Proton‐Coupled Electron Transfer in Soybean Lipoxygenase: Hydrogen Tunneling, Electrostatics, and Conformational Motions

Abstract

The enzyme soybean lipoxygenase (SLO) serves as a prototype for hydrogen tunneling reactions because of its unusually large deuterium kinetic isotope effect (KIE) of 80 for the wild‐type enzyme and over 500 for a double mutant. The proton‐coupled electron transfer (PCET) reaction catalyzed by SLO has been studied with a vibronically nonadiabatic PCET theory that includes the quantum mechanical effects of the electrons and the transferring proton, as well as the proton donor‐acceptor and solvent motions. The calculations reproduced the experimentally observed magnitude and temperature dependence of the KIE for wild‐type SLO and a series of single mutants. The large KIE was found to arise from the small overlap between the reactant and product proton vibrational wave functions. Theoretical modeling of the double mutant illustrated that the colossal KIE of over 500 arises from a combination of a non‐optimal active site conformation corresponding to longer equilibrium donor‐acceptor distances and reduced sampling of shorter donor‐acceptor distances. Recent quantum mechanical/molecular mechanical (QM/MM) free energy simulations using a finite temperature string method combined with umbrella sampling have provided additional insights about the conformational motions, as well as the significance of electrostatics in this enzymatic reaction. These calculations highlight the critical roles of hydrogen tunneling, electrostatics, and protein conformational motions in enzyme catalysis.Support or Funding InformationNational Institutes of Health Grant GM056207This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.