Citrate synthase is regulated by another metabolite that has the opposite effect (i. e., stimulates citrate synthase). Choose the description of a condition where a positive regulator activates citrate synthase.

(A) Low [ATP/ADP] ratio. ADP is an allosteric activator of citrate synthase.
(B) High [ATP/ADP] ratio.
(C) High ATP substrate levels favor product formation.
(D) High [ NADH / NAD ] ratio. NADH is a positive allosteric activator.
(E) High [citrate]. Citrate is a positive allosteric activator.

Option A, Low [ATP/ADP] ratio. ADP is an allosteric activator of citrate synthase.

Explanation:

Allosteric activators are the ones that have ability to enhance the activity of a protein. These activators change the shape of active site by binding to some other site. Pyruvate dehydrogenase complex reaction is inhibited when the ratio of ATP to ADP gets high. When acetyl-CoA combines with  oxaloacetic acid to form citrate, it is inhibited by high level of ADP, ATP and NADH. Since ATP and ADP are final product of citric acid cycle, these work as allosteric activator and hence reduce the rate of cycle  

Hence, option A is correct

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