What Second Messengers Do GPCR Signals Trigger in Cells?
Activation of a single G protein can affect the production of hundreds or even thousands of second messenger molecules. (Recall that second messengers — such as cyclic AMP [cAMP], diacylglycerol [DAG], and inositol 1, 4, 5-triphosphate [IP3] — are small molecules that initiate and coordinate intracellular signaling pathways.) One especially common target of activated G proteins is adenylyl cyclase, a membrane-associated enzyme that, when activated by the GTP-bound alpha subunit, catalyzes synthesis of the second messenger cAMP from molecules of ATP. In humans, cAMP is involved in responses to sensory input, hormones, and nerve transmission, among others. Phospholipase C is another common target of activated G proteins. This membrane-associated enzyme catalyzes the synthesis of not one, but two second messengers — DAG and IP3 — from the membrane lipid phosphatidyl inositol. This particular pathway is critical to a wide variety of human bodily processes. For instance, thrombin receptors
