Cyclic AMP-dependent G-protein coupled receptors. Some types of transmembrane receptors are naturally coupled to an internal protein called G-protein. This G-protein is composed of three subunits, alpha, beta, and gamma subunits. The alpha subunit in its inactive state is bound to a GDP molecule.
When a ligand, either an internal molecule or a drug, binds to the extracellular domain of these receptors, it activates it. As a result of this activation, GTP molecule replaces the GDP molecule bound to alpha subunit. The alpha-GTP subunit dissociates from the other subunits of G protein and binds to and activates the membrane-attached adenylal cyclase enzyme.
One of the main functions of active adenylacyclase enzyme is to activate second messenger system through catalyzing the conversion of ATP into cyclic AMP. In turn, cyclic AMP converts the inactive protein kinase P, abbreviated as pKa, into its active form. The active pKa phosphorylates and activates large numbers of cellular enzymes that induce cascades of biological effects.
Thus, single ligand can induce huge number of cellular reactions.