Pyrimidine synthesis: orotate phosphoribosyltransferase: 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 · Pyrimidine synthesis: orotate phosphoribosyltransferase

Sample item

In the de novo pyrimidine pathway, which enzyme transfers a ribose phosphate from PRPP to the ring after it has been built, forming the first nucleotide of the pathway?

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

The answer and the reasoning

B. Orotate phosphoribosyltransferase

Unlike purines, pyrimidines are built as a free ring and ribose phosphate is attached afterwards, using PRPP, to give orotidine monophosphate. Decarboxylation then gives UMP. The enzymes listed as other options act earlier in building the ring.

Option by option

  • A. Dihydroorotase

    Dihydroorotase closes the ring to make dihydroorotate.

  • B. Orotate phosphoribosyltransferase (correct)

    Correct. It attaches ribose phosphate to the preformed ring, giving the first pyrimidine nucleotide.

  • C. Aspartate transcarbamoylase

    Aspartate transcarbamoylase is the regulated early step joining carbamoyl phosphate and aspartate.

  • D. Dihydroorotate dehydrogenase

    Dihydroorotate dehydrogenase oxidises the ring on the inner mitochondrial membrane.

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 B, Orotate phosphoribosyltransferase. Unlike purines, pyrimidines are built as a free ring and ribose phosphate is attached afterwards, using PRPP, to give orotidine monophosphate.

Why is "Dihydroorotase" not the answer?

Dihydroorotase closes the ring to make dihydroorotate.

Why is "Aspartate transcarbamoylase" not the answer?

Aspartate transcarbamoylase is the regulated early step joining carbamoyl phosphate and aspartate.

Why is "Dihydroorotate dehydrogenase" not the answer?

Dihydroorotate dehydrogenase oxidises the ring on the inner mitochondrial membrane.

What concept does this question test?

It tests Pyrimidine synthesis: orotate phosphoribosyltransferase in Biochemistry. Decarboxylation then gives UMP. The enzymes listed as other options act earlier in building the ring.

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 Pyrimidine synthesis: orotate phosphoribosyltransferase?

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.