Quantum Catalysis in Enzymes

Enzyme-catalysed reactions span a wide range of reaction types and mechanisms. In many cases the rate-determining step is the transfer of a proton, hydride ion, or hydrogen atom; such reactions are almost always dominated by quantummechanical tunnelling. The effective barrier for tunnelling is highly dependent on the evolution of zero-point energy along the reaction path, and zero-point energy is one of the multidimensional effects that one must include in a reliable treatment of quantum-mechanical tunnelling. Even when tunnelling is negligible, changes in the zero-point energy of participating vibrational modes when the system passes from the reactant state to the transition state can have accelerating, or, less often, decelerating effects on reaction rates, and these kinds of vibrational effects, as well as the change in thermal vibrational energy of low-frequency quantised vibrational modes, are very important for studying kinetic isotope effects, which are one of the chief experimental tools for elucidating reaction mechanisms and probing the nature of transition states. Theoretical methods for systematically including the quantum effects of multidimensional tunnelling and quantised vibrations in the description of enzyme-catalysed reactions have been