How are G-protein linked receptors activated?

G-protein linked receptors (GPCRs) are activated when a specific ligand, such as a hormone or neurotransmitter, binds to the extracellular region of the receptor protein. This binding event induces a conformational change in the receptor, which triggers the activation of an associated heterotrimeric G protein. The G protein consists of two parts: an alpha subunit and beta-gamma subunits. The activated alpha subunit can interact with and regulate the activity of various effector proteins (such as adenylyl cyclase or phospholipase C) depending on the specific type of G protein involved. These effector proteins generate second messenger molecules, such as cyclic AMP (cAMP) or inositol trisphosphate (IP3), which propagate the signal within the cell.