Enzyme inhibition: organophosphate poisoning: 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 · Enzyme inhibition: organophosphate poisoning

Sample item

A farmer after spraying a pesticide has miosis, salivation, bradycardia and fasciculations, and red cell cholinesterase is low. What is the mechanism of enzyme inhibition?

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

The answer and the reasoning

C. Irreversible phosphorylation of a serine at the active site of acetylcholinesterase

Organophosphates phosphorylate the active-site serine of acetylcholinesterase, so acetylcholine accumulates at muscarinic and nicotinic synapses. The bond becomes irreversible after ageing, so oximes such as pralidoxime work best when given early, with atropine to block muscarinic effects. Carbamates inhibit in a reversible, carbamylating way.

Option by option

  • A. Reversible competitive binding that is overcome by excess substrate

    Reversible competitive inhibition describes drugs such as edrophonium, not organophosphates.

  • B. Binding of an allosteric inhibitor that changes enzyme shape

    Allosteric inhibition is not the mechanism, since the inhibitor attacks the catalytic serine.

  • C. Irreversible phosphorylation of a serine at the active site of acetylcholinesterase (correct)

    Correct. Covalent phosphorylation of the active-site serine inhibits irreversibly.

  • D. Reduced synthesis of acetylcholinesterase by transcriptional repression

    Organophosphates do not act by repressing transcription of the 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 C, Irreversible phosphorylation of a serine at the active site of acetylcholinesterase. Organophosphates phosphorylate the active-site serine of acetylcholinesterase, so acetylcholine accumulates at muscarinic and nicotinic synapses.

Why is "Reversible competitive binding that is overcome by excess substrate" not the answer?

Reversible competitive inhibition describes drugs such as edrophonium, not organophosphates.

Why is "Binding of an allosteric inhibitor that changes enzyme shape" not the answer?

Allosteric inhibition is not the mechanism, since the inhibitor attacks the catalytic serine.

Why is "Reduced synthesis of acetylcholinesterase by transcriptional repressi…" not the answer?

Organophosphates do not act by repressing transcription of the enzyme.

What concept does this question test?

It tests Enzyme inhibition: organophosphate poisoning in Biochemistry. The bond becomes irreversible after ageing, so oximes such as pralidoxime work best when given early, with atropine to block muscarinic effects. Carbamates inhibit in a reversible, carbamylating way.

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 Enzyme inhibition: organophosphate poisoning?

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.