5-09: The enhancement of enzymatic hydrolysis of lignocelluloses by accessory enzymes such as xylanase: Is it an additive or synergistic effect?

In order to achieve an economically feasible bioethanol production process, it is important to efficiently and effectively convert both the cellulose and hemicellulose to fermentable sugars. Therefore, when steam pretreatment is used, mild severity conditions are required, to avoid or at least minimize sugars degradation. Under these milder pretreatment conditions, some of the hemicellulose will typically remain in association with the cellulosic-rich material. We and other researchers have found that the residual hemicellulose, consisting mostly of xylan in hardwood and agricultural residues, can significantly impede enzymatic hydrolysis if the cellulase preparation used is deficient in hemicellulases. Most previous studies on this area have made use of uneconomically high dosages of xylanase-enriched preparations which is difficult to justify because of increased enzyme costs. It is also still not clear whether the beneficial effect of the xylanase addition is a result of a cooperative interaction (synergism) with cellulases or a merely  an additive effect as the increase hydrolyzability with xylanase supplementation is typically associated with a corresponding substantial increase in protein loading. In the work that will be presented, we have assessed the type and extent of the interaction between xylanase and cellulase enzymes in an attempted to improve hydrolysis efficiency of pretreated lignocellulose at low protein loadings. Xylanases were shown to have a strong synergistic interaction with cellulases depending on the ratio of cellulase:xylanase used. Under near optimal conditions this dramatically decreased the cellulase loading required for efficient conversion of the cellulosic component of steam pretreated corn stover.