Trimeric G-protein

Trimeric G Proteins are usually found coupled to a receptor protein and attached to the cytoplasmic face of the plasma membrane. It's function is to couple the receptor molecule to either an ion channel or enzyme. Trimeric G Proteins are composed of three sub-units, alpha beta and gamma. In its inactive state the alpha sub-unit is bound to GDP, when the coupled receptor is activated this alpha sub-unit releases this GDP molecule. A GTP molecule takes the place of the GDP on the alpha sub-unit and the protein under goes a large conformational change; the G protein is now in it's activated state. The sub-units dissociate into an alpha sub-unit and a beta-gamma complex which then go on to activate their target molecules which may be enzymes, this therefore carries the signal forward and the result is a cell response .