Metabolism

Question 1

What are the three steps in protein breakdown and what enzymes are required?

Answer 1

1) Transamination
- transaminases
ALT and AST
2) Formation of ammonia by oxidative deamination
Glutamate dehydrogenase and sometimes Glutamine Synthase for free ammonia
3) Formation of Urea (requires the arginase enzyme

Question 2

What occurs in the transamination reaction?

Answer 2

ALT alanine+ α-ketogluterate→pyruvate+glutamate
AST aspartate+ α-ketogluterate oxoloacetate +glutamate
Amino acid will react with alpha ketoglutarate to form alpha keto acid and glutamate. There are AST (aspartate transaminases) and ALT (alanine transaminases). Pyruvate and oxaloacetate can be converted to make glucose. High levels of AST and ALT will indicate liver damage.

Question 3

What is the role of transaminases in the reaction and what do they require to function?

Answer 3

Transaminases transfer an amino group from the amino acid to the alpha keto acid and require a pirodoxal phosphate required from vitamin B6

Question 4

What amino acids do not undego transamination?

Answer 4

Threonine and lysine

Question 5

What indicates liver damage?

Answer 5

High levels of AST and ALT

Question 6

How is ammonia produced?

Answer 6

By oxidative deamination using glutamate dehydrogenase. Occurs in the matrix of the mitochondria
Free ammonia can combine in tissues with glutamate and ATP forming glutamine and ADP. This reaction requires glutamine synthase and is a way of transporting more than one amine residue.
Glutamate+N〖H4〗^++ATP □(→┬(Glutamine Synthase ) ) Glutamine+ADP

Question 7

What molecule is the main acceptor of nitrogen what does it do?

Answer 7

Glutamate, will donate nitrogen for biosynthesis of amino acids, nucleotides and sugars

Question 8

What happens in the urea cycle?

Answer 8

CO2 + NH4+ -> carbomyl phosphate
Carbomyl phosphate + Ornithine -> citrulline
Citrulline + Aspartate -> to produce arginino succinate
Arginino succinate will split into arginine + fumarate
Arginine is converted to urea using enzyme arginase

Question 9

What happens to fumarate?

Answer 9

Fumarate will be converted to malate and then oxaloacetate which will further react with alpha amino acids to produce aspartate and alpha ketoacid

Question 10

What is the glucose alanine cycle?

Answer 10

Pyruvate produced in the muscle will react with glutamate to produce alanine and alpha keto glutarate which will then be transported to the liver and converted back into Glutamate and pyruvate and converted to urea and pyruvate is used in to make glucose by gluconeogenesis

Question 11

How are amino acids broken down in the muscle?

Answer 11

In muscle proteins are broken down as an energy source for contracting muscle. The muscle has transaminases, but it does not have enzymes to produce urea therefore transported to the liver using the glucose-alanine cycle. Nitrogen will be transferred via glutamate and pyruvate to alanine. Alanine will then be transported to the liver where it is converted back to glutamate and pyruvate. Glutamate will then produce ammonia and form urea and pyruvate will be converted into glucose.

Question 12

What are the types of amino acids?

Answer 12

Ketogenic aa = involved in the production of ketone bodies

Glucogenic amino acids = feed into the points of the TCA cycle

Question 13

Are proteins stored?

Answer 13

No proteins can either be structural or functions there are not specific protein stores. Amino acids are supplied by the diet and then excess protein will be broken down and secreted

Question 14

What is a normal nitrogen balance?

Answer 14

The loss and intake or protein is balanced

Question 15

What is a positive nitrogen balance?

Answer 15

The intake of nitrogen exceeds the amount lost resulting in an increase in the amino acid pool. Occurs in children (during periods of growth) and in excersice (enhanced by anabolic steroids)

Question 16

What is a negative nitrogen balance?

Answer 16

The amount of nitrogen excreted from the body is greater than the nitrogen consumed
Associated with burns, serious tissue injuries fevers and hyperthyroidism

Also a response due to catabolic hormones and lack of anabolic hormones

Question 17

What are the functions of lipids?

