Citric acid cycle: citrate synthase: 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 · Citric acid cycle: citrate synthase

Sample item

In the first reaction of the tricarboxylic acid cycle, a two-carbon acetyl unit is condensed with a four-carbon acceptor to form a six-carbon tricarboxylic acid, and the activity of this enzyme is often used to estimate mitochondrial content. Which enzyme catalyses the reaction?

AAconitaseBATP citrate lyaseCCitrate synthaseDIsocitrate dehydrogenase

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

The answer and the reasoning

C. Citrate synthase

Citrate synthase condenses acetyl-CoA with oxaloacetate in the mitochondrial matrix, releasing CoA, and is inhibited by ATP, NADH, succinyl-CoA and citrate. Because it is a matrix enzyme, its activity serves as a marker of mitochondrial mass. ATP citrate lyase is the cytosolic enzyme that does the reverse cleavage.

Option by option

A. Aconitase
Aconitase interconverts the six-carbon acids by isomerisation, after the condensation.
B. ATP citrate lyase
ATP citrate lyase works in the cytosol to cleave the six-carbon product into acetyl-CoA and oxaloacetate.
C. Citrate synthase (correct)
Correct. This enzyme performs the entry condensation of the cycle.
D. Isocitrate dehydrogenase
Isocitrate dehydrogenase acts later, oxidatively decarboxylating isocitrate.

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 C, Citrate synthase. Citrate synthase condenses acetyl-CoA with oxaloacetate in the mitochondrial matrix, releasing CoA, and is inhibited by ATP, NADH, succinyl-CoA and citrate.

Why is "Aconitase" not the answer?

Aconitase interconverts the six-carbon acids by isomerisation, after the condensation.

Why is "ATP citrate lyase" not the answer?

ATP citrate lyase works in the cytosol to cleave the six-carbon product into acetyl-CoA and oxaloacetate.

Why is "Isocitrate dehydrogenase" not the answer?

Isocitrate dehydrogenase acts later, oxidatively decarboxylating isocitrate.

What concept does this question test?

It tests Citric acid cycle: citrate synthase in Biochemistry. Because it is a matrix enzyme, its activity serves as a marker of mitochondrial mass. ATP citrate lyase is the cytosolic enzyme that does the reverse cleavage.

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 Citric acid cycle: citrate synthase?

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.