Cholera toxin:
Ans. is 'c' i.e. Causes continued activation of adenylate cyclase Cholera toxin (CT) is a potent protein enterotoxin, chiefly responsible for the virulence of V. cholera.It is elaborated by the organism following its colonization of the small intestine .Effect of cholera toxin leads to watery diarrhoea, the characteristic feature of cholera.Mechanism of Cholera Toxin ActionCholera toxin consists of 2 subunits a monomeric enzymatic moiety (A subunit) a pentameric binding moiety (B subunit)The B-subunit, serves as a 'landing pad' and binds to GM1 ganglioside, a glycolipid on the surface of epithelial cells of the small intestine. This facilitates the entry of A-subunit into the epithelial cells.The entry of A-subunit into the cells leads to activation of its A1 fragment.Activated A1 interacts with cytosolic proteins called ADP-ribosylation factors (ARF)The ARF-A1 complexes catalyze ADP-ribosylation of a G-protein called GSa.The ADP-ribosylated Gsa in turn binds to and stimulates adenylate cyclase. Activated adenylate cyclase generates high levels of intracellular cyclic AMP (cAMP) from ATP.cAMP inhibits the absorptive sodium transport system (decreases sodium & chloride reabsorption) and activates the secretory chloride transport system (increases chloride and bicarbonate secretion)This leads to accumulation of sodium chloride in the intestinal lumen.Accumulated sodium chloride draws water into the lumen due to increased osmolality leading to watery diarrhoea.Cholera |Na+ &Cl- | Cl- & HC03-
Increases the levels of intracellular cylic GMP
Acts through the receptor the opiates
Causes continued activation of adenylate cyclase
Inhibits the enzyme phosphodiesterase
Bacteria