G-protein coupled receptors are named after the guanosine nucleotides GTP and GDP (guanosine triphosphate and -diphosphate) with which they interact. The G-protein exists of 3 subunits: α, β, and γ. The first is associated with the receptor; the latter has a lipophilic tail in the cell membrane. While at rest, GDP is bound to the α-subunit of the G-protein. Ligand binding leads to a conformational change of the receptor. This results in replacement of the GDP by GTP and thus activation of the receptor. Ligands of GPCRs typically bind within the transmembrane domain. Agonists are ligands that shift the receptor in favour of active states; inverse agonists are ligands that shift the receptor into an inactive state; neutral antagonists are ligands that do not affect the state. Agonist binding results in replacement of the GDP by GTP
and thus activation of the receptor. The α-subunit then dissociates from the receptor and activates a membrane bound enzyme. Further signal transduction depends on the type of G protein. The enzyme converts a substrate into a so-called second messenger. This second messenger further transduces the signal and evokes a cellular response. The last step describes the hydrolysis of GTP into GDP + phosphate, the inactivation, and return of the α-subunit to the G-protein coupled receptor. Drugs interacting with G-protein coupled receptors are of various kinds, but are mostly related with neurotransmission and hormone action. The next graphics introduce the most important G-proteins.
Also check here for an animation of the G-protein cycle.