G-protein coupled receptors (GPCRs) represent the largest single family of integral membrane receptors. GPCRs are so named due to their interaction with heterotrimeric guanine nucleotide binding proteins (G-proteins), which transduces a signal across the cell membrane. Ligand binding to the receptor causes a conformational change that ultimately results in the receptor binding the alpha subunit of the G-protein. This, in turn, catalyzes the exchange of GTP for GDP bound to G, followed by dissociation of Gα-GTP from Gβγ and both subunits from the receptor. The alpha subunit of the G-protein then couples with an enzyme that catalyzes the production of second messenger molecules. The resulting change in the concentration of the second messenger causes further downstream effector events, including ion channel opening and the generation of other messengers. GPCRs share a common seven helical transmembrane domain, an extracellular amino terminus, and an intracellular carboxyl terminus. GPCRs are classified into several families based on sequence similarities.

Ligands: light, odorants, tastes, small molecule neurotransmitters, peptides, glycoprotein hormones, proteases, and others.

G-proteins: Gs, Gi, Gq, G11, and G13 (20 known Gα, 6 Gβ, 11 Gγ)

Classified by sequence homology: