phosphoribosylation of orotate to form oritidine 5'-phosphate (OMP) Authored: Jassal, B, 2003-06-17 09:00:25 EC Number: 2.4.2.10 Edited: D'Eustachio, P, 0000-00-00 00:00:00 Pubmed18184586 Pubmed6893554 Pubmed8631878 Pubmed9042911 Reactome Database ID Release 4373567 Reactome, http://www.reactome.org ReactomeREACT_33 The synthesis of orotidine 5'-monophosphate (OMP) from orotate and 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) is catalyzed by the orotate phosphoribosyltransferase activity of the bifunctional "uridine monophosphate synthetase (orotate phosphoribosyl transferase and orotidine 5'-decarboxylase)" protein. The reaction itself is freely reversible, but is pulled in the forward direction in vivo by the irreversible hydrolysis of pyrophosphate. While purified human protein has not been characterized in detail, the close similarity of the human gene to that encoding the well-studied hamster protein, and the demonstration that mutations in the human gene are associated with failure to convert orotate to UMP in vivo, provide convincing evidence that the human uridine monophosphate synthetase protein indeed catalyzes these two reactions (McClard et al. 1980; Suchi et al. 1997). The active form of the human protein is a dimer (Yablonski et al. 1996; Wittmann et al. 2008). orotate + 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) <=> orotidine 5'-monophosphate (OMP) + pyrophosphate