Heterotrimeric G-proteins are made up of alpha, beta, and gamma subunits. The chemical qualities of the alpha subunit allow it to bind easily to one of two guanine subunits, GDP or GTP. The protein thus has two functional formations. When GDP is bound to the alpha subunit, the alpha subunit remains bound to the beta-gamma subunit to form an inactive trimeric protein. However, when GTP binds to the alpha subunit, the alpha subunit dissociates from the beta-gamma complex and is then free to interact with other effector molecules.
The unique chemical qualities of G-proteins are what allow metabotropic receptors to indirectly control the opening and closing of ion channels located at other places on the postsynaptic membrane. When there is no dopamine in the synaptic cleft, GDP remains bound to the alpha subunit and the G-protein is inactive. However, the binding of dopamine at the extracellular recognition site of the receptor causes the receptor to change shape. This conformational change causes the alpha subunit to give up its GDP in exchange for GTP. Once bound to GTP, the alpha subunit dissociates from the beta-gamma complex. The free alpha-GTP complex then binds with adenylate cyclase, a transmembrane protein which converts adenosine triphosphate (ATP) to adenosine 3,5 monophosphate (cyclic AMP, or cAMP).