Molecular biology and genetics: pertussis toxin and Gi: 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: pertussis toxin and Gi

Sample item

Pertussis toxin raises cAMP in airway cells by modifying a G protein. Which modification explains this?

AADP-ribosylation of Gs alpha, which blocks its intrinsic GTPase activityBADP-ribosylation of Gq alpha, which activates phospholipase C continuouslyCDephosphorylation of the beta-adrenergic receptor, which prevents desensitisationDADP-ribosylation of Gi alpha, which removes its inhibition of adenylate cyclase

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

The answer and the reasoning

D. ADP-ribosylation of Gi alpha, which removes its inhibition of adenylate cyclase

Pertussis toxin ADP-ribosylates Gi alpha so that it cannot be activated by receptors. Inhibitory input to adenylate cyclase is lost, so cAMP rises. The modification of Gs alpha instead is the mechanism of cholera toxin.

Option by option

A. ADP-ribosylation of Gs alpha, which blocks its intrinsic GTPase activity
Modification of Gs alpha is the action of cholera toxin.
B. ADP-ribosylation of Gq alpha, which activates phospholipase C continuously
Gq alpha signalling through phospholipase C is not the target of pertussis toxin.
C. Dephosphorylation of the beta-adrenergic receptor, which prevents desensitisation
Pertussis toxin acts on the G protein, not by dephosphorylating the receptor.
D. ADP-ribosylation of Gi alpha, which removes its inhibition of adenylate cyclase (correct)
Correct. Modified Gi can no longer inhibit adenylate cyclase.

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 D, ADP-ribosylation of Gi alpha, which removes its inhibition of adenylate cyclase. Pertussis toxin ADP-ribosylates Gi alpha so that it cannot be activated by receptors.

Why is "ADP-ribosylation of Gs alpha, which blocks its intrinsic GTPase activ…" not the answer?

Modification of Gs alpha is the action of cholera toxin.

Why is "ADP-ribosylation of Gq alpha, which activates phospholipase C continu…" not the answer?

Gq alpha signalling through phospholipase C is not the target of pertussis toxin.

Why is "Dephosphorylation of the beta-adrenergic receptor, which prevents des…" not the answer?

Pertussis toxin acts on the G protein, not by dephosphorylating the receptor.

What concept does this question test?

It tests Molecular biology and genetics: pertussis toxin and Gi in Biochemistry. Inhibitory input to adenylate cyclase is lost, so cAMP rises. The modification of Gs alpha instead is the mechanism of cholera toxin.

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: pertussis toxin and Gi?

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.