The mechanism for ODCase appears to involve the formation of a zwitterion of OMP and a ylid on decarboxylation. Thiamin pyrophosphate catalyzes various decarboxylation and transfer reactions involving ketone groups because the thiazolium ring with its positively charged N atom can, on the loss of a proton from the adjacent C-2, generate a ylid which adds to carbonyl groups to produce a substrate ylid. The unusual aspect, then, of the ODCase reaction is that the substrate itself becomes the ylid, presumably by gaining a proton from ODCase, which results in a positive charge on the N-1 atom of the pyrimidine ring. It is a zwitterion in the transition state which momentarily becomes a ylid on decarboxylation of OMP which then yields the product, UMP. There is no known cofactor for the ODCase reaction. It will be of interest to discover the groups on the enzyme that aid in formation of the zwitterion and the ylid. Further work on the crystal structure and on the production of altered enzymes (where specific amino acids suspected to be important for the reaction are changed) should reveal more details about this important and novel reaction.
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