sentences:  Carbamoyl phosphate synthase (CPSase) is a heterodimeric enzyme composed of a small and a large subunit (with the exception of CPSase III, which is composed of a single polypeptide that may have arisen from gene fusion of the glutaminase and synthetase domains).CPSase has three active sites, one in the small subunit and two in the large subunit.The small subunit contains the glutamine binding site and catalyses the hydrolysis of glutamine to glutamate and ammonia, which is in turn used by the large chain to synthesize carbamoyl phosphate.