Identification of the IMP binding site in the IMP dehydrogenase from Tritrichomonas foetus

The IMP dehydrogenase from Tritrichomonas foetus has been identified as a potential target for antitritrichomonial chemotherapy. The gene encoding this enzyme was expressed in transformed Escherichia coli, and the recombinant protein was purified to homogeneity with an average yield of 3 mg of protein per liter of bacterial culture. Kinetic characterizations verified that the recombinant enzyme is in the authentic native state. 6-Cl-IMP, an irreversible inhibitor of the enzyme, was found to protect cysteine residue 319 of the enzyme against carboxymethylation by iodoacetamide. Radiolabeled IMP was covalently bound to the enzyme during the enzyme-catalyzed reaction via the formation of a specific adduct with cysteine residue 319. It is thus postulated that the conversion of IMP to XMP catalyzed by the IMP dehydrogenase from T. foetus is mediated by a nucleophilic attack of cysteine-319 in the enzyme protein to IMP at, most likely, its 2-position to facilitate a hydride transfer to NAD, resulting in the formation of a covalent intermediate between substrate and enzyme.