<p>This entry contains two related enzymes:<ol><li>Aspartate carbamoyltransferase (<db_xref db="EC" dbkey="2.1.3.2"/>) (ATCase) catalyzes the conversionof aspartate and carbamoyl phosphate to carbamoylaspartate, the second stepin the <i>de novo</i> biosynthesis of pyrimidine nucleotides [<cite idref="PUB00002421"/>]. In prokaryotesATCase consists of two subunits: a catalytic chain (gene pyrB) and aregulatory chain (gene pyrI), while in eukaryotes it is a domain in a multi-functional enzyme (called URA2 in yeast, rudimentary in Drosophila, and CADin mammals [<cite idref="PUB00000720"/>]) that also catalyzes other steps of the biosynthesis ofpyrimidines.</li><li>Ornithine carbamoyltransferase (<db_xref db="EC" dbkey="2.1.3.3"/>) (OTCase) catalyzes the conversionof ornithine and carbamoyl phosphate to citrulline. In mammals this enzymeparticipates in the urea cycle [<cite idref="PUB00000710"/>] and is located in the mitochondrialmatrix. In prokaryotes and eukaryotic microorganisms it is involved in thebiosynthesis of arginine. In some bacterial species it is also involved in thedegradation of arginine [<cite idref="PUB00001352"/>] (the arginine deaminase pathway).</li></ol>It has been shown [<cite idref="PUB00004607"/>] that these two enzymes are evolutionary related. Thepredicted secondary structure of both enzymes are similar and there are someregions of sequence similarities. One of these regions includes threeresidues which have been shown, by crystallographic studies [<cite idref="PUB00004604"/>], to beimplicated in binding the phosphoryl group of carbamoyl phosphate and may also play a role in trimerization of the molecules [<cite idref="PUB00007942"/>]. The carboxyl-terminal, aspartate/ornithine-binding domain is is described by <db_xref db="INTERPRO" dbkey="IPR006131"/>. </p>