Amino acid metabolism: serine hydroxymethyltransferase: 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: serine hydroxymethyltransferase

Sample item

A pyridoxal phosphate-dependent enzyme converts serine to glycine and transfers the removed carbon to tetrahydrofolate, forming N5,N10-methylene tetrahydrofolate. Which enzyme is this?

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

The answer and the reasoning

C. Serine hydroxymethyltransferase

Serine hydroxymethyltransferase is the major source of one-carbon units for the folate pool. The product, N5,N10-methylene tetrahydrofolate, is used for thymidylate synthesis or can be converted to other one-carbon forms. The reaction is reversible and also makes serine from glycine.

Option by option

  • A. Glycine cleavage system

    The glycine cleavage system breaks glycine down to carbon dioxide and ammonia while forming the same folate derivative, and it does not act on serine.

  • B. Cystathionine beta-synthase

    Cystathionine beta-synthase condenses serine with homocysteine and is not a folate enzyme.

  • C. Serine hydroxymethyltransferase (correct)

    Correct. This PLP-dependent enzyme interconverts serine and glycine and charges tetrahydrofolate.

  • D. Serine dehydratase

    Serine dehydratase deaminates serine to pyruvate and does not involve tetrahydrofolate.

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, Serine hydroxymethyltransferase. Serine hydroxymethyltransferase is the major source of one-carbon units for the folate pool.

Why is "Glycine cleavage system" not the answer?

The glycine cleavage system breaks glycine down to carbon dioxide and ammonia while forming the same folate derivative, and it does not act on serine.

Why is "Cystathionine beta-synthase" not the answer?

Cystathionine beta-synthase condenses serine with homocysteine and is not a folate enzyme.

Why is "Serine dehydratase" not the answer?

Serine dehydratase deaminates serine to pyruvate and does not involve tetrahydrofolate.

What concept does this question test?

It tests Amino acid metabolism: serine hydroxymethyltransferase in Biochemistry. The product, N5,N10-methylene tetrahydrofolate, is used for thymidylate synthesis or can be converted to other one-carbon forms. The reaction is reversible and also makes serine from glycine.

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: serine hydroxymethyltransferase?

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.