Conformational changes in orotidine 5′-monophosphate decarboxylase: "remote" residues that stabilize the active conformation

The structural factors responsible for the extraordinary rate enhancement (∼10¹⁷) of the reaction catalyzed by orotidine 5′-monophosphate decarboxylase (OMPDC) have not been defined. Catalysis requires a conformational change that closes an active site loop and "clamps" the orotate base proximal to hydrogen-bonded networks that destabilize the substrate and stabilize the intermediate. In the OMPDC from Methanobacter thermoautotrophicus, a "remote" structurally conserved cluster of hydrophobic residues that includes Val 182 in the active site loop is assembled in the closed, catalytically active conformation. Substitution of these residues with Ala decreases k_cat/K_m with a minimal effect on k_cat, providing evidence that the cluster stabilizes the closed conformation. The intrinsic binding energies of the 5′-phosphate group of orotidine 5′-monophosphate for the mutant enzymes are similar to that for the wild type, supporting this conclusion.