Amino acid metabolism: glucose-alanine cycle: 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 · Amino acid metabolism: glucose-alanine cycle

Sample item

During a 48-hour fast, skeletal muscle breaks down protein and releases large amounts of alanine into blood. What is the main biochemical purpose of this release?

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

The answer and the reasoning

C. It carries amino nitrogen to the liver safely and supplies carbon for gluconeogenesis

In muscle, alanine aminotransferase transfers the amino group of glutamate to pyruvate, forming alanine. In the liver the reverse reaction yields pyruvate for gluconeogenesis and glutamate, which releases ammonia into the urea cycle. This glucose-alanine cycle moves nitrogen non-toxically and recycles carbon skeletons into glucose.

Option by option

  • A. It carries free ammonia to the kidney for excretion as ammonium

    Alanine carries the amino group rather than free ammonia, and the main handling is hepatic urea synthesis.

  • B. It transports lactate from muscle to be oxidised in the brain

    Lactate transport is the Cori cycle, a different pathway, and alanine is not a lactate carrier.

  • C. It carries amino nitrogen to the liver safely and supplies carbon for gluconeogenesis (correct)

    Correct. Alanine is a non-toxic carrier of nitrogen and a gluconeogenic substrate.

  • D. It supplies acetyl-CoA to the liver for ketone body synthesis

    Alanine is glucogenic because it forms pyruvate, and it is not a ketogenic amino acid.

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, It carries amino nitrogen to the liver safely and supplies carbon for gluconeogenesis. In muscle, alanine aminotransferase transfers the amino group of glutamate to pyruvate, forming alanine.

Why is "It carries free ammonia to the kidney for excretion as ammonium" not the answer?

Alanine carries the amino group rather than free ammonia, and the main handling is hepatic urea synthesis.

Why is "It transports lactate from muscle to be oxidised in the brain" not the answer?

Lactate transport is the Cori cycle, a different pathway, and alanine is not a lactate carrier.

Why is "It supplies acetyl-CoA to the liver for ketone body synthesis" not the answer?

Alanine is glucogenic because it forms pyruvate, and it is not a ketogenic amino acid.

What concept does this question test?

It tests Amino acid metabolism: glucose-alanine cycle in Biochemistry. In the liver the reverse reaction yields pyruvate for gluconeogenesis and glutamate, which releases ammonia into the urea cycle. This glucose-alanine cycle moves nitrogen non-toxically and recycles carbon skeletons into glucose.

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 Amino acid metabolism: glucose-alanine cycle?

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.