Orotate phosphoribosyl transferase (OPRTase) is involved in the biosynthesis of pyrimidine nucleotides. Alpha-D-ribosyldiphosphate 5-phosphate (PRPP) and orotate are utilised 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 (<db_xref db="EC" dbkey="4.1.1.23"/>). The combined activity of <db_xref db="EC" dbkey="2.4.2.10"/> and <db_xref db="EC" dbkey="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. Orotate phosphoribosyltransferase