Module 3: The citric acid cycle

Question 1

How is pyruvate converted to acetyl CoA

Answer 1

1. pyruvate produced by glycolysis and transported to mitochondrial matrix
2. pyruvate dehydrogenase complex (an enzyme) catlyzes the oxidative decarboxylation of pyruvate.
   - pyruvate – (NAD -> PDH-> NADH) (-> CO2) -> acetyl-CoA

Question 2

Why is pyruvate converted to acetyl-CoA? What two metabolic processes (cycles) are connected from this?

Answer 2

Pyruvate is converted to acetyl-CoA in order to connect glycolysis to the citric acid cycle.

Question 3

Where does the citric acid cycle occur?

Answer 3

mitochondrial matrix

Question 4

True/false: the citric acid cycle is an anaerobic process

Answer 4

False, it’s aerobic

Question 5

True/False: pyruvate dehydrogenase reaction is irreversible

Answer 5

True

Question 6

How is Pyruvate Dehydrogenase complex regulated?

Answer 6

• covalently
• PDH switched off when energy is high
• inhibited by: ATP, Acetyl-CoA, NADH
• PDH stimulated when energy is low
• Activated by: ADP, pyruvate

Question 7

What are the phases of the citric acid cycle?

Answer 7

Phase 1: oxidation of carbons to produce CO2
Phase 2: regeneration to continue cycle

Question 8

What happens during phase 1 of the citric acid cycle?

Answer 8

2 carbon atoms are oxidized, energy-rich electrons gathered as NADH

Question 9

What happens during phase 2 of the citric acid cycle?

Answer 9

Oxaloacetate is regenerated and more energy-rich electrons are harvested

Question 10

What is the net reaction of the citric acid cycle?

Answer 10

Acetyl CoA + 3NAD+ + FAD + ADP + P <—> CO2 + 3NADH + FADH2 + ATP + H+ + CoA

Question 11

What are the 3 regulatory enzymes of the citric acid cycle?

other than PDH

Answer 11

• Citrate synthase: inhibited by citrate
• Isocitrate dehydrogenase: inhibited by ATP and NADH; activated by ADP
• a-ketoglutarate dehydrogenase complex: inhibited by ATP, succinyl CoA, NADH

Question 12

What roles do the biosynthetic precursors of the citric acid cycle have.

After leaving TCA

Answer 12

• oxaloacetate: can be converted into aspartate into amino acids into purines and pyrimidines
• Succinyl CoA: makes cholorphyll
• a-ketoglutarate: can be converted to glutamate and other amino acids and purines
• Citrate: converted to fatty acids and steroids
• pyruvate carboxylase regenerates oxaloacetate from pyruvate