Lecture 28: The citric acid cycle

Question 1

What are the major sources of acetyl- CoA?

What happens to acetyl-CoA in the citric acid cycle?

Answer 1

Acetyl-CoA is made during:
- B oxidation of fatty acids
- From pyruvate during glycolysis
- The catabolism of several amino acids
= acetate is oxidised to 2 molecules of CO2 and H2O and hydrogens are transferred to NAD+ and FAH+; these coenzymes are used in the electron transport chain to generate ATP

Question 2

How is the citric acid cycle inhibited ? What are the consequences of inhibition on energy metabolism?

Answer 2

The reaction catalysed by aconitase are targeted by the poison “1080”= sodium fluoroacetate
- 1080 inhibits isomerisation of citrate —-> so citrate is not produced as the citric acid cycle is unable to be completed

Question 3

How many enzyme catalyse the reactions of the CAC?

Answer 3

8 enzymes

Question 4

What are the major reactions of the CAC?

Answer 4

1. Condensation reaction between Acetyl Co-A and oxaloacetate to form citrate
2. Isomerisation
3. Removal of the first carbon
4. Removal of the second carbon

Question 5

What happens in the condensation reaction between acetyl-CoA and oxaloacetate?

• what enzyme catalyses this?

Answer 5

The 2 carbon acetyl-CoA is attached to the 4 carbon oxaloacetate by the enzyme citrate synthase
- energy for this reaction comes form the hydrolysis of the CoA from acetyl-CoA

Question 6

What happens in the isomerisation reaction?

• what enzyme catalyses this?

Answer 6

This step occurs in 2 stages: 1st a dehydration reaction, 2nd a hydration reaction

• catalysed by the enzyme aconitase
• citrate is converted to isocitrate
• isocitrate is susceptible to decarboxylation

Question 7

What happens in the removal of the first carbon reaction ?

• what enzyme catalyses this?

Answer 7

This occurs in two stages : 1st an oxidation reaction, 2nd a decarboxylation reaction
= oxidative decarboxylation
- catalysed by isocitrate dehydrogenase
- this steps produces NADH
isocitrate —> —-> alpha-ketoglutarate

Question 8

What happens in the removal of the 2nd carbon reaction ?

• what enzyme catalyses this?

Answer 8

= oxidative decarboxylation reaction

• catalysed by alpha-ketoglutarate dehydrogenase
• Another NADH is produced
• A number of cofactors and co-enzymes are used in this reaction to produce succinly-CoA
• a-ketoglutarate is converted to succinyl-CoA

Question 9

Succinyl-CoA is then converted to succinate, process occurs after the removal of the 2nd carbon. What happens during this process?

Answer 9

• the removal of CoA releases enough energy to drive the synthesis of GTP
• 3rd substrate level phosphorylation

Question 10

Energy released from the oxidation of acetyl-CoA in the CAC is conserved as what?

Answer 10

3 x NADH
1 x FADH
1 x GTP(ATP)