G-protein coupled receptor

Meaning: A GPCR is a membrane receptor with seven transmembrane α-helices that passes an extracellular signal to a heterotrimeric G protein (α, β, γ) inside the cell. Ligand binding changes the receptor's shape, so Gα swaps GDP for GTP and separates from Gβγ. Gα-GTP then regulates an effector enzyme. Gs stimulates adenylyl cyclase (ATP → cAMP, which activates protein kinase A), Gi inhibits it, and Gq activates phospholipase C, which yields IP₃ (releases Ca²⁺ from the ER) and DAG (activates protein kinase C). Gα's own GTPase activity hydrolyzes GTP to GDP, which ends the signal.

On the MCAT: Passages describe a hormone that raises cAMP or Ca²⁺ and ask what happens if adenylyl cyclase is blocked, or if a nonhydrolyzable GTP analog is added (Gα stays active, so the signal persists). Expect cascade and amplification reasoning: one receptor activates many G proteins, and each effector enzyme makes many second messenger molecules.

Common confusion: The receptor does not make cAMP; the G protein and its effector enzyme do. GTP, not ATP, switches the G protein on, and Gα-GDP is the inactive form. Do not mix GPCRs up with receptor tyrosine kinases, which dimerize and phosphorylate tyrosine residues, or with ligand-gated ion channels, which open directly when the ligand binds.