Purine synthesis: origin of ring atoms: 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 · Purine synthesis: origin of ring atoms

Sample item

A researcher traces the origin of atoms in newly synthesised IMP. Carbon 4, carbon 5 and nitrogen 7 of the purine ring come from which compound?

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

The answer and the reasoning

C. Glycine

The purine ring is assembled from glycine (C4, C5, N7), glutamine (N3 and N9), aspartate (N1), CO2 (C6) and N10-formyl tetrahydrofolate (C2 and C8). Glycine is incorporated whole into the growing ring. This explains why folate antagonists impair purine synthesis.

Option by option

  • A. Aspartate

    Aspartate donates only N1.

  • B. Glutamine

    Glutamine donates N3 and N9.

  • C. Glycine (correct)

    Correct. The whole glycine molecule gives C4, C5 and N7.

  • D. Glutamate

    Glutamate is the product left after glutamine donates its amide nitrogen.

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, Glycine. The purine ring is assembled from glycine (C4, C5, N7), glutamine (N3 and N9), aspartate (N1), CO2 (C6) and N10-formyl tetrahydrofolate (C2 and C8).

Why is "Aspartate" not the answer?

Aspartate donates only N1.

Why is "Glutamine" not the answer?

Glutamine donates N3 and N9.

Why is "Glutamate" not the answer?

Glutamate is the product left after glutamine donates its amide nitrogen.

What concept does this question test?

It tests Purine synthesis: origin of ring atoms in Biochemistry. Glycine is incorporated whole into the growing ring. This explains why folate antagonists impair purine synthesis.

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 Purine synthesis: origin of ring atoms?

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.