Schwarz W H
Research Group Microbial Biotechnology, Technische Universität München, Freising-Weihenstephan, Germany.
Appl Microbiol Biotechnol. 2001 Sep;56(5-6):634-49. doi: 10.1007/s002530100710.
Despite its simple chemical composition, cellulose exists in a number of crystalline and amorphous topologies. Its insolubility and heterogeneity makes native cellulose a recalcitrant substrate for enzymatic hydrolysis. Microorganisms meet this challenge with the aid of a multi-enzyme system. Aerobic bacteria produce numerous individual, extra-cellular enzymes with binding modules for different cellulose conformations. Specific enzymes act in synergy to elicit effective hydrolysis. In contrast, anaerobic bacteria possess a unique extracellular multi-enzyme complex, called cellulosome. Up to 11 different enzymes are aligned on the non-catalytic scaffolding protein and thus ensure a high local concentration, together with the correct ratio and order of the components. These multi-enzyme complexes attach both to the cell envelope and to the substrate, mediating the proximity of the cells to the cellulose. Binding to the scaffolding stimulates the activity of each individual component towards the crystalline substrate. The most complex and best investigated cellulosome is that of the thermophilic bacterium Clostridium thermocellum, but a scheme for the cellulosomes of the mesophilic clostridia and the ruminococci emerges. Many crucial details of cellulose hydrolysis are still to be uncovered. Yet, a mechanistic model for the action of enzyme complexes on the surface of insoluble substrates becomes apparent and the application of enzymatic hydrolysis of cellulosic biomass can now be addressed.
尽管纤维素的化学组成简单，但它以多种晶体和无定形拓扑结构存在。其不溶性和异质性使得天然纤维素成为酶促水解的顽固底物。微生物借助多酶系统应对这一挑战。需氧细菌产生许多具有针对不同纤维素构象的结合模块的胞外酶。特定的酶协同作用以引发有效的水解。相比之下，厌氧细菌拥有一种独特的胞外多酶复合物，称为纤维小体。多达11种不同的酶排列在非催化支架蛋白上，从而确保高局部浓度以及各组分的正确比例和顺序。这些多酶复合物既附着在细胞膜上，也附着在底物上，介导细胞与纤维素的接近。与支架的结合刺激每个单独组分对晶体底物的活性。最复杂且研究最深入的纤维小体是嗜热栖热梭菌的纤维小体，但中温梭菌和瘤胃球菌的纤维小体模式也已显现。纤维素水解的许多关键细节仍有待揭示。然而，酶复合物在不溶性底物表面作用的机制模型已变得清晰，现在可以探讨纤维素生物质的酶促水解应用了。
