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 reaction catalyzed by SLO has been studied with a vibronically nonadiabatic 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 wavefunctions. 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 role of electrostatics in this enzymatic reaction.  These calculations highlight the critical role of hydrogen tunneling, electrostatics, and protein conformational motions in enzyme catalysis.

Proton-coupled electron transfer in soybean lipoxygenase: Hydrogen tunneling, electrostatics, and conformational motions

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.