<p>This family contains two related enzymes:</p><p> <ol><li>Aspartate carbamoyltransferase (<db_xref db="EC" dbkey="2.1.3.2"/>) (ATCase) catalyses the conversion of aspartate and carbamoyl phosphate to carbamoylaspartate, the second step in the <i>de novo</i> biosynthesis of pyrimidine nucleotides [<cite idref="PUB00002421"/>]. In prokaryotes ATCase consists of two subunits: a catalytic chain (gene pyrB) and a regulatory chain (gene pyrI), while in eukaryotes it is a domain in a multi-functional enzyme (called URA2 in yeast, rudimentary in Drosophila, and CAD in mammals [<cite idref="PUB00000720"/>]) that also catalyses other steps of the biosynthesis ofpyrimidines.</li><li>Ornithine carbamoyltransferase (<db_xref db="EC" dbkey="2.1.3.3"/>) (OTCase) catalyses the conversion of ornithine and carbamoyl phosphate to citrulline. In mammals this enzyme participates in the urea cycle [<cite idref="PUB00000710"/>] and is located in the mitochondrial matrix. In prokaryotes and eukaryotic microorganisms it is involved in the biosynthesis of arginine. In some bacterial species it is also involved in the degradation of arginine [<cite idref="PUB00001352"/>] (the arginine deaminase pathway).</li></ol> </p><p>It has been shown [<cite idref="PUB00004607"/>] that these two enzymes are evolutionary related. The predicted secondary structure of both enzymes are similar and there are some regions of sequence similarities. One of these regions includes three residues which have been shown, by crystallographic studies [<cite idref="PUB00004604"/>], to be implicated in binding the phosphoryl group of carbamoyl phosphate.</p>