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Doctoral thesis, 2019

Dissecting function and catalytic mechanism of fungal lytic polysaccharide monooxygenases

Liu, Bing

Abstract

Fungi use a complex and well-orchestrated enzyme machinery to degrade lignocellulose biomass, in which both hydrolytic and redox enzymes are involved. Lytic polysaccharide monoxygenases (LPMOs) are copper-dependent enzymes that cleave bonds in polysaccharides using oxidative mechanisms. LPMOs belonging to auxiliary activity family 9 (AA9) are widely distributed in the fungal kingdom. The aim of this study is to develop a better understanding of the roles of AA9 LPMOs in lignocellulose degradation with the focus on a white-rot softwood-decaying fungus Heterobasidion irregulare as well as to gain more insights into their catalytic mechanism by investigating the interaction of C1-specific AA9 LPMOs with substrate/co-substrate at molecular level. Two LPMOs from H. irregulare (HiLPMO9H and HiLPMO9I) were shown to have different substrate specificity against cellulose and glucomannan, indicating that AA9 LPMOs may be involved in degradation of different plant cell wall components during the decay of softwood by the H. irregulare (Paper I). Another H. irregulare LPMO (HiLPMO9B) was found to increase the substrate accessibility for a homologous cellobiohydrolase (HiCel7A) and the cooperation between these two enzymes were shown during crystalline cellulose degradation, indicating that AA9 LPMO may act in synergy with cellulases as importance members in the cellulose degradation system of H. irregulare (Paper II). Molecular dynamics showed that the C1-specific HiLPMO9B uses acidic residues to bind onto the cellulose surface in addition to hydrophobic residues (Paper III). Furthermore, it was shown that cyanide inhibits the activity of the C1-specific PcLPMO9D by competing with O2 binding to the enzyme. Cyanide was shown to bind to the axial position of copper coordinating sites, reflecting a possible scenario of the proposed Cu-superoxyl intermediate (Paper IV). The present study has increased our understanding of the functionalization of LPMO in basidiomycete fungi and has expanded the current view on possible substrate/co-substrate interaction at molecular level.

Keywords

lignocellulose; fungi; LPMO; biological role; catalytic mechanism

Published in

Acta Universitatis Agriculturae Sueciae
2019, number: 2019:43
ISBN: 978-91-7760-404-4, eISBN: 978-91-7760-405-1
Publisher: Swedish University of Agricultural Sciences , SLU

Authors' information

Liu, Bing
Swedish University of Agricultural Sciences, Department of Molecular Sciences

UKÄ Subject classification

Biochemistry and Molecular Biology
Structural Biology

URI (permanent link to this page)

https://res.slu.se/id/publ/100744