Cell surface receptors with indirect ligand effect

Many cell surface receptors act indirectly. When
they bind to a ligand they induce a series of intracellular
activation steps. This reaction system
consists of a receptor protein, a protein (G
protein) bound to a guanosine residue, and an
enzyme to be activated. Ligand binding alters
the receptor protein and activates the G protein
(2). This moves to the effector, e.g., an enzyme
complex (3), and activates it (4). In this way, a
second messenger is formed that triggers
further reactions in the cell, e.g., cyclic adenosine
monophosphate (cAMP) bymeans of the
enzyme adenylate cyclase