Relative Significance of the Negative Impacts of Hemicelluloses on Enzymatic Cellulose Hydrolysis Is Dependent on Lignin Content: Evidence from Substrate Structural Features and Protein Adsorption

The biomass recalcitrance of the lignocellulose cell wall constructed by its chemical components, especially hemicelluloses and lignin, has become a bottleneck for the efficient release of glucose. The presence of hemicelluloses has been considered as a major factor limiting the enzymatic digestibility of lignocellulose biomass. However, most of the reported works on the effect of hemicelluloses removal on cellulose hydrolysability were conducted via dilute acid pretreatment at high temperature (>160 degrees C), and inconsistent conclusions have been found. In the present work, we studied the effects of xylan content on enzymatic digestibility of wheat straw cellulose in the cases of high and low lignin contents. Particularly, xylan removal was achieved by sulfuric acid hydrolysis under mild conditions (120 degrees C) to minimize lignin melting and migration in the cell wall and lignin structure modification. As revealed by various structure characterizations, when no lignin was removed, xylan removal by dilute acid hydrolysis resulted in reduction of particle size, deformation of the cell shape, etching of the cell lumen surface, some fracture and slight delamination of cell wall, with associated great increase in porosity and specific surface area. These structural modifications greatly improved cellulose digestibility. However, the presence of residual lignin also showed significant negative impacts by physical blocking and nonproductive adsorption of cellulases. In the case of low lignin content (similar to 4%), cellulose fibers become liberated and significant etching, delamination, fracture and even disappearance of the walls were visualized with xylan removal, which remarkably increased the effective surface area for cellulose binding with cellulose. The finding of this work demonstrates that the limiting action of hemicelluloses seems to be not important to cellulose digestibility as that observed in high-temperature (>160 degrees C) dilute acid pretreatment. Delignification seems to be more efficient to improve cellulose accessibility for mild-condition (<120 degrees C) pretreatment. It indicates that the interaction effects between lignin and hemicelluloses as structural factors limiting cellulose digestibility should be considered for investigating the mechanisms of effects of structure features on cellulose accessibility.