Kulkarni N, Shendye A, Rao M
Division of Biochemical Sciences, National Chemical Laboratory, Pune, India.
FEMS Microbiol Rev. 1999 Jul;23(4):411-56. doi: 10.1111/j.1574-6976.1999.tb00407.x.
Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.
半纤维素分解微生物通过循环利用植物多糖的主要成分之一半纤维素，在自然界中发挥着重要作用。木聚糖酶（EC 3.2.1.8）催化木聚糖（半纤维素的主要成分）的水解。这些酶的使用可以极大地提高加工木质纤维素材料以生产液体燃料和化学品的整体经济性。最近，无纤维素酶的木聚糖酶在造纸和纸浆工业的环境友好技术开发中受到了极大关注。在产生木聚糖酶的微生物中，木聚糖的低分子量片段及其位置异构体在调节其生物合成中起关键作用。木聚糖酶和纤维素酶的产生似乎是分别调节的，尽管导致两个基因都被消除的突变的多效性表明这两种酶的合成存在一些联系。木聚糖酶存在于大量生物体中，编码它们的基因已在同源和异源宿主中克隆，目的是过量生产该酶并改变其性质以适应商业应用。木聚糖酶的序列分析揭示了不同的催化和纤维素结合结构域，以及一个单独的非催化结构域，据报道该结构域在一些木聚糖酶中赋予增强的热稳定性。对三维结构和突变体性质的分析揭示了特定酪氨酸和色氨酸残基参与底物结合位点，以及谷氨酸和天冬氨酸残基参与催化机制。许多证据表明，木聚糖酶通过双取代机制起作用，其中异头构型得以保留，尽管一些酶催化构型翻转的单取代反应。基于从氨基酸序列相似性获得的系统树，评估木聚糖酶之间的进化关系。此外，还从生物技术应用的角度评估了嗜极端微生物来源的木聚糖酶的性质。
