citric acid cycle

Question 1

What happens in the citric acid cycle?

Answer 1

Energy carriers (NADH and FADH2) are produced under aerobic conditions inside mitochondria.

2 acetyl CoA (per 1 glucose) results in 6 NADH, 2 FADH2, and 1 GTP (converted to ATP).

The citric acid cycle regenerates oxaloacetate, releases 2 CO2, doesn’t directly use O2, but requires it for the regeneration of NAD+ and FAD.

Question 2

Step 1

Answer 2

Citrate synthase catalyzes synthesis of citrate from oxaloacetate
irreversible.

Question 3

Step 2

Answer 3

Aconitase catalyzes formation of isocitrate from citrate.

Question 4

Step 3

Answer 4

First of four oxidation steps.

Isocitrate dehydrogenase catalyzes formation of alpha ketogluterate from isocitrate.

Yields CO2 and NADH.

Irreversible.

Question 5

Step 4

Answer 5

2nd oxidation step of four

a-ketoglutarate dehydrogenase complex catalyzes formation of succinyl CoA from a-ketoglutarate

Yields NADH, CO2, and compound with high energy thioester bond. Irreversible.

Question 6

Step 5

Answer 6

Succinyl CoA synthase catalyzes formation of succinate from succinyl-CoA.
Phosphate displaces CoA to make high energy intermediate.
Phosphate passed to GDP to make GTP (converted to ATP).

Question 7

Step 6

Answer 7

Succinate dehydrogenase catalyzes formation of fumarate from succinate.
Third of Four oxidation steps.
Yields FADH2.

Question 8

Step 7

Answer 8

Fumarase catalyzes conversion of fumarate to L-malate.

Question 9

Step 8

Answer 9

Malate dehydrogenase catalyzes formation of oxaloacetate (regenerated) from L-Malate
NAD+ reduced to NADH