title: Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases.
abstract: Strategies for O2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9-11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O2 binding with very little reorganization energy.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305;
authors.forename: Christian H
authors.initials: CH
authors.lastname: Kjaergaard
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305;
authors.forename: Munzarin F
authors.initials: MF
authors.lastname: Qayyum
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305;
authors.forename: Shaun D
authors.initials: SD
authors.lastname: Wong
authors.affiliation: Novozymes, Inc., Davis, CA 95618;
authors.forename: Feng
authors.initials: F
authors.lastname: Xu
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Glyn R
authors.initials: GR
authors.lastname: Hemsworth
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Walton
authors.affiliation: Department of Chemistry, University of Hull, Kingston upon Hull HU6 7RX, United Kingdom;
authors.forename: Nigel A
authors.initials: NA
authors.lastname: Young
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Gideon J
authors.initials: GJ
authors.lastname: Davies
authors.affiliation: Department of Chemistry, University of York, York YO10 5DD, United Kingdom;
authors.forename: Paul H
authors.initials: PH
authors.lastname: Walton
authors.affiliation: Novozymes A/S, 2880 Bagsværd, Denmark; and.
authors.forename: Katja Salomon
authors.initials: KS
authors.lastname: Johansen
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305;Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94309.
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94309.
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.affiliation: Department of Chemistry, Stanford University, Stanford, CA 94305;Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94309 edward.solomon@stanford.edu.
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D002384:Catalysis; D020134:Catalytic Domain; D002686:Chitin / Q000737:chemistry; D003198:Computer Simulation; D003300:Copper / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005656:Fungal Proteins / Q000737:chemistry*; D006899:Mixed Function Oxygenases / Q000737:chemistry*; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D011134:Polysaccharides / Q000737:chemistry*; D013053:Spectrophotometry; D013481:Superoxides / Q000737:chemistry; D055327:Thermoascus / Q000201:enzymology*; D013816:Thermodynamics; D014965:X-Rays
chemical_list: D005656:Fungal Proteins; D011134:Polysaccharides; D013481:Superoxides; D002686:Chitin; D003300:Copper; D006899:Mixed Function Oxygenases; D010100:Oxygen
keywords: DFT; X-ray absorption spectroscopy; biofuels; dioxygen activation