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orotate phosphoribosyltransferase Orotate phosphoribosyltransferase (OPRTase) is involved in the biosynthesis of pyrimidine nucleotides. Alpha-D-ribosyldiphosphate 5-phosphate (PRPP) and orotate are utilized to form pyrophosphate and orotidine 5'-monophosphate (OMP) in the presence of divalent cations, preferably Mg2+. In a number of eukaryotes, this protein is fused to a domain that catalyses the reaction (EC 4.1.1.23). The combined activity of EC 2.4.2.10 and EC 4.1.1.23 is termed uridine 5'-monophosphate synthase. The conserved Lys (K) residue at position 101 of the seed alignment has been proposed as the active site for the enzyme. [Purines, pyrimidines, nucleosides, and nucleotides, Pyrimidine ribonucleotide biosynthesis]
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