G-proteins, or GTP-binding proteins, are heterotrimeric proteins that consist of three subunits; alpha, beta, and gamma. The alpha subunit of a G-protein has two functions. First, the alpha subunit can bind either guanosine 5'-diphosphate (GDP) or guanosine 5'-triphosphate (GTP). Second, the alpha subunit has a slow, intrinsic GTPase activity which hydroylzes bound GTP to GDP. In the inactive form, all three subunits form a complex and bind GDP at the nucleotide site. However, when a G-protein is stimulated by a horomone such as epinephrine (adrenaline), the alpha subunit binds GTP in exchange for GDP. The binding of GTP allows the alpha subunit to dissociate from the beta and gamma subunits and associate with adenylyl cyclase. This new formation activates adenylyl cyclase which in turn can now produce 3',5'-cyclic adenosine monophosphate, or cyclic AMP (cAMP) from ATP.
Due to the intrinsic GTPase activity of the G-protein's alpha subunit, eventually GTP is hydrolyzed to GDP which promotes the dissociation of the alpha subunit from adenylyl cyclase, and no more cAMP is produced from ATP. The alpha subunit now reassociates with the beta and gamma subunits to reform an inactive G-protein.
G-proteins are part of a transmembrane signaling pathway. When a horomone binds to its transmembrane receptor, it induces a conformational change within the receptor that stimulates the G-protein. This stimuli causes the alpha subunit to bind GTP instead of GDP and activate adenylyl cyclase. All three participants in this pathway are membrane-associated. The first messenger in this pathway is the horomone itself. However, cAMP is known as the second messenger because it acts as an intracellular messenger from the horomone.