The ornithine carbamoyltransferases are a family of enzymes that catalyse the production of citrulline from carbamoyl-phosphate and ornithine [<cite idref="PUB00001795"/>]. This reaction is part of the arginine biosynthetic pathway, and in some organisms the reaction is also part of the arginine deaminase pathway [<cite idref="PUB00001352"/>]. In most prokaryotes, and some lower eukaryotes, the enzyme is found in the cytoplasm, but in higher eukaryotes, such as mammals, the enzyme is foundin the mitochondrial matrix and functions as part of the urea cycle. Inmost organisms analysed to date, the enzyme has been shown to consist of a trimer of identical or nonidentical subunits [<cite idref="PUB00001353"/>]. Extensive similarity has been found within the ornithine carbamoyltransferase family, some of theconserved areas being important in catalysis [<cite idref="PUB00003720"/>]. <p>The C-terminal region of these enzymes shows a degree of similarity to the aspartate carbamoyltransferases, which are also known to bind carbamoyl-phosphate [<cite idref="PUB00003736"/>]. This region contains a highly conserved Cys residue (in aHis-Cys-Lys-Pro motif) implicated in ornithine binding [<cite idref="PUB00001353"/>].</p> Ornithine carbamoyltransferase