Involved in the urea cycle of ureotelic animals where the enzyme plays an important role in removing excess ammonia from the cell. 2 ATP + hydrogencarbonate + NH4(+) = 2 ADP + carbamoyl phosphate + 2 H(+) + phosphate Requires N-acetyl-L-glutamate (NAG) as an allosteric activator. N-acetyl-L-beta-phenylglutamate (Phe-NAG) can also activate CPSase I, but with an activation constant that is 2-fold higher than that for NAG. The activation constant Ka of N-acetyl-L-glutamate for the reaction is 0.11 mM. Can form homooligomers (monomers as predominant form and dimers). Primarily in the liver and small intestine. The type-1 glutamine amidotransferase domain is defective. 50% of the mature protein that was isolated had Leu-39 as its N-terminal residue and 50% had Ser-40 suggesting two adjacent processing sites. However, the possibility of proteolytic removal of Leu-39 during the isolation of the enzyme cannot be excluded. Undergoes proteolytic cleavage in the C-terminal region corresponding to the loss of approximately 12 AA residues from the C-terminus (PubMed:23649895). Succinylated at Lys-287 and Lys-1291. Desuccinylated at Lys-1291 by SIRT5, leading to activation (By similarity). Glutarylated. Glutarylation levels increase during fasting. Deglutarylated by SIRT5 at Lys-55, Lys-219, Lys-412, Lys-889, Lys-892, Lys-915, Lys-1360 and Lys-1486, leading to activation.