Talk about the activation of GPCR.

Ligand activates the receptor, which then binds g protein and stimulates release of GDP and exchange it with GTP, so activated GTP-bound a subunit dissociates from bg complex which then carries the signal to intracellular target which can be an enzyme or an ion channel. Activation is terminated by GTP hydrolysis into GDP where a reassociates with bg. some of them produce second messengers (cAMP activating PKA, PLC (phospholipase C) activates DAG (activate PKC) and IP3 (release Ca2+ from ER))