Action of a-subunit of G protein is:

Ans. is 'a' i.e. Breakdown of GTP to GDP This question is an indirect repetition from May 08 (q no. 22 of biochemistry). a-subunit of G protein has inherent GTPase activity which converts GTP into GDP.G proteinsG proteins are so-called because they bind the guanine nucleotidesGDP and GTP. They are heterotrimers (i.e., made of three different subunits)The three subunits are:Ga, which carries the binding site for the nucleotide. At least 21 different kinds of Ga molecules are found in mammalian cells.GbGgHow They WorkIn the inactive state G protein has GDP bound to its Ga subunit.When a hormone or other ligand binds to the associated G protein coupled receptor (GPCR), the GDP is exchanged for GTPGTP activates Ga causing it to dissociate from GbGg (which remain linked as a dimer).Activated Ga in turn activates an effector molecule (adenylyl cyclase- an enzyme in the inner face of the plasma membrane which catalyzes the conversion of ATP into the "second messenger" cyclic AMP).HormoneGDP The beta and gamma subunit do not separate from each other, and GbGg dimer also activates a variety of effectors.* The actions of the Ga-GTP complex are short lived because the a-subunit has an inherent GTPase activity, resulting in the rapid hydrolysis of GTP to GDP. This leads to reassociation of the Ga unit with the GbGg dimer. This inactivates the G protein.