G Protein-coupled Receptors

G protein-coupled receptors or GPCRs are cell surface receptors that associate with heterotrimeric G proteins and mediate cellular response to signals such as light, hormones, cytokines, and neurotransmitters.

All GPCRs contain seven transmembrane alpha-helices separated by alternating cytosolic loops that bind a G protein and extracellular loops that bind the ligand.

In the absence of ligands, a GPCR is inactive.

Ligand binding induces a conformational change in the cytosolic loop of the GPCR. The activated GPCR can now bind the inactive GDP bound G protein with increased affinity to form a receptor-G protein complex.

Receptor-G protein interaction further induces conformational changes in the G protein, triggering the exchange of GDP with GTP.

Next, the GTP-bound subunit of the G protein dissociates from the GPCR and activates downstream effectors to mediate vision, smell, taste, and immune response.