Nucleotide metabolism: PRPP synthetase regulation: 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: PRPP synthetase regulation

Sample item

PRPP synthetase supplies the substrate for the first committed step of purine synthesis. Which statement correctly describes its control in normal cells?

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

The answer and the reasoning

C. Inhibited by ADP and GDP, and activated by inorganic phosphate

PRPP synthetase is allosterically inhibited by purine nucleoside diphosphates such as ADP and GDP, and activated by inorganic phosphate. Mutations that make it resistant to inhibition or increase its activity cause PRPP excess and gout.

Option by option

  • A. Inhibited by PRPP itself through covalent phosphorylation

    PRPP does not inhibit its own synthetase by covalent phosphorylation.

  • B. Activated by AMP and GMP, and inhibited by inorganic phosphate

    This reverses the controls; phosphate is an activator and purine nucleotides are inhibitory.

  • C. Inhibited by ADP and GDP, and activated by inorganic phosphate (correct)

    Correct. Purine diphosphates inhibit the enzyme and phosphate activates it.

  • D. Activated by uric acid acting as a feedforward signal

    Uric acid is the end product and is not a regulator of PRPP synthetase.

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, Inhibited by ADP and GDP, and activated by inorganic phosphate. PRPP synthetase is allosterically inhibited by purine nucleoside diphosphates such as ADP and GDP, and activated by inorganic phosphate.

Why is "Inhibited by PRPP itself through covalent phosphorylation" not the answer?

PRPP does not inhibit its own synthetase by covalent phosphorylation.

Why is "Activated by AMP and GMP, and inhibited by inorganic phosphate" not the answer?

This reverses the controls; phosphate is an activator and purine nucleotides are inhibitory.

Why is "Activated by uric acid acting as a feedforward signal" not the answer?

Uric acid is the end product and is not a regulator of PRPP synthetase.

What concept does this question test?

It tests Nucleotide metabolism: PRPP synthetase regulation in Biochemistry. Mutations that make it resistant to inhibition or increase its activity cause PRPP excess and gout.

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: PRPP synthetase regulation?

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.