LPMOs have a copper-containing active site with an N-terminal histidine (Proc. Nat. Acad. Sci., 2011). The active site was discovered in 2011 and is known as the histidine brace. LPMOs catalyse the oxidation of recalcitrant polysaccharides such as cellulose. Our current research interests involve understanding the catalytic mechanisms of these enzymes using a combination of structure, spectroscopy and theory. See reviews: Curr. Opin. Chem. Biol. 2016, and Nature Catalysis 2018.
The active sites of most LPMOs contain a tyrosine that occupies the axial position in the copper ion's coordination sphere. Its role has been enigmatic, not least because it is has been a long-standing question as to why the tyrosine is not oxidised (O-H bond strength ~ 85 kcal mol-1) in preference to the substrate (C-H bond strength ~ 100 kcal mol-1). Our recent work has shown that the addition of H2O2 to the reduced form of an AA9 LPMO leads to the formation of a Cu(II)-tyrosyl complex, which is strongly coloured. This complex does not form in the presence of substrate--it is also inactive for the oxidation of substrate. This work indicates that a hole-hopping pathway operates in AA9 LPMOs to protect the active site from oxidative damage, which we have examined using Marcus theory. The pathway passes through a chain of conserved tryptophan and tyrosine residues (see figure), with a mean residence time of only 6 ms. Once this pathway itself is compromised by oxidation, then the Cu(II)-tyrosyl compex forms.
How do LPMOs interact with their natural polysaccharide substrates? This is a difficult question to answer since the solid/heterogeneous nature of the substrate precludes use of many of the normal techniques for the study of enzymes. To overcome this limitation we have recently used EPR spectroscopy to study LPMOs bound to cellulose fibres extracted from celery. These fibres are an excellent source of uniaxially-orientated cellulose, thus permitting the use of orientation-dependent EPR spectroscopy to give information on how LPMOs interact with their natural subsrate. This work is published in Dalton Transactions.
Figure. Space-filling model of AA9 LPMO binding to celery fibril.
- Insights from semi-oriented EPR spectroscopy studies into the interaction of lytic polysaccharide monooxygenases with cellulose, L Ciano, A Paradisi, G Hemsworth, M Tovborg, G J Davies, P H Walton, Dalton Transactions, 2020, 49, 3413-3422.
- A fungal family of lytic polysaccharide monooxygenase-like copper proteins, A Labourel, K Frandsen, F Zhang, N Brouilly, S Grisel, M Haon, L Ciano, D Ropartz, M Fanuel, F Martin, D Navarro, M-N Rosso, T Tandrup, B Bissaro, K Johansen, A Zerva, P H Walton, B Henrissat, L Leggio, J-G Berrin, Nature Chem. Biol. 2020, 16, 345-350.
- Formation of a copper(II)-tyrosyl complex at the active site of lytic polysaccharide monooxygenases following oxidation by H2O2, A Paradisi, E M Johnston, M Tovborg, C R Nicoll, L Ciano, A Dowle, J McMaster, Y Hancock, G J Davies, P H Walton, J. Am. Chem. Soc., 2019, 141, 18585-18599.
- Molecular Mechanisms of Oxygen Activation and Hydrogen Peroxide Formation in Lytic Polysaccharide Monooxygenases, B Wang, P H Walton, C Rovira ACS Catalysis, 2019, 9(6), 4958-4969.
- Bracing copper for the oxidation of C-H bonds, L Ciano, G J Davies, W B Tolman, P H Walton Nature Catalysis, 2018, 1(8), 571-577.
- An ancient family of lytic polysaccharide monooxygenases with roles in arthropod development and biomass digestion. F Sabbadin, G R Hemsworth, L Ciano, B Henrissat, P Dupree, T Tryfona, R D S Marques, S T Sweeney, K Besser, L Elias, G Pesante, Y Li, A A Dowle, R Bates, L D Gomez, R Simister, G J Davies, P H Walton, N C Bruce, S J McQueen-Mason, Nature Commun., 2018, 9, 756.
- Lytic xylan oxidases from wood-decay fungi overcome biomass recalcitrance. M Couturier, S Ladevèze, G Sulzenbacher, L Ciano, M Fanuel, C Moreau, A Villares, B Cathala, F Chaspoul, K Frandsen, A Labourel, I Herpoël-Gimbert, S Grisel, M Haon, N Lenfant, H Rogniaux, D Ropartz, G Davies, M-N Rosso, P H Walton, B Henrissat, J-G Berrin, Nature Chem. Biol., 2018, 14(3), 306-310.
- Structural and electronic determinants of lytic polysaccharide monooxygenase reactivity on polysaccharide substrates. T J Simmons, K E H Frandsen, L Ciano, T Tryfona, N Lenfant, J C Poulsen, L F L Wilson, T Tandrup, M Tovborg, K Schnorr, K S Johansen, B Henrissat, P H Walton, L Lo Leggio, P Dupree, Nature Commun., 2017, 8, 1064.
Paul Walton obtained his PhD in 1990 (University of Nottingham, UK), followed by two years as a NATO/SERC postdoctoral fellow at the University of California, Berkeley, USA. He joined the Department of Chemistry at York as a faculty member in 1993. Between 2004 and 2010 he was chair of department. His main research area is bioinorganic chemistry, in which he has made contributions to the understanding of copper oxidases, including the discovery of the histidine brace. He is the recipient of multiple awards: the Royal Society of Chemistry's Higher Education Teaching Award, the RSC's 2016 Joseph Chatt Award for outstanding multidisciplinary research, the IChemE's Global Award for energy research, and the Royal Society's inaugural Athena Prize for gender equality work (runner up). He has also been the editor of Dalton Transactions (2004-2008), chair of Heads of Chemistry UK, chair of the Royal Society of Chemistry's Diversity Committee and is one the RSC's 175 Faces of Chemistry. Paul is an internationally-known advocate of equality in sciences and lectures widely on the subject.