T cell receptor activation. Molecular model of a major histocompatibility complex (MHC) class II molecule (red, top left) presenting an antigen (green) to a T cell receptor (violet) and triggering a signalling cascade. binding to a G-protein-coupled receptor (GPCR, blue, left) The binding of the antigen causes the alpha subunit of a G-protein (blue) coupled to the receptor to exchange its bound molecule of GDP (guanosine triphosphate, yellow, centre left) for a molecule of GTP (guanosine triphosphate, yellow, centre) and to be released. The G-alpha subunit binds to the enzyme adenylyl cyclase (blue, right), activating it. Adenylyl cyclase catalyses the conversion of ATP (adenosine triphosphate, leftmost red molecule at right) to cyclic adenosine monophosphate (cAMP, red, lower right of ATP) and pyrophosphate (red, above cAMP). cAMP regulates numerous cell functions.