Citric acid cycle

Question 1

What are the reactions in the citric acid cycle?

Which are exergonic? Endergonic?

Answer 1

Preliminary: pyruvate converted to Acetyl CoA- EXERGONIC

1) Acetyl CoA -> citrate- EXERGONIC
2) Citrate isomerization to form isocitrate- ENDERGONIC
3) Isocitrate oxidized to form alpha-ketoglutarate. Produces CO2 and NADH. - EXERGONIC
4) Alpha-ketoglutarate is oxidized to succinyl CoA. Produces CO2 and NADH. -EXERGONIC
5) Succinyl CoA is cleaved to form succinate. Substrate level phosphorylation forms ATP - REVERSIBLE REACTION (Depends on ATP/GTP and ADP/GDP)
6) Succinate is oxidized to fumarate. FADH2 is produced. -EXERGONIC
7) Fumarate is hydrated to form malate. -REVERSIBLE
8) Malate is oxidized to oxaloacetate. NADH is produced. - ENDERGONIC

Final: CoA regenerated from oxaloacetate

Overall, EXERGONIC.

Question 2

What is the source of the energy harvested in the CAC?

Answer 2

Acetyl group of AcetylCoA

Question 3

What does it mean to say that CAC is amphibolic?

Answer 3

Has both anabolic and catabolic reactions

Question 4

What are the anabolic reactions in CAC? What effects does this have?

Answer 4

Excess citrate -> oxaloacetate, acetylCoA -> FAs, glucose, pyrimidines

Succinyl CoA -> heme

alpha-ketoglutarate -> glutamate, other AAs, purines

Can drain intermediates and reduce energy production. Needs to be regulated

Question 5

What are the catabolic reactions in CAC?

Answer 5

Carbons oxidized to CO2

Question 6

How can you determine energy status of a cell?

Answer 6

Relative levels of ATP/ADP and NADH/NAD+ reflect cells energy status.

Question 7

How is CAC regulated?

Answer 7

3 irreversible enzymes:

1) Citrate synthase–regulated by Acetyl CoA levels, ATP/ADP and NADH/NAD ratio
2) Isocitrate dehydrogenase–regulated by isocitrate levels, ATP/ADP and NADH/NAD ratio

3) alpha-Ketoglutarate dehydrogenase
regulated by alpha-ketoglutarate levels, AMP, and NADH levels

Question 8

How does calcium affect CAC?

Answer 8

Stimulates ATP production by stimulating enzymes that regulate pace of the CAC

Question 9

How is citrate metabolized? What effects can it have on a cell?

Answer 9

Plays a role in oxaloacetate, pyruvate, malate formation

Can lead to NADPH production for fatty acid biosynthesis

In cytoplasm, can inhibit glycolysis

Question 10

What diseases are associated with errors in CAC?

Answer 10

Encephalopathies