Transition-state and ground-state structures and their interaction with the active-site residues in catechol O-methyltransferase

The methylation of catecholate, catalyzed by catechol O-methyltransferase, has been studied by means of ab initio quantum mechanical calculations. The uncatalyzed reaction proceeds via a strongly interacting reactant complex of catecholate and methyl donor and encounters a significant activation bar rier. The enzyme active site dictates an alternative orientation of reactan ts, which leads to a large reduction of activation energy. The contribution of three active-site groups to catalysis has been evaluated from MP2/6-31G(d,p) interaction energy profiles and ONIOM MP2:HF energies. The calculati ons indicate that Tyr68 and peptide carbonyl groups of Met40 and Asp141 int eract with the reactant complex more strongly than with the transition stat e. These results suggest that the enormous rate enhancements brought about by catechol O-methyltransferase do not arise from preferential interactions of the transition state with the enzyme. Instead, the catalytic power of t his enzyme stems from orienting the reactants into a conformation where lit tle structural rearrangement is needed to form the transition state.