The impact of lignin characteristics on cellulase adsorption and enzymatic hydrolysis

The lignin-derived inhibition with lignin content, distribution, and structure has been considered one of the major recalcitrant factors during enzymatic hydrolysis of lignocellulosic biomass.  Possible mechanisms such as physical and chemical surface blockage of lignin, non-productive adsorption of cellulases onto lignin, and enzyme deactivation due to soluble derivatives degraded from lignin have been suggested as lignin-induced inhibitions.  In particular, cellulase affinity with lignin is reported to be attributed to hydrophobic, electrostatic, ionic and hydrogen bonding interactions, but the detailed mechanisms have not been fully understood due to complicated characteristics of lignin.  It was reported that hydrophobic ring-stacking interaction between aromatic amino acids in cellulase binding module and glucose rings in cellulose is key for cellulase-cellulose binding affinity, and similar interactions between cellulase and lignin could occur.  Based on our previous data, the degree of condensation and the amount of phenolic hydroxyl groups in lignin have a significant correlation with cellulase adsorption and enzymatic digestibility. Therefore, the lignin-derived inhibition could be largely influenced by linkages between aromatics rings and functional groups in aromatic rings. In this study, insoluble and soluble phenolic compounds, which are basic constituents and degradation products of lignin, were used as materials.  Cellulase adsorption onto insoluble phenolics and enzymatic hydrolysis of bleached pulps with phenolics were conducted.  Cellulase adsorption onto insoluble phenolics increased with increasing phenolic hydroxyl groups, and decreased with acetylations of hydroxyl groups.  The mechanisms of lignin-induced inhibitions will be discussed depending on solubility, size of structural linkages, and functional groups.