Quantum Variational Transition State Theory for Hydrogen Tunneling in Enzyme Catalysis

Experiments in recent years have shown that there is a large kinetic isotope effect in the rate of transfer of hydrogen or deuterium in enzymatic reactions of soybean lipoxygenase-1. The kinetic isotope effect (KIE) is only weakly temperature dependent but varies significantly in the presence of mutants whose functional groups are located rather far from the reaction center. In this paper we suggest that variational transition state theory as applied to dissipative systems, above the crossover temperature between deep tunneling and thermal activation, may be used as a paradigm for understanding the dynamics of these reactions. We find that the theory fits the experimental data rather well. The effects of different mutants are readily interpreted in terms of the friction they exert on the reaction center. Increasing the distal functional group increases the friction and thus lowers the kinetic isotope effect.