Question

The cholera toxin, an AB exotoxin, attaches an ADP-ribose group to the host's stimulatory G factor (Gs). The normal function of Gs is to stimulate the host's adenylate cyclase, which produces the second messenger molecule cAMP. This toxin-mediated ADP-ribosylation of Gs has which of the following effects?a. inactivation of Gs, causing a decrease in cAMP levels and resulting in decreased ion transport from the infected host cell.b. constant activation of Gs, causing an increase in cAMP levels and resulting in decreased ion transport from the infected host cell.c. inactivation of Gs, causing a decrease in cAMP levels and resulting in increased ion transport from the infected host cell.d. constant activation of Gs, causing an increase in cAMP levels and resulting in increased ion transport from the infected host cell.

Answer 1

b) option is correct

Explanation:

• Some bacterial toxins cause disease by  altering the activity of G protein, cholera toxin is one of them
• Cholera toxin catalyse ADP ribosylation of Gs and blocks GTPase activity thus Gs GTP become permanently active
• Constitutive activation of Gs protein continuously induce adenylyl cyclase, cytosolic cAMP level rises that leads to activation of protein kinase A (pKA)
• Activated pKA catalyse phosphorylation of two transmembrane proteins of intestinal epithelial cells:
• CFTR cause excessive outflow of Cl- ion and Na+ H+ exchange cause efflux of Na+ ion, both enters in gut and form Na+ Cl-
• Na+Cl- leads to outflow of water from the gut, resulting in diarrhea and dehydration and this condition may cause death of organisms due to loss of water and ions