Answer 17

Membranes
Uptake of lipid soluble vitamins
Energy store
Precursor of steroid hormones

Question 18

Why is fat an important energy store?

Answer 18

1g of fat produces 37kj of energy

whereas protein only produces 17kj and carbohydrate 16kj

Question 19

How is acetyl CoA transported?

Answer 19

Acetyl CoA cannot be transported out of the mitochondria therefore it combines with oxaloacetate to produce citrate which is then transported out of the mitochondria to the liver cytoso

Question 20

What is the citrate malate antiport and what does it produce?

Answer 20

Citrate will be converted back into acetyl CoA and oxaloacetate which is then converted into malate and then pyruvate to be transported back into the mitochondria. This requires NADH and will produce NADPH which will be used in the elongation step of FA synthesis

Question 21

What is the first step of FA synthesis

Answer 21

Activation by citrate will activate the Acetyl CoA carboxylase enzyme which will convert acetyl CoA and ATP into Malonyl CoA, ADP and Pi

Question 22

What does Acetyl CoA carboxylase require?

Answer 22

Activation by citrate (positive feed forward)
Vitamin biotin
Activated by insulin (blood glucose levels are low therefore energy is needed from other sources
EXPRESSION IS INCREASED BY high carbohydrate and low fat

Question 23

What decreases the activity of Acetyl-coA carboxylase?

Answer 23

Palmitate (product inhibition
Inhibited by glucagon (carboxylase is inhibited by phosphorylation glucagon stimulates phosphorylation therefore inhibits the enzyme
EXPRESSION OF ACETYL COA CARBOXYLASE IS DECREASED by low carbohydrate and high fat

Question 24

What is the elongation step in FA synthesis? `

Answer 24

Malonyl CoA (C3) binds to an ACP covalently. This forms malonyl ACP. Acetyl CoA (C2) also binds and is activated by ACP. The two react forming Acetoacyl (C4) ACP which is then reduced x2 using NADPH produced in the citrate malate antiport or the pentose phosphate pathway, aswell as dehydrated to produce butyryl ACP (C4) this then reacts with another malonyl ACP (C3) to produce a C6 molecule.

• the process will always involved the sequential addition of 2C units
• Enzyme required is fatty acid synthase

Question 25

What are the features of fatty acid synthase?

Answer 25

Fatty acid synthase exists as a dimer = makes fatty acid synthesis efficient as continual movement of reactant to product
Forms a multi functional complex
Acetyl CoA is passed from one active site to the next

Question 26

What are the features of cholesterol?

Answer 26

Rigid hydrophobic molecule, insoluble in water
Precursor of steroid hormones, sterols and bile salts
Transported in the circulation as cholesteryl esters
Cannot be oxidised to O2 or H2O = no energy
Membrane component = fluidity
Cholesterol is mainly synthesised in the ER

Question 27

What are the steps of fatty acid degradation and where do they happen?

Answer 27

Mobilisation = in the adipocyte 
Activation = in the hepatocyte cytosol 
Degradation = in the liver mitochondria

Question 28

What happens in mobilisation? (FA degradation)

Answer 28

cAMP mediated response will activate cAMP which will activate pKA which will phosphorylate triacylglycerol lipase.
Triacylglycerol lipase will break down triglycerides into its components of glycerol, fatty acids and cholesterol.
Glycerol converted to Glyceraldehyde-3-phosphate and is used in glycolysis (partially) or gluconeogenesis (mainly)
Fatty acids will be transported to the liver and activated by acyl CoA synthase

Question 29

What happens in the activation step of FA degradation?

Answer 29

Long chain fatty acids will be converted to acyl CoA-ATP OMM using Acyl CoA synthase.
Acyl CoA produced will then be transported to the mitochondria bound to the alcohol carnitine forming Acyl carnitine. In the mitochondria Acyl will bind back with a CoA molecule

Question 30

What happens in the degradation step of FA degradation?

Answer 30

Acyl CoA will be degraded by a series of steps forming Acetyl CoA, NADH and FADH2.

