Molecular biology and genetics: cholera toxin and Gs: 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 · Molecular biology and genetics: cholera toxin and Gs

Sample item

In severe cholera, the toxin of Vibrio cholerae causes massive intestinal secretion by raising cAMP in enterocytes. Which post-translational change in a signalling protein causes this?

AADP-ribosylation of Gs alpha, which locks it in the GTP-bound active stateBADP-ribosylation of elongation factor 2, which halts protein synthesisCPhosphorylation of Gs alpha by protein kinase A, which enhances GTP bindingDProteolytic cleavage of adenylate cyclase, which activates it permanently

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

The answer and the reasoning

A. ADP-ribosylation of Gs alpha, which locks it in the GTP-bound active state

Cholera toxin ADP-ribosylates the alpha subunit of Gs, inhibiting its GTPase activity. Gs therefore stays active and keeps adenylate cyclase on, raising cAMP, activating CFTR and causing chloride and water secretion. Oral rehydration solution works because glucose-coupled sodium uptake is preserved.

Option by option

A. ADP-ribosylation of Gs alpha, which locks it in the GTP-bound active state (correct)
Correct. ADP-ribosylation blocks Gs GTPase, so adenylate cyclase stays active.
B. ADP-ribosylation of elongation factor 2, which halts protein synthesis
Elongation factor 2 is modified by diphtheria toxin, not cholera toxin.
C. Phosphorylation of Gs alpha by protein kinase A, which enhances GTP binding
Cholera toxin acts by ADP-ribosylation, not by PKA phosphorylation of Gs.
D. Proteolytic cleavage of adenylate cyclase, which activates it permanently
Adenylate cyclase is not cleaved; its activity rises through Gs.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, ADP-ribosylation of Gs alpha, which locks it in the GTP-bound active state. Cholera toxin ADP-ribosylates the alpha subunit of Gs, inhibiting its GTPase activity.

Why is "ADP-ribosylation of elongation factor 2, which halts protein synthesis" not the answer?

Elongation factor 2 is modified by diphtheria toxin, not cholera toxin.

Why is "Phosphorylation of Gs alpha by protein kinase A, which enhances GTP b…" not the answer?

Cholera toxin acts by ADP-ribosylation, not by PKA phosphorylation of Gs.

Why is "Proteolytic cleavage of adenylate cyclase, which activates it permane…" not the answer?

Adenylate cyclase is not cleaved; its activity rises through Gs.

What concept does this question test?

It tests Molecular biology and genetics: cholera toxin and Gs in Biochemistry. Gs therefore stays active and keeps adenylate cyclase on, raising cAMP, activating CFTR and causing chloride and water secretion. Oral rehydration solution works because glucose-coupled sodium uptake is preserved.

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 Molecular biology and genetics: cholera toxin and Gs?

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.