Nucleotide metabolism: nitrogen sources in the purine ring: 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 · Nucleotide metabolism: nitrogen sources in the purine ring

Sample item

Isotope-labelled amino acids are used to trace the purine ring. Which amino acid provides nitrogen 1 of the ring?

AAspartateBGlutamineCSerineDGlycine

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

The answer and the reasoning

A. Aspartate

The purine ring is built from glycine (C4, C5 and N7), glutamine (N3 and N9), aspartate (N1), N10-formyl tetrahydrofolate (C2 and C8) and carbon dioxide (C6). Aspartate donates its amino nitrogen to form N1.

Option by option

A. Aspartate (correct)
Correct. Aspartate supplies N1.
B. Glutamine
Glutamine supplies N3 and N9, not N1.
C. Serine
Serine supplies one-carbon units through tetrahydrofolate and does not give a ring nitrogen.
D. Glycine
Glycine supplies C4, C5 and N7, not N1.

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, Aspartate. The purine ring is built from glycine (C4, C5 and N7), glutamine (N3 and N9), aspartate (N1), N10-formyl tetrahydrofolate (C2 and C8) and carbon dioxide (C6).

Why is "Glutamine" not the answer?

Glutamine supplies N3 and N9, not N1.

Why is "Serine" not the answer?

Serine supplies one-carbon units through tetrahydrofolate and does not give a ring nitrogen.

Why is "Glycine" not the answer?

Glycine supplies C4, C5 and N7, not N1.

What concept does this question test?

It tests Nucleotide metabolism: nitrogen sources in the purine ring in Biochemistry. Aspartate donates its amino nitrogen to form N1.

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 Nucleotide metabolism: nitrogen sources in the purine ring?

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.