answer 355

• Synthesis of 5C sugars -> These are used in the biosynthesis of:
  
  • ATP
  • CoA
  • NAD, FAD
  • RNA, DNA
• Synthesis of NADPH:
  
  • Antioxidant
  • Used in lipogenesis

Question 356

What is the starting molecule of the pentose phosphate pathway?

Answer 356

Glucose 6-phosphate (G6P) -> That’s why it is considered a branch of glycolysis, because glycolysis can feed into it (although glycogenolysis and gluconeogenesis can too)

Question 357

Give a general overview of the structure of the pentose phosphate pathway.

Answer 357

• Glucose 6-phosphate (G6P) enters the irreversible oxidative reactions
• The irreversible oxidative pathway ends in ribulose-5-phosphate
• Ribulose-5-phosphate enters the reversible non-oxidative interconversions, which produces ribose-5-phosphate and a series of products that could enter glycolysis

Question 358

What are the 2 main enzymes involved in the interconversions between sugars of the non-oxidative phase of the pentose phosphate pathway?

Answer 358

• Transketolase : Moves 2 carbon units
  
  • Requires thiamine pyrophosphate as a prosthetic group
  • Similar in structure to E1 subunit of pyruvate dehydrogenase
• Transaldolase: Moves 3 carbon units
  
  • Forms a Schiff base between the enzyme and one of the sugars
  • Similar in mechanism to aldolase in glycolysis

Question 359

Draw a diagram to show the points of action of transketolase and transaldolase in the pentose phosphate pathway.

Answer 359

Question 360

In the pentose phosphate pathway, the 6 carbon G6P is converted to 5 carbon sugars. What happens to the extra carbon?

Answer 360

It is released as CO₂.

Question 361

How many NADPH molecules does the pentose phosphate pathway produce?

Answer 361

2

Question 362

Draw a diagram to show where the products of the pentose phosphate pathway go.

Answer 362

Question 363

Compare the function of glycolysis and the pentose phosphate pathway.

Answer 363

Pentose phosphate is more of an anabolic patehway, while glycolysis is catabolic and aims to generate lots of ATP.

Question 364

Show how glycolysis and the pentose phosphate pathway are interlinked.

Answer 364

Question 365

Describe the regulation of the oxidative phase of the pentose phopshate pathway.

Answer 365

Controlled by glucose-6-phosphate dehydrogenase, which is:

• Stimulated by NADP⁺
• Inhibited by NADPH

Question 366

Describe the regulation of the non-oxidative phase of the pentose phopshate pathway.

Answer 366

The reactions are freely reversible and so the flux through them is controlled by the levels of their substrates and products.

Question 367

What are the 3 modes of operation of the pentose phosphate pathway?

Answer 367

• Mode 1
  
  • When there is a need for lots of ribose sugars
  • e.g. In cells producing RNA
• Mode 2
  
  • When there is an equal need for NADPH and ribose sugars
  • e.g. In rapidly dividing cells (since NADPH is required to make deoxyribose from ribose)
• Mode 3
  
  • When there is a need for lots of NADPH
  • e.g. In cells synthesising fatty acids or experiencing oxidative stress

Question 368

Explain the mode of operation of the pentose phosphate pathway when there is a need for lots of ribose sugars.

Answer 368

Question 369

Explain the mode of operation of the pentose phosphate pathway when there is an equal need for NADPH and ribose sugars.

Answer 369

Question 370

Explain the mode of operation of the pentose phosphate pathway when there is a need for lots of NADPH.

Answer 370

Question 371

What is oxidative stress?

Answer 371

An imbalance between anti-oxidants and molecules such as peroxides and free radicals that can cause damage to components of the cell, including proteins, lipids and DNA.

Question 372

Describe the role of the pentose phosphate pathway in antioxidant pathways.

Answer 372

• Pentose phopshate pathway produces NADPH
• NADPH can be used to reduce glutathione disulfide (oxidised form) into glutathione (reduced form)
• Glutathione is an antioxidant that acts by reducing peroxides to water -> This process regenerates gluthione disulfide, so NADPH must reduce it again

Question 373

What is the function of fatty acid synthesis?

Answer 373

It allows us to store excess carbohydrate and protein energy.

Question 374

Is fatty acid synthesis the opposite of fatty acid oxidation?

Answer 374

• Chemically they are opposites
• But mechanistically the two processes are different
• This allows for reciprocal regulation

Question 375

What are fatty acids synthesised and where are they then taken to?

Answer 375

• Synthesised in the cytosol of the liver
• Transported to the adipose tissue for storage as TAG (triglycerides)

Question 376

Describe the general principle of how fatty acid synthesis occurs from carbohydrates and proteins.

Answer 376

• Involves the elongation of a fatty acid chain using malonyl-CoA
• Synthesis is powered by ATP & NADH
• Occurs mostly in the cytoplasm of liver and lactating mammary gland

Question 377

Can humans convert fatty acids to carbohydrates?

Answer 377

No, but they can do the reverse.

Question 378

Compare how fatty acid synthesis is different to fatty acid oxidation.

Answer 378

Question 379

In fatty acid oxidation, the fatty acid is broken down into 2C units of acetyl-CoA. What units form fatty acids in fatty acid synthesis? How are these molecules formed?

Answer 379

• Acetyl-CoA is converted to malonyl-CoA using acetyl-CoA carboxylase
• Acetyl-CoA and malonyl-CoA are converted to acetyl-ACP and malonyl-ACP by acetyl transacylase and malonyl transacylase repectively
• Acetyl-ACP and malonyl-ACP are the starting substrates for fatty acid synthesis, with an additional malonyl-CoA being incorporated with each turn of the cycle

Question 380

What is the only molecule that is really needed to start the process of fatty acid synthesis?

