Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases

Kjaergaard, Christian H.; Qayyum, Munzarin F.; Wong, Shaun D.; Xu, Feng; Hemsworth, Glyn R.; Walton, Daniel J.; Young, Nigel A.; Davies, Gideon J.; Walton, Paul H.; Johansen, Katja Salomon; Hodgson, Keith O.; Hedman, Britt; Solomon, Edward I.

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.

Citation

Kjaergaard, C. H., Qayyum, M. F., Wong, S. D., Xu, F., Hemsworth, G. R., Walton, D. J., Young, N. A., Davies, G. J., Walton, P. H., Johansen, K. S., Hodgson, K. O., Hedman, B., & Solomon, E. I. (2014). Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases. Proceedings of the National Academy of Sciences of the United States of America, 111(24), 8797-8802. https://doi.org/10.1073/pnas.1408115111

• Journal Article Type: Article
• Acceptance Date: May 6, 2014
• Online Publication Date: Jun 2, 2014
• Publication Date: Jun 17, 2014
• Deposit Date: Feb 18, 2016
• Publicly Available Date: Feb 18, 2016
• Journal: Proceedings of the national academy of sciences
• Print ISSN: 0027-8424
• Publisher: National Academy of Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 111
• Issue: 24
• Pages: 8797-8802
• DOI: https://doi.org/10.1073/pnas.1408115111
• Keywords: X-ray absorption spectroscopy, DFT, Dioxygen activation, Biofuels
• Public URL: https://hull-repository.worktribe.com/output/471711
• Publisher URL: http://www.pnas.org/content/111/24/8797
• Additional Information: This is an author's accepted manuscript of an article published in Proceedings of the national academy of sciences, 2014, v.111 issue 24.
• Contract Date: Feb 18, 2016