<prompt> True about G protein coupled receptors is:G proteins bind to hormones on the cell surfaceAll the three subunits alpha, beta and gamma should bind to each other for G proteins to actG proteins act as inhibitory and excitatory because of difference in alpha subunitG protein is bound to GTP in resting state  <response>: Ans. C G proteins act as inhibitory and excitatory because of difference in alpha subunitRef: KDT 8the p54 G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers i.e., made of three different subunits The three subunits are: Ga, Gb and Gg. In the inactive state G protein has GDP bound to its Ga subunit. When a hormone or other ligand binds to the associated receptor GPCR, the GDP is exchanged for GTP. GTP activates Ga causing it to dissociate from GbGg which remain linked as a dimer. Activated Ga 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 a-subunits. Gs and Gq are stimulatory whereas Gi is inhibitory G-protein. 