title: Deep tunneling dominates the biologically important hydride transfer reaction from NADH to FMN in morphinone reductase.
abstract: The temperature dependence of the primary kinetic isotope effect (KIE), combined temperature-pressure studies of the primary KIE, and studies of the alpha-secondary KIE previously led us to infer that hydride transfer from nicotinamide adenine dinucleotide to flavin mononucleotide in morphinone reductase proceeds via environmentally coupled hydride tunneling. We present here a computational analysis of this hydride transfer reaction using QM/MM molecular dynamics simulations and variational transition-state theory calculations. Our calculated primary and secondary KIEs are in good agreement with the corresponding experimental values. Although the experimentally observed KIE lies below the semiclassical limit, our calculations suggest that approximately 99% of the reaction proceeds via tunneling: this is the first "deep tunneling" reaction observed for hydride transfer. We also show that the dominant tunneling mechanism is controlled by the isotope at the primary rather than the secondary position: with protium in the primary position, large-curvature tunneling dominates, whereas with deuterium in this position, small-curvature tunneling dominates. Also, our study is consistent with tunneling being preceded by reorganization: in the reactant, the rings of the nicotinamide and isoalloxazine moieties are stacked roughly parallel to each other, and as the system moves toward a "tunneling-ready" configuration, the nicotinamide ring rotates to become almost perpendicular to the isoalloxazine ring.
authors.affiliation: Manchester Interdisciplinary Biocentre, School of Chemical Engineering and Analytical Science, University of Manchester, 131 Princess Street, Manchester M1 7DN, UK.
authors.forename: Jiayun
authors.initials: J
authors.lastname: Pang
authors.forename: Sam
authors.initials: S
authors.lastname: Hay
authors.forename: Nigel S
authors.initials: NS
authors.lastname: Scrutton
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Sutcliffe
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D005486:Flavin Mononucleotide / Q000737:chemistry*; D009243:NAD / Q000737:chemistry*; D010088:Oxidoreductases / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D001426:Bacterial Proteins; D009243:NAD; D005486:Flavin Mononucleotide; D010088:Oxidoreductases; C087820:morphinone reductase