Aspartate carbamoyltransferase, eukaryotic <p>Aspartate carbamoyltransferase (ATCase) catalyses the formation of carbamoyl-aspartate in the pyrimidine biosynthesis pathway, by the association of aspartate and carbamoyl-phosphate. This is the commitment step in the <taxon tax_id="562">Escherichia coli</taxon> pathway and is regulated by feedback inhibition by CTP, the final product of the pathway.</p><p>The structural organisation of the ATCase protein varies considerably between different organisms. In bacteria such as E. coli, <taxon tax_id="602">Salmonella typhimurium</taxon> and<taxon tax_id="615">Serratia marcescens</taxon>, the ATCase is a dodecamer of 2 catalytic (c) trimersand 3 regulatory (r) dimers. The catalytic domains are coded for by thepyrB gene, and the regulatory domains by pyrI. In Gram-positive bacteriasuch as <taxon tax_id="1423">Bacillus subtilis</taxon>, ATCase exists as a trimer of catalytic subunits, butunlike in E. coli, it neither contains nor binds to regulatory subunits. Ineukaryotes, ATCase is found as a single domain in a multifunctional enzymethat contains activity for glutamine amidotransferase, carbamoylphosphatesynthetase, dihydroorotase, and aspartate carbamoyltransferase.</p>