Prade R A
Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater 74078-0289, USA.
Biotechnol Genet Eng Rev. 1996;13:101-31. doi: 10.1080/02648725.1996.10647925.
Xylan is the main carbohydrate found in the hemicellulosic fraction of plant tissues and accounts for one third of all renewable organic carbon available on earth. Xylanase, the major component of an enzymatic consortium, acts in nature by depolymerizing xylan molecules into monomeric pentosan units that are used by bacterial and fungal populations as a primary carbon source. Xylanase producers have been isolated from all ecological niches where plant material is deposited, and microorganisms often contain multiple loci encoding overlapping xylanolytic functions. The numerical excess of genes and the extensive sharing of structural features within beta-glycanase families suggests that extensive gene duplication and conversion events have occurred during xylanase evolution. Hydrolysis of beta-glycosidic linkages is sponsored by a general acid catalytic reaction common to all glycanases, whereas substrate recognition is specified by subsites that interact with adjacent glycosyl units. Under natural conditions xylanases are inducible by the products of their own action and subject to carbon catabolite repression. Bleaching paper pulps with xylanases is the first successful commercial application for these enzymes. The recovery of cellulosic textile fibers is the next logical application and bioconversion of biomass into fuels and chemicals, remains the ultimate target. Recent developments have shown that metabolic pathways can be transferred from one organism to another and proteins can be modified to gain conformational stability, suggesting that naturally occurring systems can be custom engineered to the situation in the fermentation tank. Thus, biotechnologies developed to transform biomass into marketable products that gradually substitute materials derived from non-renewable resources are becoming commercially worthwhile.
木聚糖是植物组织半纤维素部分中主要的碳水化合物，占地球上所有可再生有机碳的三分之一。木聚糖酶是酶复合体的主要成分，在自然界中，它通过将木聚糖分子解聚为单体戊聚糖单元来发挥作用，细菌和真菌群体将这些单体戊聚糖单元用作主要碳源。木聚糖酶产生菌已从所有有植物材料沉积的生态位中分离出来，而且微生物通常含有多个编码重叠木聚糖分解功能的基因座。β-聚糖酶家族内基因数量过多以及结构特征广泛共享，这表明在木聚糖酶进化过程中发生了广泛的基因复制和转换事件。β-糖苷键的水解由所有聚糖酶共有的一般酸催化反应促成，而底物识别则由与相邻糖基单元相互作用的亚位点决定。在自然条件下，木聚糖酶可被其自身作用的产物诱导，并受到碳分解代谢物阻遏。用木聚糖酶漂白纸浆是这些酶的首个成功商业应用。回收纤维素纺织纤维是下一个合理应用，而将生物质转化为燃料和化学品仍然是最终目标。最近的进展表明，代谢途径可以从一种生物体转移到另一种生物体，并且蛋白质可以被修饰以获得构象稳定性，这表明可以根据发酵罐中的情况对天然存在的系统进行定制工程设计。因此，为将生物质转化为可销售产品而开发的生物技术正逐渐替代来自不可再生资源的材料，变得具有商业价值。
