Visible light-exposed lignin facilitates cellulose solubilization by lytic polysaccharide monooxygenases

Degradation of plant biomass, comprised of cellulose and polyaromatic lignin, is promoted by light. Here, the authors show that light promotes lignin-catalyzed generation of hydrogen peroxide, which is used by redox enzymes to degrade cellulose. Lytic polysaccharide monooxygenases (LPMOs) catalyze oxidative cleavage of crystalline polysaccharides such as cellulose and are crucial for the conversion of plant biomass in Nature and in industrial applications. Sunlight promotes microbial conversion of plant litter; this effect has been attributed to photochemical degradation of lignin, a major redox-active component of secondary plant cell walls that limits enzyme access to the cell wall carbohydrates. Here, we show that exposing lignin to visible light facilitates cellulose solubilization by promoting formation of H2O2 that fuels LPMO catalysis. Light-driven H2O2 formation is accompanied by oxidation of ring-conjugated olefins in the lignin, while LPMO-catalyzed oxidation of phenolic hydroxyls leads to the required priming reduction of the enzyme. The discovery that light-driven abiotic reactions in Nature can fuel H2O2-dependent redox enzymes involved in deconstructing lignocellulose may offer opportunities for bioprocessing and provides an enzymatic explanation for the known effect of visible light on biomass conversion.

Automated summary

The degradation of plant biomass is promoted by light through the generation of hydrogen peroxide, which is used by enzymes to degrade cellulose. This light-driven abiotic reaction provides opportunities for bioprocessing and offers an enzymatic explanation for the effect of visible light on biomass conversion.