Role of Enzyme-Ribofuranosyl contacts in the ground state and transition state for orotidine 5′-phosphate decarboxylase: A role for substrate destabilization?

The crystal structure of yeast orotidine 5′-monophosphate decarboxylase (ODCase) complexed with the inhibitor 6-hydroxyuridine 5′-phosphate (BMP) reveals the presence of a series of strong interactions between enzyme residues and functional groups of this ligand. Enzyme contacts with the phosphoribofuranosyl moiety of orotidine 5′-phosphate (OMP) have been shown to contribute at least 16.6 kcal/mol of intrinsic binding free energy to the stabilization of the transition state for the reaction catalyzed by yeast ODCase. In addition to these enzyme-ligand contacts, active site residues contributed by both subunits of the dimeric enzyme are positioned to form hydrogen bonds with the 2′- and 3′-OH groups of the ligand's ribosyl moiety. These involve Thr-100 of one subunit and Asp-37 of the opposite subunit, respectively. To evaluate the contributions of these ribofuranosyl contacts to ground state and transition state stabilization, Thr-100 and Asp-37 were each mutated to alanine. Elimination of the enzyme's capacity to contact individual ribosyl OH groups reduced the k_ca/K_m value of the T100A enzyme by 60-fold and that of the D37A enzyme by 300-fold. Removal of the 2′-OH group from the substrate OMP decreased the binding affinity by less than a factor of 10, but decreased k_cat by more that 2 orders of magnitude. Upon removal of the complementary hydroxymethyl group from the enzyme, little further reduction in k_catlK_m for 2′-deoxyOMP was observed. To assess the contribution made by contacts involving both ribosyl hydroxyl groups at once, the ability of the D37A mutant enzyme to decarboxylate 2′-deoxyOMP was measured. The value of k_cat/K_m for this enzyme-substrate pair was 170 M^-1 s^-1, representing a decrease of more than 7.6 kcal/mol of binding free energy in the transition state. To the extent that electrostatic repulsion in the ground state can be tested by these simple alterations, the results do not lend obvious support to the view that electrostatic destabilization in the ground state enzyme-substrate complex plays a major role in catalysis.