Metabolism Flashcards
Define cellular respiration
Series of catabolic processes by which CHO, fats and proteins are broken down to yield ATP through a series of redox reactions using O2 as the oxidising agent.
What are the oxidizing agents used in cellular respiration
O2 itself is too reactive to be used directly
Intermediate electron carriers are used :
- NAD+
- FAD +
What is the eponymous name for glycolysis
Embden - Meyerhof pathway
What enzymes convert glucose to glucose 6 phosphate, in which cells do these enzymes act, what hormones/factors influence these enzymes and why is glucose converted into glucose 6 phosphate within the cytoplasm
Glucokinase (hepatocytes) –> increase by insulin + decreased in starvation/diabetes
Hexokinase (all other cells) –> not increased by insulin + not decreased by starvation/diabetes
Conversion of glucose to glucose 6 phosphate in the cell maintains a concentration gradient for ongoing entry of glucose into the cell.
Conversion to glucose 6 phosphate also makes the molecule more polar and more difficult for it to diffuse out of the cell
What is the fate of glucose 6 phosphate
- To proceed to the pentose phosphate pathway (ppp)
- To proceed into the glycolysis pathway
- Proceed into glycogensis / glycogenolysis
What are the products of glycolysis from 1 molecule of glucose
2 x ATP (Net)
2 x pyruvate
2 x NADH
Does glycolysis produce CO2
No
Why does lactate accumulate under anaerobic conditions?
NAD+ is required for the conversion of Glyceraldyde 3 Phosphate to 1.3 bisphosphoglycerate.
Under aerobic conditions:
1. NADH formed during glycolysis can exchange electrons with NAD+ from within the mitochondrial matrix (as the electron transport chain is active) NAD+ is replenished and glycolysis can continue
Under anaerobic conditions
- Pyruvate is converted to lactate which converts NADH to NAD+ allowing glycolysis to continue.
- NAD+ is not available to exchange electrons with NADH made by glycolysis as the electron transport chain is not active.
This explains the accumulation of lactate in under local or systemic anaerobic conditions
What are the fates of lactate
- Pyruvate (If PaO2 restored)
- Cori cycle –> to liver –> pyruvate –> glucose (gluconeogenesis)
- Fermentation (in organisms without a liver). Lactate is converted to ethanol.
What is the Cori cycle
Lactate transported to the liver from tissues.
Converted to pyruvate and then glucose
What intermediate within the process of glycolysis affects the oxyhaemagolbin dissociation curve. How is this curve affected
1.3 bisphosphoglycerate isomerizes to 2,3 bisphosphoglycerate (2,3 BPG)
2,3 BPG shifts the P50 of the OHDC to the right reducing affinity of Hb for O2 at tissues starved of O2 (undergoing glycolysis)
How is hyperlactataemia classified
Cohen and Woods Classification
Type A
- Tissue hypoperfusion
- Anaerobic muscular activity
- Reduced O2 delivery
Type B B1 (associated with disease) - Leukaemia / lymphoma - Pancreatitis - Hepatic / Renal failure - Short bowel - Thiamine deficiency
B2 (Drugs and toxin)
- Beta agonist
- Metformin
- Alcohols
- INH
- Nitroprusside
B3 (Inborn errors metabolism)
- Pyruvate carboxylase deficiency
- Oxidative phosphorylation enzym defects
- G6PD
What is the Citric Acid Cycle? Draw it
Complex cycle of metabolic intermediates that occurs within the inner mitochondrial matrix and produces:
- Electron donors: NADH and FADH2
- CO2
- ATP
What is Pasteur’s point?
Mitochondrial oxygen tension of 0.4 kPa. Below this level, the electron transport chain ceases to operate and anaerobic generation of ATP ensues.
Where do the substrates for the citric acid cycle come from
Acetyl CoA
- Pyruvate (from glycolysis)
- Beta-oxidation
Keto-acids formed from the deamination of amino acids
Vitamin B5 required for CoA
Draw the citric acid cycle
See page 371 chambers
What is the electron transport chain
This is the final step of CHO, fate and protein catabolism. There are 5 protein complexes on the inner surface of the inner mitochondrial membrane which use electron donors: NADH and FADH2 to produce ATP.
