Differential Quantum Tunneling Contributions in Nitroalkane Oxidase Catalyzed and the Uncatalyzed Proton Transfer Reaction

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 106; no. 49; pp. 20734 - 20739
• Main Authors: Major, Dan T., Heroux, Annie, Orville, Allen M., Valley, Michael P., Fitzpatrick, Paul F., Gao, Jiali, Schramm, Vern L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 08.12.2009; National Acad Sciences
• Subjects: ACCELERATION; Acetates - chemistry; AQUEOUS SOLUTIONS; Atoms; Biocatalysis; Biological Sciences; CATALYSIS; Chemical reactions; Coordinate systems; Crystallography, X-Ray; Deuterium - metabolism; DEUTERONS; Dioxygenases - metabolism; Enzymatic reactions; Enzyme kinetics; ENZYMES; Ethane - analogs & derivatives; Ethane - chemistry; Ethane - metabolism; FREE ENERGY; Hydrogen; ISOTOPE EFFECTS; KINETICS; Models, Molecular; national synchrotron light source; Nitroparaffins - chemistry; Nitroparaffins - metabolism; NUCLEAR PHYSICS AND RADIATION PHYSICS; ORIGIN; OXIDASES; Oxidation; Physical Sciences; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Proton-Motive Force; PROTONS; Quantum mechanics; Quantum Theory; Quantum tunneling; SUBSTRATES; Thermodynamics; TRANSFER REACTIONS; TUNNELING; WATER
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0911416106

The proton transfer reaction between the substrate nitroethane and Asp-402 catalyzed by nitroalkane oxidase and the uncatalyzed process in water have been investigated using a path-integral free-energy perturbation method. Although the dominating effect in rate acceleration by the enzyme is the lowering of the quasiclassical free energy barrier, nuclear quantum effects also contribute to catalysis in nitroalkane oxidase. in particular, the overall nuclear quantum effects have greater contributions to lowering the classical barrier in the enzyme, and there is a larger difference in quantum effects between proton and deuteron transfer for the enzymatic reaction than that in water. Both experiment and computation show that primary KIEs are enhanced in the enzyme, and the computed Swain-Schaad exponent for the enzymatic reaction is exacerbated relative to that in the absence of the enzyme. In addition, the computed tunneling transmission coefficient is approximately three times greater for the enzyme reaction than the uncatalyzed reaction, and the origin of the difference may be attributed to a narrowing effect in the effective potentials for tunneling in the enzyme than that in aqueous solution.