adenylyl cyclase
Adenylyl cyclase is the factory that stamps out the cell's alarm signal. Sitting in the membrane, it stays quiet until a hormone-activated G protein tells it to start working — then it rapidly churns out cyclic AMP from the cell's own ATP.
Mechanistically, when a hormone binds a G protein-coupled receptor and activates a stimulatory G protein (Gs), the G protein switches adenylyl cyclase on. The enzyme converts ATP into cyclic AMP, raising its level inside the cell and launching the downstream cascade through protein kinase A. An inhibitory G protein (Gi) does the opposite, turning the enzyme down and lowering cyclic AMP.
Adenylyl cyclase sits at a critical control point, taking input from many receptors and converting it into a single common currency — cyclic AMP. Because it can be pushed up by some signals and down by others, the cell can integrate competing hormonal messages at this one enzyme to decide its net response.
When glucagon binds liver-cell receptors, Gs activates adenylyl cyclase; the resulting rise in cyclic AMP signals the liver to release glucose during fasting.
The enzyme that converts a hormone signal into cyclic AMP.
Cholera toxin causes severe diarrhea by locking the Gs protein in its “on” state, leaving adenylyl cyclase running and cyclic AMP sky-high in gut cells.