Many aquatic organisms, including most of the fish species, are ammoniotelic, which means they produce ammonia as metabolic waste, that they generally excrete by diffusion through their gills.
Ureotelic species produce urea via the ornithine-urea cycle (OUC) in which CPS plays an important role.
Carbamoyl phosphate synthetase I is mostly used by terrestrial vertebrates, and it appears that some aquatic species rely on CPS III to deal with urea production.
[3] CPS III has been described in cichlids of the Alcolapia genus,[2] lungfishes,[3][4][5] the Gulf toadfish Opsanus beta,[6] the rainbow trout Oncorhyncus mykiss,[7][8] the Atlantic halibut Hippoglossus hippoglossus,[9] the largemouth bass Micropterus salmoides,[10] the common carp Cyprinus carpio,[11] and in elasmobranchs such as the spiny dogfish Squalus acanthia[12][13] for example.
In fishes, the urea cycle is only found in a few teleosts, mostly air breeders or species living in very specific environments such as alkaline water,[14] and in elasmobranchs.
It is believed that the C-terminal region contains the binding site for the fixation of the allosteric effector N-acetylglutamate (NAG) which is required for CPS III to function.
Thus, these two cysteine residues appear to probably play a crucial role in the allosteric activity of CPS III.
One hypothesis is that CPS II appeared first after the fusion of genes coding for a glutaminase and an ammonia-dependent synthetase.