Small amount of acetyl CoA will be used in TCA cycle in the presence of glycolysis
Complete oxidation of palmitate = yields 106 ATP
Odd chain length yields propionyl CoA in last round
Odd numbered double bonds removed by = isomerase
Even numbered double bonds removed by = reductase + isomerase
Acetyl CoA will be used in ketogenesis in the production of ketone bodies which are used when the body is in starvation, or taken up by tissues cardiac and renal cortex depending on the flow of carbohydrates in glycolysis.

Question 31

What is ketogenesis

Answer 31

Production of ketone bodies from acetyl CoA
Acetyl CoA converted to HMG-CoA
HMG-CoA converted to acetoacetate
Acetoacetate reduced to 3-beta-hydroxybutyrates or non enzymatically acetone

Taken up by many tissues mainly cardiac muscle and renal cortex dependant on the flow of carbohydrates in glycolysis it will be used in the cells for OXPHOS and generation of ATP.

In starvation 70% comes from ketone bodies

Question 32

What is the hormonal regulation of the fat metabolism involving insulin?

Answer 32

Insulin

inc. glycolysis in the liver
inc. FA synthesis in liver (promotes the enzyme Acetyl CoA carboxylase)
inc. TG in adipose tissue

Question 33

What is the hormonal regulation of the fat metabolism involving glucagon?

Answer 33

Increases TG mobilisation = increases breakdown of FA by Triacylglycerol lipase
(also stimulates phosphorylation of acetyl CoA carboxylase which will inhibit the enzyme)

Question 34

What is the comparison between synthesis and degradation of fatty acids?

Answer 34

Reciprocally regulated 
Synthesis in the cytosol   
Intermediates linked to acyl carrier protein, acetyl CoA is passed from one active site to the next 
Sequential addition of 2C units 
Reductant NADPH 
Fatty acid synthase enzyme complex 

Degradation in mitochondria
Intermediates linked to Co enzyme A
Sequential removal of 2C units
Oxidants FAD and NAD (become FADH2 and NADH)
Carried out by individual enzymes
Triacylglycerol lipase, Acyl CoA synthase
Requires carnitine for transport to mitochondria cytosol

Question 35

What is catabolism?

Answer 35

The breakdown of complex molecules to release energy or carry out mechanical work

Question 36

What is anabolism?

Answer 36

The synthesis of new molecules from less complex molecules

Question 37

Why should we study metabolism?

Answer 37

Understand the metabolic basis of disease e.g diabetes, artherosclerosis and gall stones
Can use the changes in metabolites to aid diagnosis

Question 38

How much ATP is used during excersice?

Answer 38

0.5kg per minute

Question 39

What is the mass of ATP in the body?

Answer 39

100g must be broken down and regenerated to meet energy demands

Question 40

What are the enzymes used in glycolysis?

Answer 40

Hexokinase (all tissues)/ Glucokinase (liver)
Phosphofructokinase converts fructose-6-phosphate to fructose-1,6-biphosphosphate
Pyruvate kinase
converts phosphenol pyruvate to pyruvate

Question 41

How is hexokinase regulated?

Answer 41

Hexokinase is inhibited by its own product G6P

Question 42

How is PFK regulated in the muscles?

Answer 42

Citrate = start of the TCA cycle indicates that the precursors are abundant = negative feedback 
ATP = high levels of ATP means that less glucose needs to be converted to pyruvate 
H+ = build up of lactate will cause decrease in H+ ions and inhibit PFK activity 

AMP = positive regulator of PFK and indicator of energy status in the muscle. Two molecules of ADP will combine to form ATP and AMP. This indicates that the cell is starced for energy and needs to promote the affinity for fructose-6-phosphate

Question 43

How is PFK regulated in the liver?

Answer 43

More complex as there are more functions
Citrate
ATP
Not inhbited by H+ as the liver does not produce lactate and therefore unaffected by a low pH

Question 44

Why do glucokinase and hexokinase have different Km values?

Answer 44

Glucokinase has a high Km value which means it has a low affinity for glucose and is only active at high concentrations of glucose providing extra capacity for the glucose by phosphorylating and storing glucose as glycogen.