InterPro domain: IPR006274

Carbamoyl phosphate synthase (CPSase) is a heterodimeric enzyme composed of a small and a large subunit (with the exception of CPSase III, see below). CPSase catalyses the synthesis of carbamoyl phosphate from biocarbonate, ATP and glutamine or ammonia, and represents the first committed step in pyrimidine and arginine biosynthesis in prokaryotes and eukaryotes, and in the urea cycle in most terrestrial vertebrates [ 1 , 2 ]. This entry represents the small subunit of the glutamine-dependent form ( 6.3.5.5 ) of carbamoyl phosphate synthase. The small subunit catalyses the hydrolysis of glutamine to ammonia, which in turn used by the large chain to synthesize carbamoyl phosphate. The C-terminal domain of the small subunit of CPSase has glutamine amidotransferase activity.

In animals CPSase small subunit is part of a fusion protein, CAD, which combines enzymatic activities of the pyrimidine pathway (glutamine-dependent carbamyl phosphate synthetase (GLN-CPSase), aspartate transcarbamylase (ATCase), and dihydroorotase (DHOase)) [ 3 ]. In fungi, the CAD-like protein Ura2 is a fusion protein with CPSase and ATCase activity, but without DHOase activity, which is provided by a separate protein [ 4 ].

1. The amidotransferase family of enzymes: molecular machines for the production and delivery of ammonia. Biochemistry 38, 7891-9
2. Carbamoyl phosphate synthetase: an amazing biochemical odyssey from substrate to product. Cell. Mol. Life Sci. 56, 507-22
3. Structure, functional characterization, and evolution of the dihydroorotase domain of human CAD. Structure 22, 185-98