Correct option is B
The cholera toxin causes diarrhea by continuously activating adenyl cyclase, which leads to an increase in intracellular cyclic AMP (cAMP) levels. Cholera toxin does this by ADP-ribosylating the Gαs subunit of the G-protein complex. This modification inhibits the GTPase activity of Gαs, locking it in an active state. The active Gαs continuously stimulates adenylate cyclase, leading to an overproduction of cAMP. The high levels of cAMP activate protein kinase A (PKA), which in turn causes the secretion of chloride ions and water into the intestinal lumen, resulting in severe diarrhea.
Information Booster
- Cholera toxin specifically targets the G-protein complex, particularly the Gαs subunit, by ADP-ribosylation.
- The continuous activation of adenyl cyclase results in excessive production of cAMP in intestinal cells.
- Elevated cAMP levels activate protein kinase A (PKA), which stimulates ion channels and causes the secretion of chloride and bicarbonate ions.
- This ion secretion leads to a massive loss of water from the cells, causing the characteristic profuse watery diarrhea in cholera infection.
- The continuous activation of adenyl cyclase by the active form of Gαs leads to the disruption of normal cellular ion balance.
- The overall effect of this process is a severe dehydration and electrolyte imbalance, which can be fatal if not treated promptly.
Additional Information
- Option (a): Inactivating Gi protein is incorrect because cholera toxin specifically targets Gs proteins, not Gi proteins. Gi proteins are involved in inhibiting adenylate cyclase, but this is not the action of cholera toxin.
- Option (c): Locking Gs protein in an inactive state is incorrect because cholera toxin locks the Gs protein in an active state, not an inactive one. This is a key aspect of how it continuously stimulates adenylate cyclase.
- Option (d): Rapidly hydrolyzing GTP to GDP is incorrect because cholera toxin prevents the hydrolysis of GTP to GDP by modifying the Gαs subunit, thereby keeping it in an active state rather than promoting GTP hydrolysis.