Answer 380

Acetyl-CoA -> The rest of the starting substrates can be synthesised from acetyl-CoA

Question 381

Give the equation and enzyme for the formation of malonyl-CoA before fatty acid synthesis.

Answer 381

Note the investment of ATP.

Question 382

In fatty acid oxidation, the intermediates are linked to CoA. What are the intermediates linked to in fatty acid synthesis?

Answer 382

ACP (acyl carrier protein)

Question 383

Give the equation and enzymes for the formation of acetyl-ACP and malonyl-ACP before fatty acid synthesis.

Answer 383

Question 384

Compare the cofactors that help with oxidation and reduction in fatty acid synthesis and fatty acid oxidation.

Answer 384

• Fatty acid synthesis involves reduction:
  
  • NADPH
• Fatty acid oxidation involves oxidation:
  
  • NAD⁺ & FAD

Question 385

Compare the general reaction pathways of fatty acid oxidation and fatty acid synthesis, including the redox cofactors.

Answer 385

Note that these are both actually cycles. In FAO, the acyl-CoA_n-2 loops back round and enters the cycle again. In FAS, the acyl-ACP_n loops back round and joins with another malonyl-ACP.

Question 386

In fatty acid synthesis, how many carbons is the fatty acid chain is extended by with each turn of the cycle?

Answer 386

• 2 carbons
• This is not intuitive because a malonyl-ACP molecule is added with each turn, which is 3 molecules -> However, 1 carbon is lost as CO₂ as the molecule is incorporated

Question 387

Draw the pathways for the first and subsequent passes of fatty acid synthesis.

Answer 387

Question 388

Where is all the ATP invested in fatty acid synthesis from acetyl-CoA?

Answer 388

One ATP molecule is invested to convert each acetyl-CoA molecule to malonyl-CoA.

Question 389

How many carbon atoms are there in palmitate (fatty acid) and how many passes of fatty acid synthesis are required to make it?

Answer 389

• 16 carbons
• So 7 passes of the fatty acid synthesis cycle are required to make it

Question 390

Give the overall stoichiometry for the formation of a molecule of palmitate from acetyl-CoA.

Answer 390

8 Acetyl-CoA + 7 ATP + 14 NADPH + 6 H⁺ ->
Palmitate + 14 NADP⁺ + 8 CoA + 6 H₂O + 7ADP + 7P_i

Question 391

Describe the synthesis of other fatty acids.

Answer 391

• Odd chain length fatty acids -> Ggenerated through the introduction of propionyl-ACP rather than acetyl-ACP
• Longer chain fatty acids -> Produced by enzymes on the cytoplasmic face of the smooth endoplasmic reticulum
• Unsaturated fatty acids -> ER systems also introduce double bonds to long chain FAs to generate unsaturated FAs

Question 392

What causes essential fatty acids to be essential?

Answer 392

Mammals lack the enzyme to introduce double bonds beyond C-9, so linoleate (18:2 cis-D9,D12) and linolenate(18:3 cis-D9,D12,D15) are considered “essential” fatty acids.

Question 393

What is the name for the enzyme complex involved in fatty acid synthesis?

Answer 393

Fatty acid synthase (FAS)

Question 394

What is the function of fatty acid synthase (FAS)?

Answer 394

• It is a dimeric enzyme, with each of the monomers being a multi-catalytic polypeptide with many functions.
• It is only functional as a dimer.
• In total, it has 7 different enzymatic activities and a phosphopantetheine binding domain

(Don’t need to know the structure or function though!)

Question 395

Fatty acid synthesis takes place in the cytosol, but the acetyl-CoA that is required for this is found in the mitochondria. How is this overcome?

Answer 395

• Acetyl-CoA in the mitochondria combines with oxaloacetate (OAA) to form citrate
• Citrate is exported into the cytosol by a tricarboxylate transporter
• In the cytosol, citrate combines with CoA-SH to form OAA and acetyl-CoA. This is an ATP-requiring reaction and is catalysed by ATP-citrate lyase.

Question 396

What is CoA-SH?

Answer 396

CoA is often referred to in this way when it is not attached to an acetyl group (i.e. in acetyl-CoA).

Question 397

Before fatty acid synthesis, citrate is transported out of mitochondria, then combined with CoA-SH to form oxaloacetate and acetyl-CoA. The acetyl-CoA enters fatty acid synthesis. What happens to the oxaloacetate?

Answer 397

It is transported back into the mitochondria.

Question 398

Describe the key differences between fatty acid oxidation and fatty acid synthesis in terms of enzymes, cofactors and subcellular compartments.

Answer 398

Enzymes:

• In fatty acid synthesis, enzymes are joined in a single polypeptide chain (fatty acid synthase, FAS)
• In fatty acid oxidation, the enzymes are all seperate

Cofactors:

• In fatty acid synthesis, the intermediates are linked to ACP (acyl carrier protein) and the reductant is NADPH
• In fatty acid oxidation, the intermediates are linked to CoA and th oxidants are NAD⁺ and FAD

Subcellular compartments:

• Fatty acid synthesis occurs in the cytosol
• Fatty acid oxidation takes place in the mitochondria

Question 399

What is the main regulation point of fatty acid synthesis?

Answer 399

Acetyl-CoA carboxylase -> This is the enzyme that converts acetyl-CoA to malonyl-CoA

Question 400

What are the two states of acetyl-CoA carboxylase and what is the phosphorylation of each? Why is this important?

Answer 400

• Inactive -> Phosphorylated
• Active -> Not phosphorylated

This is a regulation point in fatty acid synthesis.

Question 401

Describe the regulation of acetyl-CoA carboxylase in order to regulate fatty acid synthesis.

