Question

How can adenylate cyclase be activated? How can adenylate cyclase cause diarrhea?

Answer 1

Adenylate cyclase is activated by the alpha subunit of G protein. When ligands bind at GPCR they activate the G protein present in the cytosolic side of the cell membrane. The alpha subunit of G protein bound with the GTP gets active which activates adenylyl cyclase.

If adenylyl cyclase once activated by the G protein synthesizes cAMP which activates the CFTR located on the membrane of cells. Once activated they allow Cl- to move it from inside the cells to the lumen of the intestine. Now if G proteins are activated for always (mediated by cholera toxin) they will lock the adenylyl cyclase in active state that will raise the concentration of cAMP which in turn will activate the CFTR allowing the excess loos of Cl- with water and produces diarrhea.