What is the function of Complex 1, 2, 3, 4 in the electron transport chain. What co-factors are required and what occurs during this process
To pump H+ from the inner mitochondrial matrix into the inter-membrane space to establish an H+ concentration gradient.
Cofactors:
- Co-enzyme Q –> transfers electrons from complex 1 to complex 3 and from complex 2 to complex 3.
- Cytochrome C –> transfers electrons from complex 3 to complex 4
What is another name for complex 4 in the ETC and what is its unique role. Why is this complex of particular importance
Cytochrome c oxidase
–> It transfers the collected electrons to Oxygen (O2) forming water (H2O)
This is the part of the ETC that is affected by cyanide poisoning. Cyanide binds to the complex 4 haem group, preventing it from binding O2.
Describe the final complex and process of the ETC
Once a H+ gradient has been established by electron transfer between complexes 1 to 4, H+ ions flow down the concentration gradient through complex 5 or ATP synthase. During this process ATP is generated. This is called oxidative phosphorylation
What is meant by uncoupling of oxidative phosphorylation
Oxidative phosphorylation is usually coupled, that is, H+ movement across the inner mitochondrial membrane is used to generate ATP.
In brown adipose tissue, pores can be opened that allow H+ to move into the inner mitochondrial matric without passing through ATP synthase (complex 5). This is called uncoupling, where oxidation and phosphorylation ar no longer strictly matched. The energy released during H+ movement generates heat instead of ATP. This is an important mechanism of thermogenesis in neonates
How many molecules of ATP are created by NADH versus FADH2
NADH –> 3 ATP
FADH –> 1 ATP
How much ATP is generated from a molecule of glucose during aerobic and anaerobic metabolism?
1 molecule of glucose makes:
Anaerobic = 2 ATP
Aerobic = 36 ATP
What is meant by “fats have a high energy value”
They produce more than twice the amount of ATP than equivalent masses of CHO or protein.
Summarise the events that take place during beta-oxidation
Takes place in the inner mitochondrial matrix.
Catabolism of fatty acids that involves removing successive two-carbon units from the fatty acid, each event producing one molecule of acetyl CoA which enters the citric acid cycle.
How are free fatty acids stored. Describe the structure of this molecule
Triglyceride. Three fatty acids esterified with glycerol. When needed triglycerides are hydrolysed by lipases to regenerate free fatty acids and glycerol
The fatty acids undergo beta oxidation, what happens to the glycerol molecule
Hepatocytes transform glycerol into glucose during gluconeogenesis
How do short, medium and long chain fatty acids enter the inner matrix of the mitochondria for beta oxidation to take place
Short and medium fatty acids –> small enough to enter on their own
Long chain fatty acids require the carnitine shuffle. –> i.e. are bound to a carrier in order to cross the mitochondrial membrane.
Name the three common ketone bodies and how and when are these formed
When carbohydrates are scarce (e.g. starvation) or are unable to enter the cell (diabetic ketoacidosis), beta oxidation becomes the main source of energy.
This results in high mitochondrial Acetyl CoA concentration and the following ketone bodies are formed by condensation of two molecules of acetyl CoA.
- Acetone
- Acetoacetic acid
- Beta-hydroxybutyric acid
In times of starvation how do the liver, heart and brain respond to high ketone bodies
Liver –> converts them back into Acetyl CoA and then back into Kreb’s cycle
Heart –> Favours fatty acids as its energy source but can use ketone bodies in times of starvation
Brain –> does not normally metabolize fatty acids. Usually entirely dependent on glucose for ATP. But the brain can adapt to using ketone bodies during times of starvation. (70% of metabolic demands max)
Can RBC’s use ketone bodies as an energy source? why
No. RBCs do not have mitochondria and rely 100% on glycolysis for ATP.
Describe the products of the beta oxidation of a 12 Carbon fatty acid
6 x Acetyl CoA –> Citric acid cycle
7 x NADH ———–> ETC
7 x FADH2 ———–> ETC
All converted into approximately 100 ATPs
When are proteins catabolized?
