Indiscriminate Binding by Orotidine 5‘-Phosphate Decarboxylase of Uridine 5‘-Phosphate Derivatives with Bulky Anionic C6 Substituents

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Abstract
Orotidine 5‘-phosphate (OMP) decarboxylase appears to act upon its substrate without the intervention of metals or other cofactors and without the formation of covalent bonds between the enzyme and the substrate. Crystallographic information indicates that substrate binding forces the substrate's scissile carboxylate group into the neighborhood of several charged groups at the active site. It has been proposed that binding might result in electrostatic stress at the substrate's C6 carboxylate group in such a way as to promote decarboxylation by destabilizing the enzyme−substrate complex in its ground state. If that were the case, one would expect uridine 5‘-phosphate (UMP) derivatives with bulky anionic substituents at C6 to be bound weakly compared with UMP, which is unsubstituted at C6. Here, we describe the formation of anionic 5,6-dihydro-6-sulfonyl derivatives by spontaneous addition of sulfite to UMP and to OMP. These sulfite addition reactions, which are slowly reversible and are not catalyzed by the enzyme, result in the appearance of one (or, in the case of OMP, two) bulky anionic substituents at the 6-carbon atom of UMP. These inhibitors are bound with affinities that surpass the binding affinity of UMP. We are led to infer that the active site of OMP decarboxylase is remarkably accommodating in the neighborhood of C6. These are not the properties that one would expect of an active site with a rigid structure that imposes sufficient electrostatic stress on the substrate to produce a major advancement along the reaction coordinate.