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

A cytosolic liver enzyme assembles into long active filaments in the presence of citrate and breaks into inactive protomers when long-chain acyl-CoA accumulates. Which enzyme behaves in this way?

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

The answer and the reasoning

A. Acetyl-CoA carboxylase

Citrate is a feed-forward activator and palmitoyl-CoA a feedback inhibitor of the enzyme making malonyl-CoA, acting through polymerisation and depolymerisation. This coordinates lipogenesis with the supply of carbon and the end product. Phosphofructokinase-1 is inhibited by citrate rather than activated.

Option by option

  • A. Acetyl-CoA carboxylase (correct)

    Correct. Citrate promotes the filamentous active form and acyl-CoA end product promotes the inactive form.

  • B. Phosphofructokinase-1

    Phosphofructokinase-1 is inhibited by citrate, and it does not form filaments or take part in lipogenesis.

  • C. Fatty acid synthase

    Fatty acid synthase is a dimeric multi-enzyme complex and is not regulated by citrate polymerisation.

  • D. ATP citrate lyase

    ATP citrate lyase is regulated by phosphorylation and expression, not by filament formation with citrate.

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. Citrate is a feed-forward activator and palmitoyl-CoA a feedback inhibitor of the enzyme making malonyl-CoA, acting through polymerisation and depolymerisation.

Why is "Phosphofructokinase-1" not the answer?

Phosphofructokinase-1 is inhibited by citrate, and it does not form filaments or take part in lipogenesis.

Why is "Fatty acid synthase" not the answer?

Fatty acid synthase is a dimeric multi-enzyme complex and is not regulated by citrate polymerisation.

Why is "ATP citrate lyase" not the answer?

ATP citrate lyase is regulated by phosphorylation and expression, not by filament formation with citrate.

What concept does this question test?

It tests Fatty acid synthesis: acetyl-CoA carboxylase in Biochemistry. This coordinates lipogenesis with the supply of carbon and the end product. Phosphofructokinase-1 is inhibited by citrate rather than activated.

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 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.