Nucleotide metabolism: CAD protein in pyrimidine synthesis: 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: CAD protein in pyrimidine synthesis

Sample item

In mammals, the first three steps of de novo pyrimidine synthesis are catalysed by a single multifunctional protein. Which three activities does it carry?

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

The answer and the reasoning

A. CPS II, aspartate transcarbamoylase and dihydroorotase

The CAD protein combines carbamoyl phosphate synthetase II, aspartate transcarbamoylase and dihydroorotase. Orotate phosphoribosyltransferase and OMP decarboxylase are combined in UMP synthase, which acts later in the pathway.

Option by option

  • A. CPS II, aspartate transcarbamoylase and dihydroorotase (correct)

    Correct. CAD stands for the three activities it combines.

  • B. Orotate phosphoribosyltransferase, OMP decarboxylase and CTP synthetase

    The first two belong to UMP synthase; CTP synthetase is a separate enzyme acting on UTP.

  • C. Thymidylate synthase, dihydrofolate reductase and thymidine kinase

    These enzymes act in the thymidylate cycle and salvage, not the first steps of the de novo pathway.

  • D. CPS I, ornithine transcarbamylase and argininosuccinate synthetase

    These are urea cycle enzymes located partly in the mitochondrion and are not pyrimidine synthetic enzymes.

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 A, CPS II, aspartate transcarbamoylase and dihydroorotase. The CAD protein combines carbamoyl phosphate synthetase II, aspartate transcarbamoylase and dihydroorotase.

Why is "Orotate phosphoribosyltransferase, OMP decarboxylase and CTP syntheta…" not the answer?

The first two belong to UMP synthase; CTP synthetase is a separate enzyme acting on UTP.

Why is "Thymidylate synthase, dihydrofolate reductase and thymidine kinase" not the answer?

These enzymes act in the thymidylate cycle and salvage, not the first steps of the de novo pathway.

Why is "CPS I, ornithine transcarbamylase and argininosuccinate synthetase" not the answer?

These are urea cycle enzymes located partly in the mitochondrion and are not pyrimidine synthetic enzymes.

What concept does this question test?

It tests Nucleotide metabolism: CAD protein in pyrimidine synthesis in Biochemistry. Orotate phosphoribosyltransferase and OMP decarboxylase are combined in UMP synthase, which acts later in the pathway.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: CAD protein in pyrimidine synthesis?

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.