Which enzyme of the citric acid cycle is inhibited by feedback from adenosine triphosphate, succinyl coenzyme A and NADH together?
AThe enzyme that catalyses step one
BThe enzyme that catalyses step two
CThe enzyme that catalyses step four
DThe enzyme that catalyses step seven
Answer & Solution
Correct answer: C. The enzyme that catalyses step four
1. Steps three and four are both oxidation and decarboxylation steps that make NADH and carbon dioxide.
2. A succinyl group is the product of step four, and coenzyme A binds it to form succinyl coenzyme A.
3. The enzyme that catalyses step four is regulated by feedback inhibition of adenosine triphosphate, succinyl coenzyme A and NADH.
4. Being inhibited by its own product, succinyl coenzyme A, is the clue that fixes this on step four.
5. Step three is regulated by negative feedback from adenosine triphosphate and NADH and a positive effect of adenosine diphosphate, a different list.
6. Step one is controlled by the amount of adenosine triphosphate alone, not by the three-part list given here.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.3 Oxidation of Pyruvate and the Citric Acid Cycle_
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