Class III cellobiose dehydrogenases: the missing link?

Cellobiose dehydrogenase (CDH) is an enzyme secreted by biomass degrading fungi. The physiological function of CDH has been unknown for a long time before its physiological partner enzyme, the biomass-degrading lytic polysaccharide monooxygenase (LPMO), was discovered. CDH acts as an auxiliary enzyme to LPMO and delivers electrons and hydrogen peroxide to LPMO. With the help of CDH, LPMO cleaves plant-based polysaccharides like cellulose, starch or hemicelluloses. Many newly discovered LPMOs show different substrate specificities exerting their degrading function towards various hemicelluloses. However, the currently known CDHs all use cello-oligosaccharides like cellobiose as substrate. The oligosaccharides produced by LPMO from hemicelluloses cannot be used by the currently known CDHs. This raises the question on how the newly discovered LPMOs can be regenerated by CDH? The, for a long-time known, phylogenetic Class III CDHs, which could so far not be isolated or recombinantly expressed, might be the missing link to hemicellulose-degrading LPMOs. A cooperation between Austrian researchers from BOKUUniversity of Natural Resources and Life Sciences, Vienna and the Skryabin Institute of the Russian Academy of Sciences, Moscow, investigates the distribution of Class III CDHs in fungi and their correlation with LPMOs. Class III CDHs will be isolated from the culture collection available at the Skryabin Institute or recombinantly expressed in yeasts or fungi. Obtained CDHs will be biochemically characterized to test the following hypothesis: Class III CDHs can convert hemicellulose-derived oligosaccharides to supply hemicellulose-degrading LPMOs with electrons and hydrogen peroxide. Class III CDHs might be the missing link to explore new routes for the degradation of plant biomass polysaccharides and a positive outcome of this project will support the usage of renewables for a bio-based economy.

Cellobiose dehydrogenase (CDH) is an enzyme secreted by biomass degrading fungi. The physiological function of CDH has been unknown for a long time before its physiological partner enzyme, the biomass-degrading lytic polysaccharide monooxygenase (LPMO), was discovered. CDH acts as an auxiliary enzyme to LPMO and delivers electrons and hydrogen peroxide to LPMO. With the help of CDH, LPMO cleaves plant-based polysaccharides like cellulose, starch or hemicelluloses. Many newly discovered LPMOs show different substrate specificities exerting their degrading function towards various hemicelluloses. However, the currently known CDHs all use cello-oligosaccharides like cellobiose as substrate. The oligosaccharides produced by LPMO from hemicelluloses cannot be used by the currently known CDHs. This raises the question on how the newly discovered LPMOs can be regenerated by CDH? The, for a long-time known, phylogenetic Class III CDHs, which was for the first time recombinantly expressed in this project, was considered the missing link to hemicellulose-degrading LPMOs. A cooperation between Austrian researchers from BOKU University in an Austrian Science Fund (FWF) funded project was able to characterise a Class III CDHs from the plant pathogen fungus Fusarium solani for their first time. Biochemical and electrochemical characterisation showed, that the substrate specificity does not differ from the well-known other classes. An absolutely interesting protperty of the newly described Fusarium solani CDH is its lack of oxygen reactivity, which points towards a different interaction mechanism with LPMOs.