QM−FE and Molecular Dynamics Calculations on Catechol O-Methyltransferase: Free Energy of Activation in the Enzyme and in Aqueous Solution and Regioselectivity of the Enzyme-Catalyzed Reaction

Abstract

We present combined quantum mechanical and free energy (QM−FE) as well as molecular dynamics (MD) calculations to investigate the methyl transfer reaction catalyzed by the enzyme catechol O-methyltransferase (COMT). The calculated transition state free energy, ΔG‡, of 24.5 kcal/mol for the enzymatic reaction is in reasonable agreement with experiment (18 kcal/mol), and the level of agreement improves when the substrates are allowed to slightly deviate from their molecular mechanical energy minimized interpolated geometries. The calculated ΔG‡ for the reaction in water is 5 kcal/mol higher than that found in the enzyme when cratic free energy terms are not considered, and this difference increases to 14−18 kcal/mol when they are included, in very good agreement with the estimated rate enhancement due to enzyme catalysis of 1011 (corresponding to ΔΔG‡ ≈ 15 kcal/mol). In contrast to trypsin, studied earlier by QM−FE methods, the calculated gas phase ΔG‡ is significantly lower than observed in the enzyme or i...