Answer 401

Allosteric regulation:

• Citrate -> Promotes activation of acetyl-CoA carboxylase, activating it
• Long chain fatty acyl-CoA -> Inhibits activation of acetyl-CoA carboxylase, inactivating it

Covalent regulation (intacellular signals):

• AMP -> Stimulates AMPK (AMP-activated protein kinase), which phosphorylates acetyl-CoA carboxylase, inactivating it

Covalent regulation (hormonal):

• Insulin -> Stimulates protein phosphatase 2A, which dephosphorylates acetyl-CoA carboxylase, activating it
• Glucagon/Adrenaline -> Stimulate PKA, which phosphorylates acetyl-CoA carboxylase, inactivating it

Question 402

What is the role of malonyl-CoA in the reciprocal regulation of fatty acid oxidation and fatty acid synthesis?

Answer 402

• Malonyl-CoA inhibits CPT-1, blocking transfer of FAs into the mitochondria
• Hence, this promotes FAS alongside inhibiting FAO

Question 403

Describe the reciprocal regulation of fatty acid synthesis and fatty acid oxidation in the cell.

Answer 403

Question 404

What is the reaction type involved in forming triglycerides from fatty acids and glycerol?

Answer 404

Esterification

Question 405

What are the reactants used to form a triglyceride?

Answer 405

• 3 x Fatty acyl-CoA
• 1 x Glycerol-3-phosphate

Question 406

Draw the pathway of fatty acid esterification into a triglyceride.

Answer 406

Question 407

Glycerol-3-phosphate is require to synthesis triglycerides (not glycerol itself!). Where can this glycerol-3-phosphate be obtained from?

Answer 407

• Can be synthesized from the glycolytic intermediate DHAP -> Remember – this was part of the glycerol-phosphate shuttle mechanism to get NADH into the mitochondria
• Can also be synthesised from glycerol itself

Question 408

What is the use of the glycerol that is released during the breakdown of triglycerides?

Answer 408

It can be phosphorylated to provide a source of glucose during starvation.

Question 409

After synthesis of triglycerides in the liver, how are they transported to adipose tissue and muscles?

Answer 409

• Exported inside VLDL (very low density lipoprotein) particles -> These are made of TAGs, cholesterol and proteins, so they are very similar to chylomicrons
• VLDL particles are recognised by LPL (lipoprotein lipases) on the surface of endothelial cells in capillaries of muscle and adipose cells -> LPL hydrolyses these TAGs into FAs and glycerol
• In adipose -> These are taken up and re-esterified into TAGs
• In muscle -> The FAs are taken up for oxidation

Question 410

How is LPL (lipoprotein lipase) affected by hormones?

Answer 410

Activity and expression increased by:

• Insulin (in adipose)
• Glucagon and adrenaline (in muscles)

Question 411

What are the two main processes that are upregulated during fasting and starvation?

Answer 411

1. GLuconeogenesis
2. Ketogenesis + Ketolysis

Question 412

Describe the etymology of the word glucogneogenesis.

Answer 412

• Glycos = sugar
• Neo = new
• Genesis = synthesis

Question 413

What is gluconeogenesis?

Answer 413

Pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates.

Question 414

What are the different substrates for gluconeogenesis?

Answer 414

• Lactate
• Glycerol
• Amino acids -> Such as alanine and glutamine
• Other sugars

Question 415

Where does gluconeogenesis?

Answer 415

• Liver (predominantly)
• Kidney cortex (smaller amount)

Question 416

In what cellular compartment does gluconeogenesis occur?

Answer 416

There are sections in:

• Mitochondria
• Cytosol

Question 417

What are the daily glucose requirements of the brain, muscle, renal medulla and red blood cells? How does this compare to the daily intake of glucose and its different stores?

Answer 417

• Brain = 110g*
• Muscle = 30g
• Renal medulla = 30g*
• Red blood cells = 25g*

(* = cant use fatty acids)

• Daily intake of glucose = ~100g
• Stored glucogen in the liver can liberate 90g
• Blood glucose = 15g

This shows the importance of gluconeogenesis in short-term fasting.

Question 418

How long are glucose stores sufficient for upon fasting?

Answer 418

12 hours -> Beyond this, gluconeogenesis is required to make new glucose for glucose-dependne tissues

Question 419

Draw a graph to show glucose use from different sources after feeding for the next 30 hours.

Answer 419

Question 420

Draw a diagram to show how gluconeogenesis fits into other metabolic pathways.

Answer 420

Question 421

Compare glycolysis and gluconeogenesis.

Answer 421

Question 422

Which part of gluconeogenesis comes first: mitochondrial or cytoplasmic?

Answer 422

Mitochondrial

Question 423

What can the starting substrate for the mitochondrial and cytosolic parts of gluconeogenesis be considered to be?

Answer 423

• Mitochondrial -> It depends (e.g. lactate)
• Cytosolic -> Phosphoenolpyruvate

The whole purpose of the mitochondrial reaction is to funnel products into the phosphoenolpyruvate.

Question 424

Draw how the mitochondrial part of gluconeogenesis (starting with pyruvate) links into the cytosolic pathway.

Answer 424

Question 425

Pyruvate is converted into phosphoenolpyruvate in the mitochondria, so that the phosphoenolpyruvate enter the cytosolic part of gluconeogenesis. Why is such a long way round taken to do this rather than converting pyruvate straight to phosphoenolpyruvate?

Answer 425

• The conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase is an irreversible reaction
• The pyruvate is converted to oxaloacetate, but there is no transporter for this in the inner mitochondrial membrane, so OAA is converted to malate for transport before being converted back to OAA and then phosphoenolpyruvate

Question 426

Draw the cytosolic part of gluconeogenesis.

Answer 426

* denotes any enzymes that are not present in glycolysis

Question 427

Why can gluconeogenesis only happen in the liver and kidneys?

