<p>This group contains a number of protein families, example are:</p><li>Archaeal and bacterial dihydroorotase (<db_xref db="EC" dbkey="3.5.2.3"/>) (DHOase)</li><li>Allantoinase (<db_xref db="EC" dbkey="3.5.2.5"/>)</li><p>Dihydroorotase belongs to MEROPS peptidase family M38 (clan MJ), where it is classified as a non-peptidase homologue. DHOase catalyses the third step in the <i>de novo</i> biosynthesis of pyrimidine, the conversion of ureidosuccinic acid (N-carbamoyl-L-aspartate) into dihydroorotate. Dihydroorotase binds a zinc ion which is required for its catalytic activity [<cite idref="PUB00002652"/>].</p><p>In bacteria, DHOase is a dimer of identical chains of about 400 amino-acid residues (gene pyrC). In higher eukaryotes, DHOase is part of a large multi-functional protein known as 'rudimentary' in <taxon tax_id="7227">Drosophila melanogaster</taxon> and CAD in mammals and which catalyzes the first three steps of pyrimidine biosynthesis [<cite idref="PUB00000720"/>]. The DHOase domain is located in the central part of this polyprotein. In yeasts, DHOase is encoded by a monofunctional protein (gene URA4). However, a defective DHOase domain [<cite idref="PUB00001777"/>] is found in a multifunctional protein (gene URA2) that catalyzes the first two steps of pyrimidine biosynthesis.</p><p>The comparison of DHOase sequences from various sources shows [<cite idref="PUB00003725"/>] that there are two highly conserved regions. The first located in the N-terminal extremity contains two histidine residues suggested [<cite idref="PUB00001777"/>] to be involved in binding the zinc ion. The second is found in the C-terminal part. Members of this family of proteins are predicted to adopt a TIM barrel fold [<cite idref="PUB00004994"/>].</p><p>Allantoinase (<db_xref db="EC" dbkey="3.5.2.5"/>) is the enzyme that hydrolyzes allantoin into allantoate. In yeast (gene DAL1) [<cite idref="PUB00005645"/>], it is the first enzyme in the allantoin degradation pathway; in amphibians [<cite idref="PUB00002839"/>] and fishs it catalyzes the second step in the degradation of uric acid. The sequence of allantoinase is evolutionary related to that of DHOases.</p> Dihydroorotase, conserved site