1BXR

Carbamoyl phosphate synthetase (CPS) catalyses the formation of carbamoyl phosphate, an intermediate in the biosynthesis of pyrimidine nucleotides and arginine, from glutamine, bicarbonate, and two molecules of MgATP. The enzyme from Escherichia coli has three separate active sites, which are connected by a molecular tunnel that is almost 100 Angstroms in length and is a heterodimeric protein that is composed of two subunits of molecular weight ~40 and ~118 kD.

In the proposed reaction mechanism, ATP phosphorylates bicarbonate to form carboxy phosphate, and glutamine is hydrolysed to glutamate and ammonia. The ammonia then reacts with the carboxy phosphate intermediate to form carbamate. In the final step, a second molecule of ATP phosphorylates carbamate to produce the ultimate product, carbamoyl phosphate. Overall, three unstable intermediates (ammonia, carboxy phosphate, and carbamate) and carbamoyl phosphate are formed in a series of four separate reactions.

The hydrolysis of glutamine occurs within the small subunit via a thioester intermediate. Meanwhile, the carboxyphosphate is produced from the phosphorylation of hydrogencarbonate by ATP at a site contained within the N-terminal half of the large subunit. This phosphorylation reaction is thought to trigger the reaction cascade. The ammonia migrates through the interior of the protein, where it reacts with carboxyphosphate to produce the carbamate intermediate. The carbamate intermediate is then transported through the interior of the protein to a second site within the C-terminal half of the large subunit, where it is phosphorylated by another ATP to yield the final product, carbamoyl phosphate.