When amino acids are plentiful
During starvation
How energy efficient is protein catabolism versus glucose catabolism
Inefficient: 1.75 g protein equivalent ATP to 1 g glucose.
Why is protein catabolism an energy inefficient process.
Oxidative deamination produces a keto acid and NH3 (ammonia). NH3 is toxic and must be removed. NH3 enters the urea (ornithine) cycle which is an energy consuming process and requires 3 ATP per urea molecule formed.
what must first be done to amino acids so that they can be used in biochemical processes and metabolism. How is this achieved?
The amino group needs to be removed to form keto acids. Keto acids have the following fates:
- Enter Citric Acid Cycle
- Transformed into glucose (gluconeogenesis)
- Synthesis of other amino acid or fatty acid
Processes used to remove amino group
- Oxidative deamination
- -> forms NH3 toxic –> must be removed by urea cycle (3 ATPs required per one urea formed) - Transamination –> aminotransferases remove the amino group and transfer to a keto acid or another amino acid to form a new amino acid
List the nine essential amino acids (that can’t by synthesized by the liver)
HLLP ! TV TIM (too much TV, hopefully essential amino acids can help)
Histidine
Lysine
Leucine
Phenylalanine
Threonine
Valine
Tryptophan
Isoleucine
Methionine
How much glycogen is stored in the liver and how long can this maintain plasma glucose for? What mechanism takes over thereafter to maintain blood glucose levels
100 g
24 hours
Gluconeogenesis
How much glycogen is stored within the muscles and can this be released as glucose into circulation
200 g
Cannot be released. Ca only be used for metabolic processes within the muscle
Describe glycogenesis
Follwoing CHO based meal, insulin stimulates glycogen synthase to link multiple Glucose-6-Phosphate molecules together to make glycogen.
What is glycogenolysis
When blood glucose falls, glucagon and adrenalin stimulate glycogen phosphorylase to reform glucose from glycogen
What causes glycogenolysis in liver versus muscle
Liver: glucagon and adrenalin
Muscle: Growth Hormone
Why can glucose released from glycogen stored in the muscles not be released into the systemic circulation?
Muscle cells do not contain Glucose - 6 - phosphosphatase, the enzyme required to convert Glucose 6 phosphate into glucose. the liver cells contain this enzyme
What is a chylomicron
These are ultra-low density lipoproteins that containL triglycerides, cholesterol, proteins and phospholipids. They transport dietary (EXOGENOUS) lipids from the intestines to other tissues in the body.
What is lipogenesis. What initiates lipogenesis
Hepatocytes synthesize triglyceride in an anabolic process called lipogenesis.
Meal –> high insulin levels –> Once liver glycogen stores are full –> any excess CHO or amino acids are converted to fatty acids and glycerol which are esterified to give triglyceride.
What is the fate of triglycerides once synthesized in the liver by lipogenesis.
Packaged as VLDL and released into circulation
What is the difference between a chylomicron and VLDL
Chylomicron = ultra-low density lipoprotein which transports exogenous lipids from the intestine to other tissues in the body
VLDL = Very Low Density Lipoprotein which transports endogenous (synthesized via lipogenesis in the liver) lipids to other tissues in the body.
Where is the metabolic store of protein for use in times of starvation
The muscle mass
What is gluconeogenesis
Energy consuming anabolic process in which glucose is synthesized from non-carbohydrate precursors
Where does gluconeogenesis occur
Mainly in the liver
Small contribution by the kidney
Which tissues depend on glucose for energy
Brain - predominantly (can use ketone bodies in starvation for 70% of metabolic processes)
RBCs - rely 100% on glucose for ATP
List the molecules used as substrates for gluconeogenesis
- Lactate
- Pyruvate
- Glycerol
- Amino acids
- All the intermediates of the citric acid cycle
(Citrate, Isocitrate, alpha-ketoglutarate, succinyl CoA, succinate, Fumurate, Malate, Oxaloacetate)
Is gluconeogenesis the reverse of glycolysis
No. It is a separate biochemical pathway
How does metformin work
Metformin
- Inhibits gluconeogenesis
- Reduces glucose uptake from GIT
- Improves insulin sensitivity
What is the pentose phosphate pathway
Also called the hexose monophosphate shunt.
