Question

Rate limiting step in TCA cycle is catalyzed by:

• A. Alpha-ketoglutarate synthase
• B. Fumarase
• C. Aconitase
• D. Thiokinase

Hint:

The true regulatory enzyme is isocitrate dehydrogenase; pick the alpha-ketoglutarate step from the given choices.

Answer 1

The Correct Option is A

Step 1: The classical rate-limiting and most important regulatory enzyme of the TCA cycle is isocitrate dehydrogenase, which is allosterically controlled by ADP and ATP. Among the options given, this enzyme is not listed, so the recall key points to the alpha-ketoglutarate handling enzyme as the intended best answer.

Step 2: Why this option is chosen. The other named enzymes catalyze steps that are not regarded as flux-controlling. Fumarase simply hydrates fumarate to malate, aconitase isomerises citrate to isocitrate, and thiokinase (succinyl-CoA synthetase) performs substrate-level phosphorylation. None of these is a committed regulatory point.

Step 3: Therefore, by exclusion among the choices offered, option a is marked as correct in this recall paper. The conceptually true rate-limiting enzyme remains isocitrate dehydrogenase.