Simulating large nuclear quantum mechanical corrections in hydrogen atom transfer reactions in metalloenzymes.

The role of nuclear quantum mechanical effects in enzyme catalysis has recently attracted significant interest both from theoretical and experimental points of view. From a theoretical point of view, it is undoubtedly a challenge to try to account for the observed tunneling in the protein by microscopic simulations without adjustable parameters. One of the most spectacular examples of nuclear quantum mechanical effects is the reaction of lipoxygenase, which is characterized by a very large kinetic isotope effect and, thus, provides an excellent benchmark for simulation approaches. In the present study, we report a microscopic simulation of the large kinetic isotope effect in soybean lipoxygenase and its temperature dependence. This is, to the best of our knowledge, the first time that a very large nuclear quantum mechanical contribution to the activation free energy of a hydrogen atom transfer reaction and its temperature dependence have been evaluated by microscopic simulation. The simulation reproduces quite well the experimental kinetic information and the corresponding difference between the classical and quantum mechanical activation free energies for the H and D transfer reactions.