Questions: Citric Acid Cycle: Mechanism and Stoichiometry

Question 1

How many NADH molecules are produced per single turn of the citric acid cycle (per acetyl-CoA oxidized)?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (C)

Three NADH are produced per turn: one at isocitrate dehydrogenase, one at α-ketoglutarate dehydrogenase, and one at malate dehydrogenase. A common error is confusing the yield per acetyl-CoA (3 NADH) with the yield per glucose, which requires two turns (6 NADH total from the cycle).

Question 2

The two carbon atoms that enter the citric acid cycle as acetyl-CoA are the same two carbons that are released as CO₂ in the same turn of the cycle.

• T. True
• F. False

Answer: False

This is a key misconception. The two carbons of acetyl-CoA are incorporated into citrate but are NOT the ones released as CO₂ in that same turn. CO₂ is released from the carbon skeleton of oxaloacetate at the isocitrate and α-ketoglutarate dehydrogenase steps. The newly added acetyl carbons are only lost as CO₂ in a subsequent turn.

Question 3

Why is the citric acid cycle described as 'catalytic' with respect to oxaloacetate?

Model answer: Oxaloacetate is consumed at the start of the cycle (by condensing with acetyl-CoA to form citrate) and regenerated at the end (by malate dehydrogenase). It is not net consumed — it acts as a molecular carrier that is continuously recycled, meaning a small amount can drive many rounds of oxidation.

This catalytic regeneration is what makes the cycle a cycle rather than a linear pathway. It also means that if oxaloacetate levels fall (e.g., during gluconeogenesis), the cycle slows — which is why anaplerotic reactions that replenish oxaloacetate (like the pyruvate carboxylase reaction) are essential for cycle flux.