Forestry Products Biotechnology/Bioenergy Group, Wood Science Department, University of British Columbia, 2424 Main Mall, Vancouver BC, V6T 1Z4, Canada.
Biotechnol Biofuels. 2011 Oct 5;4:36. doi: 10.1186/1754-6834-4-36.
We and other workers have shown that accessory enzymes, such as β-glucosidase, xylanase, and cellulase cofactors, such as GH61, can considerably enhance the hydrolysis effectiveness of cellulase cocktails when added to pretreated lignocellulosic substrates. It is generally acknowledged that, among the several factors that hamper our current ability to attain efficient lignocellulosic biomass conversion yields at low enzyme loadings, a major problem lies in our incomplete understanding of the cooperative action of the different enzymes acting on pretreated lignocellulosic substrates.
The reported work assessed the interaction between cellulase and xylanase enzymes and their potential to improve the hydrolysis efficiency of various pretreated lignocellulosic substrates when added at low protein loadings. When xylanases were added to the minimum amount of cellulase enzymes required to achieve 70% cellulose hydrolysis of steam pretreated corn stover (SPCS), or used to partially replace the equivalent cellulase dose, both approaches resulted in enhanced enzymatic hydrolysis. However, the xylanase supplementation approach increased the total protein loading required to achieve significant improvements in hydrolysis (an additive effect), whereas the partial replacement of cellulases with xylanase resulted in similar improvements in hydrolysis without increasing enzyme loading (a synergistic effect). The enhancement resulting from xylanase-aided synergism was higher when enzymes were added simultaneously at the beginning of hydrolysis. This co-hydrolysis of the xylan also influenced the gross fiber characteristics (for example, fiber swelling) resulting in increased accessibility of the cellulose to the cellulase enzymes. These apparent increases in accessibility enhanced the steam pretreated corn stover digestibility, resulting in three times faster cellulose and xylan hydrolysis, a seven-fold decrease in cellulase loading and a significant increase in the hydrolysis performance of the optimized enzyme mixture. When a similar xylanase-aided enhancement strategy was assessed on other pretreated lignocellulosic substrates, equivalent increases in hydrolysis efficiency were also observed.
It was apparent that the 'blocking effect' of xylan was one of the major mechanisms that limited the accessibility of the cellulase enzymes to the cellulose. However, the synergistic interaction of the xylanase and cellulase enzymes was also shown to significantly improve cellulose accessibility through increasing fiber swelling and fiber porosity and also plays a major role in enhancing enzyme accessibility.
我们和其他研究人员已经表明，辅助酶（如β-葡萄糖苷酶、木聚糖酶和纤维素酶辅因子如 GH61）在添加到预处理木质纤维素底物后，可以显著提高纤维素酶混合物的水解效果。人们普遍认为，在阻碍我们目前以低酶负荷实现高效木质纤维素生物质转化产量的几个因素中，一个主要问题是我们对作用于预处理木质纤维素底物的不同酶的协同作用不完全了解。
本报告评估了纤维素酶和木聚糖酶之间的相互作用及其在低蛋白负荷下添加时提高各种预处理木质纤维素底物水解效率的潜力。当木聚糖酶添加到达到蒸汽预处理玉米秸秆（SPCS）纤维素水解 70%所需的最低纤维素酶量时，或者用于部分替代等效纤维素酶剂量时，两种方法都导致了酶解效率的提高。然而，木聚糖酶的添加方法增加了达到显著水解改善所需的总蛋白负荷（加性效应），而用木聚糖酶部分替代纤维素酶则导致水解没有增加酶负荷（协同效应）的相似改善。当酶在水解开始时同时添加时，木聚糖酶辅助协同作用产生的增强效果更高。这种木聚糖的共水解还影响了总纤维特性（例如纤维膨胀），从而增加了纤维素酶对纤维素的可及性。这些可及性的明显增加增强了蒸汽预处理玉米秸秆的消化性，导致纤维素和木聚糖水解速度提高了三倍，纤维素酶负荷降低了七倍，优化酶混合物的水解性能显著提高。当在其他预处理木质纤维素底物上评估类似的木聚糖酶辅助增强策略时，也观察到水解效率的等效提高。
显然，木聚糖的“阻断效应”是限制纤维素酶对纤维素的可及性的主要机制之一。然而，木聚糖酶和纤维素酶之间的协同相互作用也通过增加纤维膨胀和纤维孔隙率来显著提高纤维素的可及性，并且在增强酶的可及性方面也起着主要作用。
