The receptor molecules for these hormones are inside the cells. The discovery of nitric oxide as a signal molecule provides an explanation for the ability of nitroglycerine to provide relief from the pains of angina. NO is released from nitroglycerine and relaxes blood vessels. NO often binds to the enzyme which converts GTP into cyclic GMP, an internal cell signal molecule. G protein receptors are a major pathway for converting an external signal into an intracellular message.
G proteins consist of three subunits.
The large a unit dissociates from the rest of the molecule and diffuses along the internal cell surface when a signaling molecule binds to the external receptor binding site. Binding of GTP is common in molecular switches. Adrenaline, glucagon, and ACTH are important hormones which cause changes in cyclic AMP concentrations within target cells. As you can see from fig. 15-22, one of the effects of increased concentrations of cAMP is activation of a kinase.