Lipid metabolism: citrate shuttle for fatty acid synthesis: 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 · Lipid metabolism: citrate shuttle for fatty acid synthesis

Sample item

In a well-fed liver cell, acetyl-CoA formed from pyruvate inside mitochondria is needed in the cytosol for fatty acid synthesis. How does it reach the cytosol?

AAs citrate, which is cleaved by ATP-citrate lyaseBAs malate converted to oxaloacetate and acetyl-CoACDirectly through a mitochondrial acetyl-CoA transporterDAs acetylcarnitine, formed by CPT-I

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

The answer and the reasoning

A. As citrate, which is cleaved by ATP-citrate lyase

Acetyl-CoA cannot cross the inner mitochondrial membrane. It condenses with oxaloacetate to form citrate, which is exported and cleaved in the cytosol by ATP-citrate lyase to give acetyl-CoA and oxaloacetate. Cytosolic citrate also activates acetyl-CoA carboxylase.

Option by option

A. As citrate, which is cleaved by ATP-citrate lyase (correct)
Correct. Citrate carries acetyl units out of mitochondria, and ATP-citrate lyase regenerates acetyl-CoA in the cytosol.
B. As malate converted to oxaloacetate and acetyl-CoA
Malate carries reducing equivalents and oxaloacetate cannot be split to give acetyl-CoA.
C. Directly through a mitochondrial acetyl-CoA transporter
Mitochondria have no direct acetyl-CoA transporter, which is why a shuttle is needed.
D. As acetylcarnitine, formed by CPT-I
CPT-I attaches long-chain acyl groups to carnitine for import, which is the reverse direction.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, As citrate, which is cleaved by ATP-citrate lyase. Acetyl-CoA cannot cross the inner mitochondrial membrane.

Why is "As malate converted to oxaloacetate and acetyl-CoA" not the answer?

Malate carries reducing equivalents and oxaloacetate cannot be split to give acetyl-CoA.

Why is "Directly through a mitochondrial acetyl-CoA transporter" not the answer?

Mitochondria have no direct acetyl-CoA transporter, which is why a shuttle is needed.

Why is "As acetylcarnitine, formed by CPT-I" not the answer?

CPT-I attaches long-chain acyl groups to carnitine for import, which is the reverse direction.

What concept does this question test?

It tests Lipid metabolism: citrate shuttle for fatty acid synthesis in Biochemistry. It condenses with oxaloacetate to form citrate, which is exported and cleaved in the cytosol by ATP-citrate lyase to give acetyl-CoA and oxaloacetate. Cytosolic citrate also activates acetyl-CoA carboxylase.

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 Lipid metabolism: citrate shuttle for fatty acid synthesis?

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.