G protein Mediated Receptor Family

The G protein coupled receptor is a seven transmembrane structured protein. It consists of trimer G protein of three (α, β, γ) subunits. The GTP molecules binds to α subunit and will be replaced by GDP when the ligands bind to the receptor. There are four groups of α subunits, such as G_αs, G_αi, G_αq, and G_α12. G_αs is stimulatory, G_αi is inhibitory, and G_αq acts on phospholipases.

The G_αs subunit stimulates membrane bound ADC (ADenylate Cyclase, EC.4.6.1.1) and increases cAMP concentration which leads to phosphorylation of PKA (Protein Kinase A).

The G_αi subunit may act directly on K⁺ and Ca⁺² channels. The G_αi of muscarinic ACh receptor opens K⁺ channels on heart smooth muscles. The G_αi of NE β receptor closes K⁺ channels on sympathetic nerves. In photoreceptor, the G_αi activates PDE (PhosphoDiEsterase) which decreases cGMP concentration which keeps K⁺ channels open. The G_αi of nicotinic ACh receptor acts on Ca⁺² channels and change gene expressions.