Each regulatory subunit contains a pseudosubstrate sequence comparable to the substrate domain of a target protein for Protein Kinase A, but with alanine substituting for the serine or threonine. The pseudosubstrate domain of the regulatory subunit, which lacks a hydroxyl that can be phosphorylated, binds to the active site of the catalytic subunit, blocking its activity. When each regulatory subunit binds 2 cAMP, a conformational change causes the regulatory subunits to release the catalytic subunits. The a subunit binds GTP , and can hydrolyze it to GDP . The a and g subunits have covalently attached lipid tails, that embed in the plasma membrane, binding the G-protein to the cytosolic membrane surface.
Hormone binding causes a conformational change in the receptor that is transmitted to the G-protein.
G a -GTP dissociates from the inhibitory bg subunits, and can now bind to and activate Adenylate Cyclase. Adenylate Cyclase , activated by G a -GTP, catalyzes production of cAMP . The presence of GDP on G a causes it to rebind to the inhibitory bg complex. Adenylate cyclase is no longer activated. Hormone receptor desensitization occurs.