True about G protein coupled receptors is:

Ans. (C) G proteins act as inhibitory and excitatory because of difference in alpha subunit(Ref: KDT 8th/e 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.