Answer 427

• Glucose 6-phosphatase is only expressed here.
• This is the enzyme required to convert G6P to glucose, prior to release into the blood.
• The liver has this enzyme because it is involved in glycogenolysis.

Question 428

In what part of the cell does the conversion of glucose 6-phosphate occur in gluconeogenesis (and glycogenolysis)?

Answer 428

Endoplasmic reticulum

Question 429

Draw the entire pathway for gluconeogenesis.

Answer 429

Question 430

Where is lactate supplied from and how does it enter gluconeogenesis?

Answer 430

• Supplied by the blood
• Converted to pyruvate by lactate dehydrogenase, which requires NAD.

Question 431

Where is alanine supplied from and how does it enter gluconeogenesis?

Answer 431

• Supplied by the blood
• Converted to pyruvate by alanine aminotransferase, releasing NH₃.

Question 432

Where is glutamine supplied from and how does it enter gluconeogenesis?

Answer 432

• Supplied from the muscle
• It is converted to glutamate, then alpha-ketoglutarate (losing an NH₃ in each reaction)
• Alpha-ketoglutarate enters the Krebs cycle and is converted to malate, which is in the gluconeogenesis pathway

Question 433

Where is glycerol supplied from and how does it enter gluconeogenesis?

Answer 433

• Supplied from the breakdown of triglycerides into glycerol and fatty acids
• It is converted to glycerol-3-phosphate, which is in turn converted to dihydroxyacetone phosphate

Question 434

Draw the Cori cycle and explain how it relates to gluconeogenesis.

Answer 434

• Lactate is produced by anaerobically respiring muscle, RBCs and renal medulla
• It must be converted so it is exported to the liver
• The liver resynthesises glucose via gluconeogenesis, before releasing it back into blood for uptake by tissues (completing the cycle)

Question 435

Explain why humans can’t make glucose out of fatty acids. [EXTRA]

Answer 435

Question 436

What are the acute and chronic forms of control of gluconeogenesis?

Answer 436

Acute:

• Allosteric control by metabolites
• Covalent modification by hormones

Chronic:

• Transcriptional control of gluconeogenic genes/enzymes

Question 437

What are the main enzymes in gluconeogenesis that can be regulated by allosteric and hormonal control?

Answer 437

• Pyruvate carboxylase
• Phosphoenolpyruvate carboxykinase
• Fructose 1,6-bisphosphatase
• Pyruvate kinase (NOTE: This is not directly in gluconeogenesis, but it can short-circuit the process, making it futile)

Question 438

Describe the allosteric and hormonal control of pyruvate carboxylase in gluconeogenesis.

Answer 438

• It is stimulated by acetyl-CoA

Question 439

Describe the allosteric and hormonal control of phosphoenolpyruvate carboxykinase in gluconeogenesis.

Answer 439

• It is stimulated by glucagon

Question 440

Describe the allosteric and hormonal control of fructose 1,6-bisphosphatase in gluconeogenesis.

Answer 440

• It is stimulated by citrate
• It is stimulated by glucagon (indirectly by fructose 2,6-bisphosphate)

Question 441

Describe the allosteric and hormonal control of pyruvate kinase and how this relates to gluconeogenesis.

Answer 441

• It is inhibited by glucagon
• This is important because pyruvate kinase reverts phosphoenolpyruvate to pyruvate, essentially making the start of gluconeogenesis redundant

Question 442

Describe the effects of hormones on the transcription of enzymes that are either directly or indirectly involved in gluconeogenesis.

Answer 442

Question 443

The rate of gluconeogenesis increases over the first 32 hours after fasting starts. Does this continue after this?

Answer 443

No, gluconeogenesis slows a bit. This is because ketone body metabolism begins.

Question 444

Why does gluconeogenesis decrease with prolonged starvation?

Answer 444

• There is a cost to prolonged starvation -> It is that you are using body proteins, so muscle atrophy occurs
• Ketone body metabolism takes over instead

Question 445

What are ketone bodies? Where do they come from?

Answer 445

• Small 4 carbon molecules that are water soluble
• Used as an alternative fuel to power our bodies in times of nutrient deprivation
• Cannot get them from the diet, but instead the body makes them during fasting and starvation

Question 446

Can ketone bodies be stored?

Answer 446

No

Question 447

What are the two processes in ketone body metabolism? How does the rate of both change in fasting and starvation?

Answer 447

Ketogenesis and ketolysis -> Both are upregulated in fasting and starvation

Question 448

What is ketogenesis and where does it occur?

Answer 448

• Production of ketone bodies
• Occurs in the mitochondria in the liver

Question 449

What is ketolysis and where does it occur?

Answer 449

• Breakdown and utilisation of ketone bodies as a fuel
• Occurs in the mitochondria in peripheral tissues (except the liver)

Question 450

For which tissues is ketolysis particularly important?

Answer 450

• Brain
• Nerve cells

Question 451

Does ketolysis occur in the liver and RBCs? Why?

Answer 451

• Not in the liver -> Because it would be a futile cycle (since that is where ketogenesis occurs)
• Not in RBCs -> Because they do not have mitochondria

Question 452

Explain in essence what ketogenesis is.

Answer 452

It is an alternative fate for acetyl-CoA after fatty acids are converted to acetyl-CoA.

Question 453

What is the starting substrate for ketogenesis?

Answer 453

Fatty acids that have been released from the adipose tissue and taken to the liver.

Question 454

What is the first control point of ketogenesis? How does this cause upregulation of ketogenesis in fasting?

Answer 454

• HSL (hormone-sensitive lipase)
• This is the enzyme that breaks down TAGs into NEFAs and glycerol
• HSL is active in fasting because there is no insulin to inhibit HSL, so NEFAs are released into the blood for uptake by the liver

Question 455

How does ketogenesis compare to fatty acid oxidation?

