Enzymes: irreversible inhibition: 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 · Enzymes: irreversible inhibition

Sample item

Organophosphate poisoning irreversibly inhibits acetylcholinesterase by covalently modifying which amino-acid residue at its active site?

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

The answer and the reasoning

C. Serine

Organophosphates phosphorylate the active-site serine of acetylcholinesterase. Before ageing occurs, pralidoxime may reactivate the enzyme.

Option by option

  • A. Tryptophan

    Tryptophan is not the relevant active-site nucleophile.

  • B. Lysine

    Lysine is not the covalently modified residue in acetylcholinesterase poisoning.

  • C. Serine (correct)

    Correct. Active-site serine is phosphorylated.

  • D. Histidine

    Histidine participates in many catalytic triads but is not the organophosphate target here.

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, Serine. Organophosphates phosphorylate the active-site serine of acetylcholinesterase.

Why is "Tryptophan" not the answer?

Tryptophan is not the relevant active-site nucleophile.

Why is "Lysine" not the answer?

Lysine is not the covalently modified residue in acetylcholinesterase poisoning.

Why is "Histidine" not the answer?

Histidine participates in many catalytic triads but is not the organophosphate target here.

What concept does this question test?

It tests Enzymes: irreversible inhibition in Biochemistry. Before ageing occurs, pralidoxime may reactivate the enzyme.

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 Enzymes: irreversible inhibition?

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.