Fundamental Understanding and Removing Strategies of the Inhibitory Effects of Lignin on Enzymatic Saccharification of Lignocellulose

Lignin is a major inhibitor to enzymatic saccharification of lignocellulose. Lignin retards enzymatic hydrolysis lignocellulose presumably by acting as a physical barrier (restricting the access of cellulases to cellulose) and/or by irreversibly adsorbing cellulases (resulting in non-productive binding of the enzymes). However, what roles lignin structural features play in inhibiting cellulases and how lignin inhibition can be effectively removed/reduced have not been sufficiently understood. Our study was to investigate the interactions between lignin and cellulases, to fundamentally understand the relationship between lignin structure and the lignin inhibition, and to develop strategies for removing/reducing the lignin inhibition. Structural properties of lignin such as molecular weight, functional groups, and hydrophobicity were investigated and correlated to the inhibitory effect of lignin to cellulases. The results suggested that non-productive bonding of cellulases on lignin through hydrophobic adsorption was the primary mechanism of the lignin inhibition. Phenolic hydroxyl group of lignin was highly correlated to the lignin inhibition. Strategies to reduce the lignin inhibition were proposed and evaluated. It turned out that covering active sites of lignin with proteins or surfactants, chemically blocking phenolic hydroxyl groups by etherification, and reducing lignin hydrophobicity through chemical modification such as sulfonation and carboxylation could efficiently reduce/remove the inhibitory effects of lignin on enzymatic hydrolysis of lignocellulose.