GLOSSARY

A G-protein-coupled receptor (GPCR) belongs to the largest family of membrane receptors (more than 800 genes in humans), with a seven-transmembrane structure; after ligand binding, it activates a heterotrimeric G protein, relaying the signal to enzymes or ion channels.

Mechanism

• Resting state: the G protein (alpha, beta, and gamma subunits) with GDP bound to alpha.
• Hormone binding → receptor conformational change → GDP swapped for GTP on the alpha subunit.
• Alpha-GTP dissociates from beta-gamma → activates effectors.
• Termination: GTP hydrolysed to GDP by the intrinsic GTPase activity of alpha.

Types of G Protein

• Gs: activates adenylate cyclase → raises cAMP (beta-adrenergic receptor, glucagon, TSH, ACTH).
• Gi: inhibits adenylate cyclase → lowers cAMP (alpha-2 receptor, somatostatin, opioids).
• Gq: activates phospholipase C → IP₃ and DAG → raises calcium (alpha-1 receptor, ADH-V1, oxytocin).

Importance

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