Final answer:
G-proteins are activated by the binding with GTP, which is facilitated by a ligand-induced conformational change in a G-protein-linked receptor. This activation triggers a cascade of intracellular signaling events, including the generation of second messengers like cAMP and the phosphorylation of substrate molecules by protein kinases.
Step-by-step explanation:
G-proteins are typically activated by the binding with GTP (Guanosine triphosphate). When a signal molecule binds to a G-protein-linked receptor on the cell surface, it causes a conformational change in the receptor, which in turn activates the associated G-protein. This is done by the replacement of bound GDP (Guanosine diphosphate) with GTP on the G-protein. Activation of G-proteins is crucial as they help transduce extracellular signals into intracellular responses, often through the action of enzymes such as adenylyl cyclase, which converts ATP (Adenosine triphosphate) into the second messenger cAMP (cyclic AMP). cAMP activates a group of proteins known as protein kinases, which are enzymes that transfer a phosphate group from ATP to a substrate molecule in the process termed phosphorylation, altering the structural orientation and activity of the substrate, affecting various cellular processes.