Watson Brian J, Zhang Haitao, Longmire Atkinson G, Moon Young Hwan, Hutcheson Steven W
Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA.
J Bacteriol. 2009 Sep;191(18):5697-705. doi: 10.1128/JB.00481-09. Epub 2009 Jul 17.
Bacteria and fungi are thought to degrade cellulose through the activity of either a complexed or a noncomplexed cellulolytic system composed of endoglucanases and cellobiohydrolases. The marine bacterium Saccharophagus degradans 2-40 produces a multicomponent cellulolytic system that is unusual in its abundance of GH5-containing endoglucanases. Secreted enzymes of this bacterium release high levels of cellobiose from cellulosic materials. Through cloning and purification, the predicted biochemical activities of the one annotated cellobiohydrolase Cel6A and the GH5-containing endoglucanases were evaluated. Cel6A was shown to be a classic endoglucanase, but Cel5H showed significantly higher activity on several types of cellulose, was the highest expressed, and processively released cellobiose from cellulosic substrates. Cel5G, Cel5H, and Cel5J were found to be members of a separate phylogenetic clade and were all shown to be processive. The processive endoglucanases are functionally equivalent to the endoglucanases and cellobiohydrolases required for other cellulolytic systems, thus providing a cellobiohydrolase-independent mechanism for this bacterium to convert cellulose to glucose.
细菌和真菌被认为通过由内切葡聚糖酶和纤维二糖水解酶组成的复合或非复合纤维素分解系统的活性来降解纤维素。海洋细菌食糖嗜纤维菌2-40产生一种多组分纤维素分解系统，其含GH5的内切葡聚糖酶含量异常丰富。该细菌分泌的酶从纤维素材料中释放出高水平的纤维二糖。通过克隆和纯化，对一种已注释的纤维二糖水解酶Cel6A和含GH5的内切葡聚糖酶的预测生化活性进行了评估。结果表明，Cel6A是一种典型的内切葡聚糖酶，但Cel5H对几种类型的纤维素表现出显著更高的活性，表达量最高，并能从纤维素底物中持续释放纤维二糖。发现Cel5G、Cel5H和Cel5J是一个独立系统发育分支的成员，并且都被证明具有持续性。持续性内切葡聚糖酶在功能上等同于其他纤维素分解系统所需的内切葡聚糖酶和纤维二糖水解酶，从而为该细菌将纤维素转化为葡萄糖提供了一种不依赖纤维二糖水解酶的机制。
