Elucidating the Nature of Enzyme Catalysis Utilizing a New Twist on an Old Methodology:  Quantum Mechanical−Free Energy Calculations on Chemical Reactions in Enzymes and in Aqueous Solution

How do enzymes achieve very large rate enhancements compared to corresponding uncatalyzed reactions in solution? We present a computational approach which combines high-level ab initio quantum mechanical calculations with classical free energy calculations to address this question. Our calculations lead to accurate estimates of ΔG ⧧ for both trypsin and catechol O-methyltransferase-catalyzed and reference uncatalyzed reactions and give new insights into the nature of enzyme catalysis. The same methodology applied to steps in the catalytic mechanism of citrate synthase further supports the conclusion that one need not invoke special concepts such as “low-barrier hydrogen bonds” or “pK a matching” to explain enzyme catalysis.