Small chain of the glutamine-dependent form of carbamoyl phosphate synthase, CPSase II
This group of sequences represents the small chain of the glutamine-dependent form of carbamoyl phosphate synthase, CPSase II. CPSase II catalyzes the production of carbomyl phosphate (CP) from bicarbonate, glutamine and two molecules of MgATP. The reaction is believed to proceed by a series of four biochemical reactions involving a minimum of three discrete highly reactive intermediates. The synthesis of CP is critical for the initiation of two separate biosynthetic pathways. In one CP is coupled to aspartate, its carbon and nitrogen nuclei ultimately incorporated into the aromatic moieties of pyrimidine nucleotides. In the second pathway CP is condensed with ornithine at the start of the urea cycle and is utilized for the detoxification of ammonia and biosynthesis of arginine. CPSases may be encoded by one or by several genes, depending on the species. The E.coli enzyme is a heterodimer consisting of two polypeptide chains referred to as the small and large subunit. Ammonia an intermediate during the biosynthesis of carbomyl phosphate produced by the hydrolysis of glutamine in the small subunit of the enzyme is delivered via a molecular tunnel between the remotely located carboxyphosphate active site in the large subunit. CPSase IIs belong to the triad family of amidotransferases having a conserved Cys-His-Glu catalytic triad in the glutaminase active site. This group also contains the sequence from the mammalian urea cycle form which has lost the active site Cys, resulting in an ammonia-dependent form, CPSase I.