Answer 455

Ketogenesis essentially follows on from fatty acid oxidation. After uptake of NEFAs, their activation, the carnitine shuttle and finally beta-oxidation, acetyl-CoA molecules are produced. Usually, they would enter an energy-generating pathway through the Krebs cycle, but here the acetyl-CoA is diverted into ketogenesis.

Question 456

Draw the pathway for ketogenesis. [IMPORTANT]

Answer 456

Question 457

What are the three main ketone bodies?

Answer 457

• Acetoacetate
• Acetone
• β-hydroxybutyrate

Question 458

Show how the concentrations of the main ketone bodies and NEFA changes over time in fasting and starvation.

Answer 458

Question 459

Ketogenesis follows on after fatty acid oxidation produces acetyl-CoA. This acetyl-CoA usually enters the Krebs cycle. What causes it to be diverted away in ketogenesis?

Answer 459

Question 460

How are gluconeogenesis and ketogenesis related?

Answer 460

Ketogenesis only occurs when gluconeogenesis is also occuring. This is because gluconeogenesis stimulates ketogenesis.

Question 461

What affects the uptake of ketone bodies from the blood into peripheral tissues?

Answer 461

Uptake is proportional to concentration in the blood.

Question 462

Why does ketolysis not occur in the liver?

Answer 462

• Liver doesn’t express 3-Ketoacyl CoA Transferase needed for ketolysis
• This prevents futile cycling of ketones in the liver

Question 463

Draw the pathway for ketolysis. [IMPORTANT]

Answer 463

Note: The intermediates are the same as in ketogenesis.

Question 464

Why are ketone bodies used during fasting and starvation?

Answer 464

1) It is a glucose-sparing fuel (because the brain can use it instead of glucose)
2) It prevents wasting of muscle in gluconeogenesis

Question 465

Which tissues can use ketone bodies for energy?

Answer 465

• Many peripheral tissues -> Muscle, heart, kidney
• Brain

Question 466

How do ketone bodies relate to diabetes?

Answer 466

In type 1 diabetes, ketosis is uncontrolled:

• Causes much higher concentrations of ketone bodies in blood
• These keto acids drop the pH
• Associated with diabetic ketoacidosis, coma and death
• Rapid breathing – Kussmaul breathing
• Smell acetone on breath
• Acid change that’s dangerous – rather than ketone per se.

Question 467

What are the two main shuttles across the inner mitochondrial membrane involved in oxidative phosphorylation? What is the purpose of these?

Answer 467

• Malate-aspartate shuttle
• Glycerol-3-phosphate shuttle

The purpose of these is to transport NADH from the cytosol across the inner mitochondrial membrane, which is impermeable to NADH.

Question 468

Compare where the malate-aspartate shuttle and glycerol-3-phosphate shuttle happen and why.

Answer 468

Malate-aspartate shuttle:

• Liver and cardiac cells
• Because it is more efficient

Glycerol-3-phosphate shuttle:

• Muscle cells
• Because it is more rapid and it can work against an NADH concentration gradient

Question 469

The malate-aspartate shuttle and glycerol-3-phosphate both solve the problem that the inner mitochondrial membrane is impermeable to NADH. What is the difference in the way in which they work?

Answer 469

Malate-aspartate shuttle:

• NADH is converted to NAD⁺.
• Malate is transported out across the mitochondrial membrane.
• An equivalent amount of NADH is then formed in the mitochondrial matrix.
• It can then enter Complex I.

Glycerol-3-phosphate shuttle:

• NADH is converted to FADH₂ in the inner mitochondrial membrane
• Electrons pass directly to QH₂

Question 470

Draw the malate-aspartate shuttle.

Answer 470

Question 471

Draw the glycerol-3-phosphate shuttle.

Answer 471

Question 472

Describe the protein turnover, intake and excretion that occurs each day in the body.

Answer 472

• The body typically turns over around 300g of proetin per day
• The intake is around 100g per day
• The excertion is around 100g per day

Question 473

Explain the concept of nitrogen balance.

Answer 473

The protein intake each day is the same as the excretion, under normal conditions.

Question 474

Is protein an energy storage form in the body?

Answer 474

No, all protein in the body is functional.

Question 475

Describe the general principle of the metabolism of excess amino acids.

Answer 475

• Excess amino acids are separated into a nitrogen group (for disposal) and a carbon skeleton (for energy generation)
• The nitrogen group is transported to the liver and processed via the urea cycle for excretion as urea by the kidneys

Question 476

What happens to each of the two parts of an excess amino acid?

Answer 476

• Nitrogen group -> For excretion (via conversion to urea)
• Carbon skeleton -> For energy generation

Question 477

In general, how is the nitrogen group of an excess amino acid metabolised?

Answer 477

• It is transported to the liver
• It is processed via the urea cycle for excretion by the kidneys

Question 478

In general, how is the carbon skeleton of an excess amino acid metabolised?

Answer 478

The carbon skeleton enters the TCA to generate ATP (or other molecules).

Question 479

Draw a diagram to show an overview of amino acid metabolism (i.e. which things can form and be formed from amino acids).

Answer 479

Question 480

What are the different classifications of amino acids by their availability?

Answer 480

• Essential
• Non-essential
• Semi-essential (i.e. conditionally essential)

Note: The same classification does not relate to breakdown of amino acids.

Question 481

What are the essential, conditionally-essential and non-essential amino acids?

Answer 481

Question 482

What are the essential amino acids?

Answer 482

Question 483

What are the conditionally-essential amino acids?

Answer 483

Question 484

What are the non-essential amino acids?

Answer 484

Question 485

Draw a diagram to show the digestion and absorption of amino acids [Note: This is covered more in chapter 9].

Answer 485

Question 486

Give some examples of amino acid transporters and their tissue location. [EXTRA?]

Answer 486

Question 487

How are amino acids taken up into cells?

