right|thumb|300px|G-protein-coupled receptor mechanism If a receptor in an active state encounters a G protein, it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to GTP.
right|thumb|300px|G-protein-coupled receptor mechanism If a receptor in an active state encounters a G protein, it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's aff?inity for ligands. Activated G proteins are bound to GTP.