Ninety percent of lignocellulose-degrading fungi contain genes encoding lytic polysaccharide monooxygenases (LPMOs). These enzymes catalyze the initial oxidative cleavage of recalcitrant polysaccharides after activation by an electron donor. Understanding the source of electrons is fundamental to fungal physiology and will also help exploit LPMOs for biomass processing. Using genome data and biochemical methods, we characterized and compared different extracellular electron sources for LPMO: cellobiose dehydrogenase, phenols procured from plant biomass or produced by fungi, and GMC oxidoreductases that regenerate LPMO-reducing diphenols. These data demonstrate that all three electron transfer systems are functional and that their relative importance during cellulose degradation depends on fungal lifestyle. The availability of extracellular electron donors is obligatory to activate fungal oxidative attack on polysaccharides.

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