Nucleotide metabolism: reducing power for ribonucleotide reductase: 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: reducing power for ribonucleotide reductase

Sample item

Ribonucleotide reductase converts ribonucleoside diphosphates to deoxyribonucleoside diphosphates. Which immediate reductant supplies electrons to the enzyme, and how is it regenerated?

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

The answer and the reasoning

A. Thioredoxin, regenerated by NADPH

The enzyme's cysteine residues are oxidised to a disulfide at each turnover. Thioredoxin reduces them and is itself reduced by thioredoxin reductase using NADPH, so deoxyribonucleotide synthesis ultimately depends on NADPH from the pentose phosphate pathway.

Option by option

  • A. Thioredoxin, regenerated by NADPH (correct)

    Correct. Thioredoxin donates the electrons and NADPH regenerates it.

  • B. Ascorbate, regenerated by glutathione

    Ascorbate is a cofactor for hydroxylases and is not the donor for this enzyme.

  • C. NADH, regenerated by lactate dehydrogenase

    NADH from lactate dehydrogenase is not the electron donor for ribonucleotide reductase.

  • D. FADH2, regenerated by succinate

    FADH2 carries electrons into the respiratory chain and does not reduce this enzyme directly.

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, Thioredoxin, regenerated by NADPH. The enzyme's cysteine residues are oxidised to a disulfide at each turnover.

Why is "Ascorbate, regenerated by glutathione" not the answer?

Ascorbate is a cofactor for hydroxylases and is not the donor for this enzyme.

Why is "NADH, regenerated by lactate dehydrogenase" not the answer?

NADH from lactate dehydrogenase is not the electron donor for ribonucleotide reductase.

Why is "FADH2, regenerated by succinate" not the answer?

FADH2 carries electrons into the respiratory chain and does not reduce this enzyme directly.

What concept does this question test?

It tests Nucleotide metabolism: reducing power for ribonucleotide reductase in Biochemistry. Thioredoxin reduces them and is itself reduced by thioredoxin reductase using NADPH, so deoxyribonucleotide synthesis ultimately depends on NADPH from the pentose phosphate 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: reducing power for ribonucleotide reductase?

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.