Hydrogen isotope tracing in the reaction of orotidine-5'-monophosphate decarboxylase

The mechanism of the enzyme orotidine-5(')-monophosphate decarboxylase (OMP decarboxylase, ODCase) is not fully characterized; some of the proposed mechanisms suggest the possibility of hydrogen rearrangement (shift from C5 to C6 or loss of H5 to solvent) during catalysis. In this study, we sought mechanistic information for the ODCase reaction by examining the extent of hydrogen exchange in the product uridine-5(')-monophosphate, in combination with ODCase, at the H5 and H6 positions. In a subsequent experiment, partially deuterated OMP was prepared, and the extent of 2H5 rearrangement or loss to solvent was examined by integration of 1H nuclear magnetic resonance signals in the substrate and the resulting enzymatically decarboxylated product. The absence of detectable hydrogen exchange in these experiments limits somewhat the possible mechanisms for ODCase catalysis.