TCA cycle: calcium and regulation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · TCA cycle: calcium and regulation

Sample item

During vigorous muscle contraction, mitochondrial calcium rises and TCA cycle flux increases. Which pair of enzymes is activated by calcium and inhibited by NADH?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

Isocitrate dehydrogenase (activated also by ADP) and alpha-ketoglutarate dehydrogenase are regulated by calcium, and both are inhibited by NADH and ATP. Together with citrate synthase they catalyse the largely irreversible steps. Calcium therefore matches oxidative flux to contractile demand.

Option by option

  • A. Citrate synthase and succinate dehydrogenase

    Citrate synthase is regulated by substrate availability and citrate; succinate dehydrogenase is not a calcium target.

  • B. Aconitase and succinate thiokinase

    These steps are not the principal sites of calcium or NADH control.

  • C. Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase (correct)

    Correct. Both NAD-linked decarboxylating dehydrogenases are calcium sensitive.

  • D. Malate dehydrogenase and fumarase

    These catalyse near-equilibrium reactions and are not major regulatory points.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. Isocitrate dehydrogenase (activated also by ADP) and alpha-ketoglutarate dehydrogenase are regulated by calcium, and both are inhibited by NADH and ATP.

Why is "Citrate synthase and succinate dehydrogenase" not the answer?

Citrate synthase is regulated by substrate availability and citrate; succinate dehydrogenase is not a calcium target.

Why is "Aconitase and succinate thiokinase" not the answer?

These steps are not the principal sites of calcium or NADH control.

Why is "Malate dehydrogenase and fumarase" not the answer?

These catalyse near-equilibrium reactions and are not major regulatory points.

What concept does this question test?

It tests TCA cycle: calcium and regulation in Biochemistry. Together with citrate synthase they catalyse the largely irreversible steps. Calcium therefore matches oxidative flux to contractile demand.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study TCA cycle: calcium and regulation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.