<prompt> True about G protein coupled subunitG-protein binds to hormone on the cell surfaceAll the three subunits alpha,beta and gamma should bind to eachother G-protein to actG-protein acts as inhibitor and excitator due to difference in alpha subunitG-protein is bound to GTP in resting state  <response>: Ref Harer 26e p458: Liglpincott Badem 3e p93 Ganong 22 p41: KDT 6ep46 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. 