The biological degradation of cellulose.
The biological degradation of cellulose.
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DOI：
10.1111/j.1574-6976.1994.tb00033.x
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发表时间：
1994
影响因子：
11.3
通讯作者：
P. Béguin;J. Aubert
中科院分区：
文献类型：
--
作者：
P. Béguin;J. Aubert
纤维素分解微生物通过回收植物产生的最丰富的碳水化合物纤维素在生物圈中发挥重要作用。纤维素是一种简单的聚合物，但它形成不溶性的结晶微纤维，对酶水解具有高度抗性。所有已知能有效降解纤维素的生物都能产生一系列具有不同特异性的酶，它们协同作用。在分子水平上对纤维素分解酶的研究揭示了有助于其活性的一些特征。尽管有相当大的多样性，序列比较表明，纤维素酶的催化核心属于有限数量的家庭。在每个家族内，现有数据表明，各种酶共享共同的折叠模式、相同的催化残基和相同的反应机制，即具有构型反转的单取代或导致在异头碳处保留β-构型的双取代。不同家族的纤维素酶和木聚糖酶具有越来越多的三维结构，这将为相关酶的分子模拟提供范例。除了催化结构域之外，许多纤维素分解酶含有不参与催化但参与底物结合、多酶复合物形成或可能附着于细胞表面的结构域。据推测，这些结构域通过防止酶从基质表面被洗掉，通过将水解集中在限制区域（其中基质被多个切割事件协同地去稳定化），以及通过促进纤维素分解生物体回收可溶性降解产物，来帮助结晶纤维素的降解。在大多数纤维素分解生物中，纤维素酶合成在易代谢的可溶性碳源存在下被抑制，而在纤维素存在下被诱导。纤维素酶的诱导似乎受到由纤维素生成的可溶性产物的影响，所述可溶性产物由组成型合成的低水平的纤维素分解酶产生。这些产物可能通过转糖基化反应转化为真正的诱导物。纤维素酶或半纤维素酶的几种应用正在被开发用于纺织、食品和纸浆加工。这些应用基于通过部分水解对纤维素和半纤维素的改性。将纤维素完全水解成葡萄糖，葡萄糖可以发酵成乙醇、异丙醇或丁醇，但在经济上还不可行。然而，减少温室气体排放的需要为开发从纤维素（一种主要的可再生碳源）产生燃料的工艺提供了额外的激励。
Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.