Anatomy of a proficient enzyme: The structure of orotidine 5 '-monophosphate decarboxylase in the presence and absence of a potential transition state analog

Orotidine 5'-phosphate decarboxylase produces the largest rate enhancement that has been reported for any enzyme. The crystal structure of the recombi nant Saccharomyces cerevisiae enzyme has been determined in the absence and presence of the proposed transition state analog 6-hydroxyuridine 5'-phosp hate, at a resolution of 2.1 Angstrom and 2.4 Angstrom, respectively, Oroti dine 5'-phosphate decarboxylase folds as a TIM-barrel with the ligand bindi ng site near the open end of the barrel. The binding of 6-hydroxyuridine 5' -phosphate is accompanied by protein loop movements that envelop the ligand almost completely, forming numerous favorable interactions with the phosph oryl group, the ribofuranosyl group, and the pyrimidine ring. Lysine-93 app ears to be anchored in such a way as to optimize electrostatic interactions with developing negative charge at C-6 of the pyrimidine ring, and to dona te the proton that replaces the carboxylate group at C-6 of the product. In addition, H-bonds from the active site to O-2 and O-4 help to delocalize n egative charge in the transition state, Interactions between the enzyme and the phosphoribosyl group anchor the pyrimidine within the active site, hel ping to explain the phosphoribosyl group's remarkably large contribution to catalysis despite its distance from the site of decarboxylation.