Laccase-Catalyzed Oxidation of Lignin Induces Productionof H₂O₂

Laccases are abundantin microorganisms specialized in lignocellulose degradation, but the exact role of the enzymes in this process remains poorly understood. Laccases catalyze oxidation of lignin, initially producing a phenoxy radical. The heterogeneous lignin structure canstabilize these reactive radicals, but eventually, they dissipate by uncharacterized mechanisms. We show that the laccase-catalyzed oxidation of lignin leads to formation of H₂O₂, likely via a reaction between lignin radicals and O2. Up to 25 μM of H₂O₂ was generated byeach of four fungal and bacterial laccases catalyzing oxidation of both organosolv lignin and birch wood. The generated H₂O₂ activated lytic polysaccharide monooxygenases (LPMOs) both in separate reactions, where the LPMOs were fed with laccase-generated H₂O₂, and in one-pot reactions, where laccases and LPMOs acted simultaneously. These observations are new, and our results reveal a potentially critical connection between conversion of lignin and polysaccharides, suggesting that laccases generate H₂O₂ for other lignocellulose-depolymerizing enzymes such as lignin peroxidases and LPMOs. This new role for laccases maybe exploited in application settings.