Anabolic carbohydrate pathway with 2 functions:
- Produce pentose sugars for NUCLEIC ACID SYNTHESIS
- Produce NADPH for intracellular REDUCTION REACTIONS (i.e. reverse of oxidation)
Describe the pentose phosphate shunt
- Glucose - 6 - phosphate
Acted on by glucose - 6 - phosphate dehydrogenase (G6PD) to make: - NADPH
If NADP+ levels are high –> G6PD is activated to make more NADPH.
NADPH is used to reduce glutathione which is an antioxidant used to prevent oxidative damage by ROS.
What is the function of NADPH
- Used to reduce glutathione which is then used as antioxidant to prevent cellular damage from ROS.
- Used in RBCs to maintain the Fe2+ (Ferrous) state.
What happens in G6PD deficiency
Pentose Phosphate Pathway cannot be utilized. Decreased reducing power. –> damage from ROS and Methaemaglobinaemia (Fe3+) formation
What is insulin and where is it produced
Peptide hormone. Produced in the beta cells of the islets of Langerhans in the pancreas
Summarise the key aspects of insulin synthesis within the beta cell of the islets of Langerhans
Precursor = Proinsulin
Proinsulin is an A and B chain joined together by two disulfide bridges and a C-Peptide.
Proinsulin is cleaved by endopeptidases and insulin and free C-peptide are packed together in vesicles.
What is the primary trigger for insulin vesicles to undergo exocytosis. Describe the cellular mechanism for insulin release
Increase in plasma glucose concentration –> facilitated diffusion (GLUT 2) of glucose into beta cells –>Increase metabolic activity of cell –> increased ATP –> ATP gated K channels are closed by high ATP –> reduced potassium flux –> membrane depolarization –> opening of voltage gated Ca channels –> Ca influx –> exocytosis of vesicles
Discuss the distinct phases of insulin release from beta cells
- Initial phase –> stored insulin is released
2. Thereafter, pre-formed insulin is depleted and insulin is released as it is synthesized
Describe the effects of the sympathetic nervous system on the Beta-islet cells and insulin release
SNS –> direct neural input: inhibits insulin release via alpha 2 adrenoreceptor
Adrenalin (released from adrenal medulla) –> stimulates beta 2 adrenoreceptors –> increase insulin secretion (muscles require insulin for glucose uptake through GLUT-4 glucose transporter.
Where are the GLUT-1, GLUT-2 and GLUT-4 glucose transporters found and which of these are sensitive to insulin
GLUT-1: Brain
GLUT-2: Liver
GLUT-3: Neurons
GLUT-4: Adipose, Skeletal Muscle, Heart
GLUT-4 is insulin dependent
GLUT-1, GLUT-2, GLUT 3 are not insulin dependent
–> Neurons, brain and Liver can take in glucose without insulin
How does insulin affect storage of metabolic substances?
- LIVER: Increased glycogenesis
- LIVER: Increased fatty acid synthesis
- ADIPOSE: Increased esterification of fatty acids and glycerol
How does insulin inhibit endogenous glucose production?
Inhibition of:
- Gluconeogenesis
- Lipolysis
- Glycogenolysis
Summarise the function of insulin
- Facilitate glucose uptake
- GLUT-4 - Storage of metabolic substrates
- LIVER: Glycogenesis and free fatty acid synthesis
- ADIPOSE: Triglycerides - Inhibit endogenous glucose production
- Inhibit glycogenolysis
- Inhibit lipolysis
- Inhibit gluconeogenesis - Promote cellular uptake
- amino acids
- Potassium
What is the difference between lipolysis and beta-oxidation
Lipolysis: Triglycerides are broken down into glycerol and free fatty acids
Beta oxidation: occurs in the inner matrix of the mitochondria - sequential removal of two carbon portions of the fatty acid chain to form Acetyl-CoA which enters the citric acid cycle.
