Final answer:
Upon ligand binding to a GPCR, the nearby G-protein binds to the receptor, leading to GTP replacing GDP on the α subunit and activation of the G-protein. The activated G-protein then dissociates into α and βγ subunits, which can activate further proteins and enzymes, initiating intracellular signaling cascades.
Step-by-step explanation:
When a ligand binds to a G-protein-coupled receptor (GPCR) on the cell surface, it triggers a conformational change in the receptor. This change allows the receptor to interact with a nearby G-protein. The G-protein, which is comprised of three subunits (α, β, and γ), binds to the activated receptor, leading to the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the α subunit of the G-protein.
Once GTP is bound, the G-protein undergoes another conformational change, causing the α subunit, now bound to GTP, to dissociate from the βγ subunits. The activated α subunit, as well as the βγ complex, may then go on to activate other membrane proteins, such as ion channels or enzymes like adenylate cyclase, leading to the generation of second messengers and subsequent intracellular signaling pathways. Eventually, intrinsic GTPase activity hydrolyzes the bound GTP to GDP, which causes the α subunit to reassociate with the βγ subunits, thus inactivating the G-protein and readying it for another cycle of activation.