Amino acid metabolism: nitric oxide synthase cofactors: 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 · Amino acid metabolism: nitric oxide synthase cofactors

Sample item

Nitric oxide synthase oxidises arginine to citrulline and nitric oxide. Which set of cofactors does it require?

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

The answer and the reasoning

A. NADPH, tetrahydrobiopterin and calmodulin

Nitric oxide synthase is a flavin- and heme-containing enzyme using NADPH and oxygen, with tetrahydrobiopterin as an essential cofactor. The endothelial and neuronal forms are activated by calcium-calmodulin.

Option by option

  • A. NADPH, tetrahydrobiopterin and calmodulin (correct)

    Correct. NADPH, tetrahydrobiopterin and calmodulin are all needed.

  • B. NADH, tetrahydrofolate and biotin

    Tetrahydrofolate and biotin serve one-carbon transfer and carboxylation, not this oxygenase reaction.

  • C. NAD+, pyridoxal phosphate and thiamine pyrophosphate

    Pyridoxal phosphate and thiamine pyrophosphate serve transamination and decarboxylation reactions.

  • D. ATP, biotin and lipoic acid

    ATP, biotin and lipoic acid belong to carboxylase and dehydrogenase complexes, not this enzyme.

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, NADPH, tetrahydrobiopterin and calmodulin. Nitric oxide synthase is a flavin- and heme-containing enzyme using NADPH and oxygen, with tetrahydrobiopterin as an essential cofactor.

Why is "NADH, tetrahydrofolate and biotin" not the answer?

Tetrahydrofolate and biotin serve one-carbon transfer and carboxylation, not this oxygenase reaction.

Why is "NAD+, pyridoxal phosphate and thiamine pyrophosphate" not the answer?

Pyridoxal phosphate and thiamine pyrophosphate serve transamination and decarboxylation reactions.

Why is "ATP, biotin and lipoic acid" not the answer?

ATP, biotin and lipoic acid belong to carboxylase and dehydrogenase complexes, not this enzyme.

What concept does this question test?

It tests Amino acid metabolism: nitric oxide synthase cofactors in Biochemistry. The endothelial and neuronal forms are activated by calcium-calmodulin.

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 Amino acid metabolism: nitric oxide synthase cofactors?

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.