Carbamoyl phosphate synthetase, large subunit connection domain superfamily

Carbamoyl-phosphate synthase (CPSase) catalyzes the ATP-dependent synthesis of carbamyl-phosphate from glutamine or ammonia and bicarbonate [1972379]. This important enzyme initiates both the urea cycle and the biosynthesis of arginine and pyrimidines. Glutamine-dependent CPSase (CPSase II) is involved in the biosynthesis of pyrimidines and purines.

In bacteria such as Escherichia coli, a single enzyme is involved in both biosynthetic pathways while other bacteria have separate enzymes. The bacterial enzymes are formed of two subunits. A small chain (carA) that provides glutamine amidotransferase activity (GATase) necessary for removal of the ammonia group from glutamine, and a large chain (carB) that provides CPSase activity. The large subunit consists of four structural units: the carboxyphosphate synthetic component, the oligomerization domain, the carbamoyl phosphate synthetic component and the allosteric domain [10089390]. Such a structure is also present in fungi for arginine biosynthesis (CPA1 and CPA2).

Two main CPSases have been identified in mammals, CPSase I is mitochondrial, is found in high levels in the liver and is involved in arginine biosynthesis; while CPSase II is cytosolic, is associated with aspartate carbamoyltransferase (ATCase) and dihydroorotase (DHOase) and is involved in pyrimidine biosynthesis. In the pyrimidine pathway in most eukaryotes, CPSase is found as a domain in a multi-functional protein, which also has GATase, ACTase and DHOase activity. Ammonia-dependent CPSase (CPSase I) is involved in the urea cycle in ureolytic vertebrates and is a monofunctional protein located in the mitochondrial matrix. The CPSase domain is typically 120 kD in size and has arisen from the duplication of an ancestral subdomain of about 500 amino acids. Each subdomain independently binds to ATP and it is suggested that the two homologous halves act separately, one to catalyze the phosphorylation of bicarbonate to carboxyphosphate and the other that of carbamate to carbamyl phosphate. The CPSase subdomain is also present in a single copy in the biotin-dependent enzymes acetyl-CoA carboxylase (ACC), propionyl-CoA carboxylase (PCCase), pyruvate carboxylase (PC) and urea carboxylase .