Answer 487

• Via active sodium-linked co-transporters
• The sodium gradient for this is generated by an Na⁺/K⁺-ATPase on the opposite membrane

Question 488

What is protein turnover?

Answer 488

• When older proteins are broken down in the body, they must be replaced.
• This concept is called protein turnover, and different types of proteins have very different turnover rates.

Question 489

Give some examples of when protein turnover must happen.

Answer 489

• Damaged or incorrectly produced/folded proteins need to be removed
• Signalling proteins need to be produced and removed as needed
• Enzymes are often up/down regulated as part of regulatory mechanisms

Question 490

Describe the protein turnover each day in different tissues.

Answer 490

Question 491

How does the body know which proteins to break down?

Answer 491

Proteins that need to be broken down are tagged with ubiquitin.

Question 492

Describe how used proteins are broken down.

Answer 492

• Proteins that need to be borken down are tagged with ubiquitin
• The tagged protein then goes into proteasomes for degradation

Question 493

What are used proteins degraded by?

Answer 493

Proteasome

Question 494

Describe how used proteins are marked by ubiquitin for degradation.

Answer 494

• Ubiquitin ligases attach the ubiquitin to the protein
• These ligases are made up of three components – E1, E2 & E3:
  
  • E1 & E2 -> Responsible for activating the ubiquitin
  • E3 -> Recognises the damaged/misfolded protein, as well as markers of an old protein

Question 495

What does component E3 of ubiquitin ligases do?

Answer 495

• Recognises damaged/misfolded proteins
• It can also recognise certain N-terminal residues which signal the “half-life” of the protein

Question 496

Give an example of a disorder of protein breakdown.

Answer 496

Angelman’s syndrome:

• Caused by a mutation in ubiquitin ligase
• Characterised by severe motor and intellectual disability

Question 497

In general, how is protein synthesis and breakdown regulated by hormones?

Answer 497

• Insulin -> Anabolic effect:
  
  • Stimulates chain initiation (effects on transcription are protein-specific)
  • Inhibits protein breakdown
• Thyroid hormones and glucocorticoids (cortisol) -> Catabolic effect
• Other steroids (anabolic steroids) -> Anabolic effect

Question 498

Give an example of an endogenous anabolic steroid hormone.

Answer 498

Testosterone

Question 499

Draw the structure of an amino acid at pH 7.

Answer 499

Question 500

Draw the structure of an amino acid at pH 1 and 11.

Answer 500

Question 501

What must happen before an amino acid can enter oxidative breakdown to be used as a fuel? Why?

Answer 501

• It must be deaminated
• This is because the α-amino group prevents catabolism

Question 502

What is deamination?

Answer 502

The removal of an amino group from an amino acid. It is done so that amino acids can enter catabolic energy-generating pathways (i.e. be oxidised).

Question 503

Where does deamination occur?

Answer 503

Liver (mostly) and kidneys

Question 504

What are the different types of deamination? Which amino acids are broken down by each?

Answer 504

• Oxidative -> e.g. Glutamate (done by glutamate dehydrogenase)
• Non-oxidative -> e.g. Serine & threonine: OH in side chain
• Hydrolytic -> e.g. Asparagine & glutamine: N in side chain

Question 505

What is the general equation for oxidative deamination? What are the cofactors and enzyme?

Answer 505

• Amino acid -> α-keto acid + NH₄⁺
• This involves the conversion of NAD(P)⁺ to NAD(P)H + H⁺.
• Enzyme: Glutamate dehydrogenase

Note: This is a reversible reaction!

Question 506

In what form in the nitrogen lost from an amino acid upon oxidative deamination? What happens to this product?

Answer 506

• It is lost as ammonium (NH₄⁺)
• This can then be incorporated into other compounds or excreted

Question 507

Which amino acids undergo oxdative deamination?

Answer 507

Only glutamate (thus the enzyme glutamate dehydrogenase).

Question 508

What enzyme catalyses oxidative deamination?

Answer 508

Glutamate dehydrogenase

Question 509

What is the name for the reverse reaction of oxidative deamination?

Answer 509

Reductive amination

Question 510

What coenzymes are required for oxidative deamination? When is each used?

Answer 510

• NAD⁺ used mostly in oxidative deamination
• NADPH used mostly in reductive amination (the opposite reaction of deamination)

Question 511

Describe the allosteric control of glutamate dehydrogenase.

Answer 511

Glutamate dehydrogenase is able to catalyse both oxidative deamination and reductive amination:

• ADP and GDP -> Stimulate oxidative deamination
• ATP and GTP -> Stimulate reductive amination

The rate of each reaction also depends on substrate concentration.

Question 512

What is an α-ketoacid?

Answer 512

• Ketoacids are organic compounds that contain a carboxylic acid group and a ketone group.
• They are the oxidised form of an amino acid.

Question 513

What is the importance of glutamate in amino acid metabolism?

Answer 513

It is the only amino acid that can undergo oxidative deamination (by glutamate dehydrogenase), so some other amino acids are “funnelled” into it in order to be oxidised.

Question 514

What is transamination?

Answer 514

• The reaction of an amino acid with an α-ketoacid to produce a different amino acid and different α-ketoacid.
• Essentially, the amino group is moved from the amino acid to the ketoacid.

Question 515

What enzyme is required for transamination?

Answer 515

Transaminases (a.k.a. aminotransferases)

Question 516

What is the purpose of transamination?

Answer 516

It funnels other amino acids into glutamate, which can be undergo oxidative deamination. The α-ketoacid can also be used in metabolism.

Question 517

In oxidative deamination, what is the carbon component and nitrogen component that are produced? What happens to each?

