Regulation of citric acid cycle

Question 1

Which enzymes are the citric acid rate-controlling enzymes

Answer 1

1. Citrate synthase
2. isocitrate dehydrogenase
3. 2-oxoglutarate dehydrogenase

Question 2

What are the three main ways enzymes in the citric cycle are regulated

Answer 2

1. Substrate availability
2. product inhibition
3. competitive feedback inhibition by intermediates further along the cycle

Question 3

How do the substrates regulate citric acid cycle

Answer 3

1. Acetyl-CoA and oxaloacetate are not present in mitochondria in concentrations that would staurate citrate synthase
2. Therefore metabolic flux varies with substrate concentration and availability

Question 4

How is the production of acetyl-CoA regulated

Answer 4

1. Activity of pyruvate dehydrogenase

Question 5

How is oxaloacetate concentration regulated

Answer 5

1. In equilibrium with malate
2. its concentration fluctuates with NADH/NAD+ concentration ratio
3. k= [oxaloacetate][NADH]/[malate][NAD+]
4. As respiration rate increases [NADH] decreases
5. This means [oxaloacetate] increases which stimulates the citrate synthase reaction

Question 6

What is the role of NAD+

Answer 6

1. [citrate] falls as wokr increases, indicating the rate of citrate removal increases more than its formation
2. The rate of citrate removal is controlled by NAD+-dependent-isocitrate dehydrogenase
3. Citrate synthase also inhibited by NADH
4. NAD+-dependent-isocitrate dehydrogenase is more sensitive to [NADH] changes than citrate synthase

Question 7

What happens when [citrate] falls upon transition from low to high work and respiration rates

Answer 7

1. citrate is a competitive inhibitor of oxaloacetate for citrate synthase
2. the fall of [citrate] caused by increased isocitrate dehydrogenase activity increases the rate of citrate formation
3. 2-oxoglutarate dehydrogenase is also strongly inhibited by its products, NADH and succinyl-CoA. Its activity increases when [NADH] decreases
4. Succinate thiokinase also competes with acetyl-CoA in the citrate synthase reaction
5. These interlocking system keeps the citric acid cycle coordinately regulated

Question 8

What are allosteric regulators of citric acid cycle enzymes

Answer 8

1. ADP
2. ATP
3. Ca2+

Question 9

How does [ADP] and [ATP] affect citric cycle

Answer 9

1. Increased workload is accompanied by increased [ADP], resulting from increased rate of ATP hydrolysis
2. ADP acts as an allosteric activator of isocitrate dehydrogenase by decreasing its Km for isocitrate
3. ATP which builds up when muscle is at rest inhibits the enzyme

Question 10

How does Ca2+ affect citric acid cycle

Answer 10

1. Ca2+ activates pyruvate dehydrogenase phosphatase which activates pyruvate dehydrogenase complex to produce acetyl-CoA
2. It also activates both isocitrate dehydrogenase and oxoglutarate dehydrogenase
3. The same signal stimulates muscle contraction and the production of ATP to fuel it