|MoBio||G Proteins||Chapter 6|
The full name of "G protein" is GTP-binding protein because in the active state it binds to GTP (guanosine triphosphate). There are two types of G proteins: heterotrimeric G proteins and monomeric G proteins (or small G proteins). G-protein-coupled receptors are coupled to heterotrimeric G proteins.
The heterotrimeric G protein consists of three subunits: α, β and γ. Based on the differences in their genes, 20 α, 6 β and 12 γ subunits have been identified. Their molecular weights are in the following ranges:
In the inactive state, the α subunit binds to GDP and the three subunits are attached together (see above figure). When the α subunit binds to GTP, its affinity to the βγ subunits is decreased, resulting in their dissociation. The separated α and/or βγ subunits can then interact with their effectors.
Cycling of G protein between active and inactive states