Answer 517

• Carbon component -> α-ketoacid, which enters energy metabolism
• Nitrogen component -> Ammonium, which enters the urea cycle

Question 518

What is the importance of α-ketoglutarate and glutamate in transamination? [IMPORTANT]

Answer 518

• In transamination, α-ketoglutarate is converted into glutamate (by receiving an amino group from the other amino acid), which can enter oxidative deamination.
• This means that the other amino acid is converted to an α-ketoacid that can enter energy metabolism

Question 519

Draw a diagram to show how amino acids are funnelled into glutamate, as well as what happens to the products.

Answer 519

Question 520

What two important transamination reactions is it worth learning?

Answer 520

Catalysed by alanine transaminase:

• alanine + α-KG ⇌ pyruvate + glutamate

Catalysed by aspartate transaminase:

• aspartate + α-KG ⇌ OAA + glutamate

Question 521

Give the equation and enzyme for the transamination of alanine.

Answer 521

Catalysed by alanine transaminase:

alanine + α-KG ⇌ pyruvate + glutamate

Question 522

Give the equation and enzyme for the transamination of aspartate.

Answer 522

Catalysed by aspartate transaminase:

aspartate + α-KG ⇌ OAA + glutamate

Question 523

What functional group is being moved in a transamination reaction?

Answer 523

Amino

Question 524

What is pyridoxal phosphate? [EXTRA]

Answer 524

The co-enzyme of aminotransferases in transamination.

Question 525

What are the amino acids that are transaminated to glutamate and what is the keto-acid produced by each? What happens to this keto-acid?

Answer 525

Question 526

What keto-acid is produced by the transamination of alanine and what happens to it?

Answer 526

• Pyruvate
• It it the product at the end of glycolysis

Question 527

What keto-acid is produced by the transamination of aspartate and what happens to it?

Answer 527

• Oxaloacetate
• Enters the TCA cycle

Question 528

What keto-acid is produced by the transamination of leucine and what happens to it?

Answer 528

• 2-oxo-3-methyl valerate
• Oxidised in muscle

Question 529

What keto-acid is produced by the transamination of isoleucine and what happens to it?

Answer 529

• 2-oxo-4-methyl valerate
• Oxidised in muscle

Question 530

What keto-acid is produced by the transamination of valine and what happens to it?

Answer 530

• 2-oxo-3-methyl butyrate
• Oxidised in muscle

Question 531

Which amino acids undergo non-oxidative deamination and why?

Answer 531

• Serine and threonine
• Because they have an OH group

Question 532

What enzymes catalyse non-oxidative deamination?

Answer 532

Dehydratases (they are called this because dehydration precedes deamination)

Question 533

Give the equation and the enzyme for non-oxidative deamination of serine.

Answer 533

• Serine -> Pyruvate + NH₄⁺
• Enzyme: Serine dehydratase

Question 534

Give the equation and the enzyme for non-oxidative deamination of threonine.

Answer 534

• Threonine -> α-ketobutyrate + NH₄⁺
• Enzyme: Threonine dehydratase

Question 535

Which amino acids undergo hydrolytic deamination and why?

Answer 535

• Glutamine and asparagine
• This is because they have a nitrogen in their variable group

Question 536

What is the equation and the enzyme for hydrolytic deamination of glutamine?

Answer 536

• Glutamine + H₂O -> Glutamate + NH₄⁺
• Enzyme: Glutaminase

Question 537

What is the equation and the enzyme for hydrolytic deamination of asparagine?

Answer 537

• Asparagine + H₂O -> Aspartate + NH₄⁺
• Enzyme: Asparaginase

Question 538

What are the different types of glutaminase? What is the role of each?

Answer 538

• Mitochondrial
• In liver -> To generate urea
• In kidneys -> To generate NH₄⁺ for acid-base balance
• In neurons -> To assist in neurotransmission

Question 539

Where does urea synthesis occur?

Answer 539

In the liver.

Question 540

Draw a diagram to show how urea synthesis relates to the cycling of amino acids around the body.

Answer 540

Question 541

Why is release of amino acids from muscle important?

Answer 541

It is necessary for gluconeogenesis and other energy metabolic pathways.

Question 542

How much protein is there in the body and how much of it is in muscle?

Answer 542

• 10-11kg of proteins
• 5-7kg in muscle

Question 543

Draw a table to show the most prevalent amino acids in skeletal muscle.

Answer 543

Question 544

Draw a summary of amino acid metabolism in muscle. [IMPORTANT?]

Answer 544

Question 545

Draw the Cahill cycle. [EXTRA]

Answer 545

Question 546

Write the equations for the interconversion between glutamate and glutamine. State the enzymes and where each occurs.

Answer 546

Question 547

Write the equation and enzyme for the formation of glutamine from glutamate.

Answer 547

Question 548

What are the main excretory forms for waste nitrogen?

Answer 548

• 90% as urea
• 10% as ammonia

Question 549

Why is more urea excreted than ammonia?

Answer 549

It is less toxic.

Question 550

Why is amminia used as an excretion form if it is toxic?

Answer 550

It is used in acid-base balance.

Question 551

What do birds and reptiles excrete instead of urea and ammonia? Why?

Answer 551

• Uric acid
• It prevents water loss

Question 552

What is the urea cycle?

Answer 552

A cycle of biochemical reactions that produces urea (NH₂)₂CO from ammonia NH₃.

Question 553

Draw the urea cycle. [IMPORTANT]

Answer 553

Question 554

In what cells does the urea cycle take place?

Answer 554

Periportal cells of the liver lobule

Question 555

What are the starting substrates for the urea cycle?

Answer 555

CO₂ and NH₄⁺

Question 556

What is the rate-limiting step of the urea cycle, what enzyme catalyses it and how can it be regulated?

Answer 556

• The conversion of CO₂ and NH₄⁺ into carbamoyl phosphate
• It is catalysed by carbomyl phosphate synthetase-I:
  
  • Activated by N-acetylglutamate

Question 557

What does the carbamoyl phosphate synthetase-I reaction require?

