True about G protein coupled subunit
(Ref Harer 26/e p458: Liglpincott Badem 3/e p93 Ganong 22/ p41: KDT 6/ep46) G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three difterent subunits) The three subunits are Galpha, Gbeta and Ggamma In the inactive state G protein has GDP bound to its Galpha subunit. When a hormone or other ligand binds to the associated receptor (GPCR), the GDP is exchanged for GTP GTP activates Galpha causing it to dissociate from GbetaGgamma (which remain linked as a dimer) Activated Galpha in turn activates an effector molecule like adenylyl cyclase, which catalyzes the conversion. of ATP to the "second messenger" cyclic AMP). Gs, Gi and Gq are different types of G-proteins due to different alpha-subunits. Gs and Gq are stimulatory whereas Gi is inhibitory G-protein.
G-protein binds to hormone on the cell surface
All the three subunits alpha,beta and gamma should bind to eachother G-protein to act
G-protein acts as inhibitor and excitator due to difference in alpha subunit
G-protein is bound to GTP in resting state
General anatomy