Laccase and LPMO-driven biocatalysis produces surface carboxylic acids on lignocellulose and promotes nanofiber production

Environmentally sustainable nanocellulose production on a large scale is limited by current chemical and mechanical approaches but may benefit from enzymatic processes. Mimicking nature, we demonstrate the power of combining laccases and lytic polysaccharide monooxygenases (LPMOs) in cellulose oxidation. Here, fungal laccases work in tandem with LPMO, employing an electron shuttle generated via radical-containing lignin fragments/laccase-generated hydrogen peroxide, to reduce LPMO, resulting in hydrolyzed and oxidized cellulose at 60 mmol [COOH]/kg, confirmed by ¹³C CP/MAS NMR and promoted nanofiber formation upon shearing. The surface display of carboxylate groups on nanofibrils was demonstrated by photothermal spectroscopy and chemical mapping via atomic force microscopy coupled with infrared spectroscopy (AFM-IR). The carboxylated nanofibers were added at 20% by weight to paper sheets and gave significant improvements in tensile, compression, and extensional stiffness indexes over untreated lignocellulose nanofibers by 22, 23.4, and 23.1%, respectively. The enzymes used in tandem provide a new route to nanocellulose production without chemical pretreatment and a conceptual advance toward sustainable processing of lignocellulose.