Answer 557

2 molecules of ATP

Question 558

Draw the carbamoyl phosphatase synthetase-I reaction and how it is regulated.

Answer 558

Question 559

Remember to revise which parts of the urea cycle take place in the mitochondria and in the cytosol.

Answer 559

Question 560

How is the urea cycle linked to the TCA cycle?

Answer 560

The aspartate that enters the urea cycle (by combining with citrulline) can come from the TCA cycle.

Question 561

Describe the regulation of the urea cycle. [IMPORTANT]

Answer 561

Acute:

• The enzymes are upregulated by glucagon and glucocorticoids (i.e. in catabolic conditions)
• Carbamoyl phosphate synthetase-I is upregulated by N-acetylglutamate

Chronic:

• High protein diet induces the enzymes

Question 562

What can defects of the urea cycle lead to?

Answer 562

Hyperammonemia (high levels of ammonium ions in the blood), since ammonium is not cleared from the blood.

Question 563

What are some of the symptoms of urea cycle defects and when do they become evident?

Answer 563

• Lethargy and vomiting may be visible a couple of days after birth
• Comma and brain damage may follow

Question 564

Give some examples of urea cycle defects and how they may be treated.

Answer 564

Question 565

What are the different categories of amino acids based on their metabolic fate?

Answer 565

• Glucogenic via pyruvate
• Glucogenic via TCA cycle intermediates
• Ketogenic via acyl-CoA
• Mixed

Question 566

Categorise the amino acids into those which are glucogenic, ketogenic or both upon deamination.

Answer 566

Question 567

What are glucogenic and ketogenic amino acids? Why are some amino acids glucogenic, some ketogenic and some both? [IMPORTANT]

Answer 567

• Glucogenic AAs:
  
  • Those that can be degraded into TCA intermediates that can enter glucogenesis via OAA
• Ketogenic AAs:
  
  • Those can be degraded directly into acetyl-CoA, which is the precursor of ketone bodies
• Glucogenic AAs can’t form ketones because there is no way to exit the TCA cycle into acetyl-CoA, while ketogenic AAs can’t form glucose because the 2 carbons from the acetyl-CoA are lost in the TCA cycle before OAA is formed.
• Some AAs are both glucogenic and ketogenic because they can be degraded into both acetyl-CoA and TCA intermediates

Question 568

After deamination, the carbon skeletal is either glucogenic or ketogenic. This is via conversion to either a TCA intermediate or an acetyl-CoA related mlecule. What are these 7 molecules?

Answer 568

Question 569

After deamination, which amino acids are converted to pyruvate?

Answer 569

• Alanine
• Cysteine
• Glycine
• Serine
• Threonine
• Tryptophan

Question 570

Draw a diagram to show which amino acids are converted to pyruvate in catabolism.

Answer 570

Note that threonine can also be converted to pyruvate by threonine dehydrogenase (with the loss of a CO₂), but it can also be converted to succinyl-CoA.

Question 571

After deamination, which amino acids are converted to fumarate?

Answer 571

• Phenylalanine
• Tyrosine

Note that these can also be converted to acetoacetate, meaning they are both glucogenic and ketogenic.

Question 572

Draw a diagram to show which amino acids are converted to fumarate in catabolism.

Answer 572

Question 573

After deamination, which amino acids are converted to oxaloacetate?

Answer 573

• Asparagine
• Aspartate

Question 574

Draw a diagram to show which amino acids are converted to oxaloacetate in catabolism.

Answer 574

Question 575

After deamination, which amino acids are converted to succinyl-CoA?

Answer 575

• Isoleucine
• Leucine
• Methionine
• Threonine
• Valine

Question 576

Draw a diagram to show which amino acids are converted to succinyl-CoA in catabolism.

Answer 576

Question 577

After deamination, which amino acids are converted to α-ketoglutarate?

Answer 577

• Arginine
• Glutamine
• Glutamate
• Histidine
• Proline

Question 578

Draw a diagram to show which amino acids are converted to α-ketoglutarate in catabolism.

Answer 578

Question 579

After deamination, which amino acids can be converted into acetyl-CoA or acetoacetyl-CoA?

Answer 579

Acetyl-CoA:

• Isoleucine
• Leucine
• Tryptophan

Acetoacetyl-CoA:

• Leucine
• Lysine
• Phenylalanine
• Tryptophan
• Tyrosine

Question 580

Draw a summary of amino acid catabolism into energy-generating products.

Answer 580

Question 581

How do muscles use amino acids as fuels?

Answer 581

• They use branched chain amino acids (BCAAs) -> These are leucine, isoleucine and valine.
• They can also use transamination to produce ketoacids that can enter metabolism. The by-product of this is typically glutamate, which is converted to alanine or glutamine, which can be taken to the liver for deamination.

[Add notes on this - Check if amino acids are transaminated]

Question 582

Remember to add flashcards on the importance of glutamate and glutamine.

Answer 582

Do it. Involved in transport of ammonia to the liver, similar to the Cahill cycle.

https://www.ncbi.nlm.nih.gov/books/NBK22475/

Question 583

Draw a summary of how muscles use amino acids as fuel and how the products are used up.

Answer 583

The ammonia is taken to the liver, kidneys and small intestine mostly in the form of glutamine.

Question 584

Which amino acids does muscle use as fuel?

Answer 584

BCAA (branched-chain amino acids) [CHECK THIS]

Question 585

Describe how glutamine is metabolised in intestinal cells.

Answer 585

Glutamine is degraded by the intestines by means of the enzyme glutaminase, yielding glutamate and ammonia. Subsequently ammonia is released in the portal vein, and the glutamate is subsequently metabolized by the intestine, mostly to α-ketoglutarate.