Why does insulin promote the cellular uptake of potassium?
Increased K uptake is a feedforward response to prevent hyperkalaemia.
Following a meal, the presence of increased glucose suggests feeding and the likelihood of increased potassium.
Which cells produce glucagon
alpha cells of the islets of Langerhans
How is glucagon secretion stimulated?
- Hypoglycaemia induced ANS activity.
2. Increased circulating adrenalin stimulates glucagon secretion
Which of the islet of Langerhans are sensitive to glucose levels
Beta cells only –> high glucose –> high ATP –> ATP gated K channels close –> reduced K flux –> Depolarisation of cell –> voltage gated Ca chennels open –> Increased intracellular calcium –> exocytosis insulin and C-peptide.
alpha cells are not sensitive to glucose levels and glucagon is released subsequent to increased ANS signals and adrenalin levels during hypoglycaemia
How is glucagon release inhibited?
- Insulin
- Somatostatin
- Increased ffa and ketone body concentrations
By what mechanisms does glucagon increase plasma glucose
- Increase gluconeogenesis
- Increase glycogenolysis
- Inhibit glycolysis –> intermediates of glycolysis are shifted to gluconeogenesis
Define basal metabolic rate
What is BMR corrected for
What is normal BMR for an adult
This is the amount of energy a patient consumes per unit time in a state of mental and physical rest in a comfortable environment, 12 hour after a meal.
BMR is corrected for age and surface area
Normal BMR adult = 200 kJ/(m^2h) or 40 kcal/(m^2h)
What factors increase and decrease BMR
Increase BMR
- Exercise
- Raised catecholamines
- Hyperthyroidism
- Pregnancy and lactation
- Placental and fetal metabolism + growth uterus and breasts. Milk production - High and low environmental temperature
- Recent meal (oxidative deamination amino acids) within 6 hours following large meal
- Children - metabolic needs of growth and thermoregulation
Reduced BMR
- Hypothyroidism
- Starvation
- Advancing age (2% per decade)
What is the BMR of neonates compared to adults and why
Double. Thermoregulation and growth
What is: 1 MET 3 METS 4 METS 8 METS > 10 METS
1 MET = equals in a state of rest, awake, fasted > 12 hours –> BMR
3 METS = walking at moderate pace
4 METS = climbing two flights of stairs without stopping
8 METS = Jogging
> 10 METS = strenuous exercise
Which tissues have a propensity for anaerobic metabolism –> mnemonic…
Think Lactate When you Can’t Make Respirations
Testes Lens WBCs Cornea Medulla (kidney) RBCs
These are the tissues with a propensity for glycolysis / anaerobic metabolism
Discuss the possible fate of pyruvate under aerobic and anaerobic conditions
Anaerobic (cytosol)
1. CORI cycle:
Enzyme: Lactate dehydrogenase
Lactate –> blood stream –> liver –> pyruvate –> gluconeogenesis –> glucose –> blood stream –> muscle –> glycolysis –> repeat
- CAHILL cycle: Alanine (as above
Enzyme: Alanine Transferase
Aerobic (mitochondria)
Oxaloacetate (Pyruvate carboxylase) –> enter TCA cycle (need Biotin = B7)
Acetyl CoA (Pyruvate dehydrogenase) –> enter TCA (need B1, B2, B3, B5, Lipoic acid
Which amino acids are:
- Ketogenic
- Ketogenic and Glucogenic
- Glucogenic
KETOGENIC
Leucine
Lysine
KETOGENIC and GLUCOGENIC Isoleucine Tyrosine Tryptophan Phenylalanine
GLUCOGENIC (AAAGG) Alanine Aspartate Asparagine Glutamine Glycine
Threonine (TV)
Valine
Cysteine (CHAMPS) Histidine Arginine Methionine Proline Serine
List the essential amino acids
HLLP TV TIM (Help TV Tim with essential amino acids)
Histidine
Lysine
Leucine
Phenylalanine
Threonine
Valine
Tryptophan
Isoleucine
Methionine
