There are several different classifications of receptors that couple signal transduction to G

G-Protein Coupled Receptors

Receptors that are coupled, inside the cell, to GTP-binding and hydrolyzing proteins (termed G-proteins). Receptors of the class that interact with G-proteins all have a structure that is characterized by 7 transmembrane spanning domains. These receptors are termed serpentine receptors. Examples of this class are the adrenergic receptors, odorant receptors, and certain hormone receptors (e.g. glucagon, angiotensin, vasopressin and bradykinin)

There are several different classifications of receptors that couple signal transduction to G-proteins. These classes of receptor are termed G-protein coupled receptors, GPCRs. Well over 1000 different GPCRs have been cloned, most being orphan receptors having no as yet identified ligand. Three different classes of GPCR are reviewed:

1. GPCRs that modulate adenylate cyclase activity. One class of adenylate cyclase modulating receptors activate the enzyme leading to the production of cAMP as the second messenger. Receptors of this class include the b-adrenergic, glucagon and odorant molecule receptors. Increases in the production of cAMP leads to an increase in the activity of PKA in the case of b-adrenergic and glucagon receptors. In the case of odorant molecule receptors, the increase in cAMP leads to the activation of ion channels. In contrast to increased adenylate cyclase activity, the a-type adrenergic receptors are coupled to inhibitory G-proteins that repress adenylate cyclase activity upon receptor activation.

2. GPCRs that activate PLC leading to hydrolysis of polyphosphoinositides (e.g. PIP₂) generating the second messengers, diacylglycerol (DAG) and inositoltrisphosphate (IP₃). This class of receptors includes the angiotensin, bradykinin and vasopressin receptors.

3. A novel class of GPCRs are the photoreceptors. This class is coupled to a G-protein termed transducin that activates a phosphodiesterase which leads to a decrease in the level of cGMP. The drop in cGMP then results in the closing of a Na⁺/Ca²⁺ channel leading to hyperpolarization of the cell.