Which enzyme catalyzes the committed, rate-limiting step of the TCA cycle, and what are its principal allosteric activators?
- A Isocitrate dehydrogenase, activated by ADP and calcium ✓
- B Citrate synthase, activated by NADH and ATP
- C Alpha-ketoglutarate dehydrogenase, activated by succinyl-CoA
- D Malate dehydrogenase, activated by AMP
Correct answer: A. Isocitrate dehydrogenase, activated by ADP and calcium
Explanation
Mitochondrial NAD-dependent isocitrate dehydrogenase is the rate-limiting, committed step of the cycle. It is activated by ADP (lowering the Km for substrates) and by calcium ions signaling contraction, while NADH and ATP allosterically inhibit it. The listed activators for citrate synthase and alpha-ketoglutarate dehydrogenase are actually their product inhibitors, making those options internally contradictory.
Reference: Lehninger Principles of Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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