Fatty acid synthesis: acetyl-CoA carboxylase: 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 · Fatty acid synthesis: acetyl-CoA carboxylase

Sample item

During prolonged exercise, AMP-activated protein kinase phosphorylates and inhibits a biotin-dependent enzyme of fatty acid synthesis. This lowers the level of the product that normally restrains carnitine palmitoyltransferase I and so frees fatty acids to enter mitochondria. Which enzyme is inhibited?

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

The answer and the reasoning

A. Acetyl-CoA carboxylase

Acetyl-CoA carboxylase makes malonyl-CoA, the first committed step of fatty acid synthesis, and malonyl-CoA inhibits carnitine palmitoyltransferase I. Phosphorylation by AMP-activated protein kinase inactivates the enzyme, lowering malonyl-CoA so that oxidation proceeds. Insulin has the opposite effect, activating it by dephosphorylation.

Option by option

  • A. Acetyl-CoA carboxylase (correct)

    Correct. Inhibiting this carboxylase lowers malonyl-CoA and releases the block on fatty acid entry to mitochondria.

  • B. Fatty acid synthase

    Fatty acid synthase uses malonyl-CoA to build palmitate and does not make the inhibitor of the carnitine shuttle.

  • C. Malic enzyme

    Malic enzyme generates NADPH for synthesis and does not use biotin or make malonyl-CoA.

  • D. ATP citrate lyase

    ATP citrate lyase supplies cytosolic acetyl-CoA and does not use biotin.

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 A, Acetyl-CoA carboxylase. Acetyl-CoA carboxylase makes malonyl-CoA, the first committed step of fatty acid synthesis, and malonyl-CoA inhibits carnitine palmitoyltransferase I.

Why is "Fatty acid synthase" not the answer?

Fatty acid synthase uses malonyl-CoA to build palmitate and does not make the inhibitor of the carnitine shuttle.

Why is "Malic enzyme" not the answer?

Malic enzyme generates NADPH for synthesis and does not use biotin or make malonyl-CoA.

Why is "ATP citrate lyase" not the answer?

ATP citrate lyase supplies cytosolic acetyl-CoA and does not use biotin.

What concept does this question test?

It tests Fatty acid synthesis: acetyl-CoA carboxylase in Biochemistry. Phosphorylation by AMP-activated protein kinase inactivates the enzyme, lowering malonyl-CoA so that oxidation proceeds. Insulin has the opposite effect, activating it by dephosphorylation.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Fatty acid synthesis: acetyl-CoA carboxylase?

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.