VTT Technical Research Centre of Finland, Espoo, Finland.
FEBS J. 2013 Jan;280(1):285-301. doi: 10.1111/febs.12069. Epub 2012 Dec 20.
A minor xylanase, named XYN IV, was purified from the cellulolytic system of the fungus Trichoderma reesei Rut C30. The enzyme was discovered on the basis of its ability to attack aldotetraohexenuronic acid (HexA-2Xyl-4Xyl-4Xyl, HexA(3)Xyl(3)), releasing the reducing-end xylose residue. XYN IV exhibited catalytic properties incompatible with previously described endo-β-1,4-xylanases of this fungus, XYN I, XYN II and XYN III, and the xylan-hydrolyzing endo-β-1,4-glucanase EG I. XYN IV was able to degrade several different β-1,4-xylans, but was inactive on β-1,4-mannans and β-1,4-glucans. It showed both exo-and endo-xylanase activity. Rhodymenan, a linear soluble β-1,3-β-1,4-xylan, was as the best substrate. Linear xylooligosaccharides were attacked exclusively at the first glycosidic linkage from the reducing end. The gene xyn4, encoding XYN IV, was also isolated. It showed clear homology with xylanases classified in glycoside hydrolase family 30, which also includes glucanases and mannanases. The xyn4 gene was expressed slightly when grown on xylose and xylitol, clearly on arabinose, arabitol, sophorose, xylobiose, xylan and cellulose, but not on glucose or sorbitol, resembling induction of other xylanolytic enzymes from T. reesei. A recombinant enzyme prepared in a Pichia pastoris expression system exhibited identical catalytic properties to the enzyme isolated from the T. reesei culture medium. The physiological role of this unique enzyme remains unknown, but it may involve liberation of xylose from the reducing end of branched oligosaccharides that are resistant toward β-xylosidase and other types of endoxylanases. In terms of its catalytic properties, XYN IV differs from bacterial GH family 30 glucuronoxylanases that recognize 4-O-methyl-D-glucuronic acid (MeGlcA) substituents as substrate specificity determinants.
一种名为 XYN IV 的小木聚糖酶从里氏木霉纤维素酶系统中被分离纯化。该酶是根据其攻击醛酸四糖（HexA-2Xyl-4Xyl-4Xyl，HexA(3)Xyl(3))的能力被发现的，能够释放还原末端的木糖残基。XYN IV 的催化特性与该真菌先前描述的内切-β-1,4-木聚糖酶 XYN I、XYN II 和 XYN III 以及木聚糖水解内切-β-1,4-葡聚糖酶 EG I 不同。XYN IV 能够降解几种不同的β-1,4-木聚糖，但对β-1,4-甘露聚糖和β-1,4-葡聚糖没有活性。它具有外切和内切木聚糖酶活性。罗得曼聚糖，一种线性可溶性β-1,3-β-1,4-木聚糖，是最佳底物。线性木寡糖仅从还原末端的第一个糖苷键被攻击。编码 XYN IV 的 xyn4 基因也被分离出来。它与糖苷水解酶家族 30 中分类的木聚糖酶具有明显的同源性，该家族还包括葡聚糖酶和甘露聚糖酶。当在木糖和木糖醇上生长时，xyn4 基因表达微弱，但在阿拉伯糖、阿拉伯醇、槐糖、纤维二糖、木聚糖和纤维素上表达明显，但在葡萄糖或山梨糖醇上不表达，类似于其他木聚糖酶从里氏木霉的诱导。在毕赤酵母表达系统中制备的重组酶表现出与从里氏木霉培养基中分离的酶相同的催化特性。这种独特酶的生理作用尚不清楚，但它可能涉及从抵抗β-木聚糖酶和其他类型内切木聚糖酶的支链寡糖的还原末端释放木糖。就其催化特性而言，XYN IV 与识别 4-O-甲基-D-葡萄糖醛酸（MeGlcA）取代基作为底物特异性决定因素的细菌 GH 家族 30 木葡聚糖酶不同。
