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Carbamoyl Phosphate Synthetase

Carbamoyl phosphate synthetase catalyzes the ATP-dependent synthesis of carbamoyl phosphate from glutamine or ammonia and bicarbonate. [1] This enzyme catalyzes the reaction of ATP and bicarbonate to produce carbonyl phosphate and ADP. Carbonyl phosphate reacts with ammonia to give carbamate. In turn, carbamate reacts with a second ATP to give carbamoyl phosphate plus ADP. It represents the first committed step in pyrimidine and arginine biosynthesis in prokaryotes and eukaryotes, and in the urea cycle in most terrestrial vertebrates.[2] Most prokaryotes carry one form of CPSase that participates in both arginine and pyrimidine biosynthesis, however certain bacteria can have separate forms.

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. The large subunit has two homologous carboxy phosphate domains, both of which have ATP-binding sites; however, the N-terminal carboxy phosphate domain catalyses the phosphorylation of biocarbonate, while the C-terminal domain catalyses the phosphorylation of the carbamate intermediate. The carboxy phosphate domain found duplicated in the large subunit of CPSase is also present as a single copy in the biotin-dependent enzymes acetyl-CoA carboxylase, propionyl-CoA carboxylase, pyruvate carboxylase and urea carboxylase. [3]