adenylyl cyclase

Adenylyl cyclase is the factory that manufactures the second messenger cyclic AMP. It sits in the cell membrane, and when a stimulatory G protein nudges it, it starts stamping out cyclic AMP molecules from the cell's plentiful supply of ATP.

Chemically, the enzyme converts ATP into 3',5'-cyclic AMP plus pyrophosphate. Its activity is a balance point: Gs proteins switch it on, Gi proteins switch it off, and in some tissues calcium and other inputs tune it further. Because it can churn out many cyclic-AMP molecules per activated receptor, it is a key amplification step in the pathway.

Several drugs converge on this enzyme. Forskolin activates it directly (a useful laboratory tool). Caffeine and theophylline do not act on the cyclase itself but on phosphodiesterase, the enzyme that destroys cyclic AMP — yet the net effect, more cyclic AMP, is similar. Understanding adenylyl cyclase explains why so many receptor drugs ultimately raise or lower the same internal signal.