Collins Tony, Meuwis Marie-Alice, Stals Ingeborg, Claeyssens Marc, Feller Georges, Gerday Charles
Laboratory of Biochemistry, Institute of Chemistry B6, University of Liège, B-4000 Liège, Belgium.
J Biol Chem. 2002 Sep 20;277(38):35133-9. doi: 10.1074/jbc.M204517200. Epub 2002 Jun 27.
Xylanases are generally classified into glycosyl hydrolase families 10 and 11 and are found to frequently have an inverse relationship between their pI and molecular mass values. However, we have isolated a psychrophilic xylanase that belongs to family 8 and which has both a high pI and high molecular mass. This novel xylanase, isolated from the Antarctic bacterium Pseudoalteromonas haloplanktis, is not homologous to family 10 or 11 enzymes but has 20-30% identity with family 8 members. NMR analysis shows that this enzyme hydrolyzes with inversion of anomeric configuration, in contrast to other known xylanases which are retaining. No cellulase, chitosanase or lichenase activity was detected. It appears to be functionally similar to family 11 xylanases. It hydrolyzes xylan to principally xylotriose and xylotetraose and is most active on long chain xylo-oligosaccharides. Kinetic studies indicate that it has a large substrate binding cleft, containing at least six xylose-binding subsites. Typical psychrophilic characteristics of a high catalytic activity at low temperatures and low thermal stability are observed. An evolutionary tree of family 8 enzymes revealed the presence of six distinct clusters. Indeed classification in family 8 would suggest an (alpha/alpha)(6) fold, distinct from that of other currently known xylanases.
木聚糖酶通常分为糖基水解酶家族10和11，并且发现它们的等电点和分子量值之间经常呈反比关系。然而，我们分离出了一种属于家族8的嗜冷木聚糖酶，它具有高的等电点和高分子量。这种从南极细菌嗜盐浮游假交替单胞菌中分离出的新型木聚糖酶，与家族10或11的酶不同源，但与家族8的成员有20%-30%的同源性。核磁共振分析表明，与其他已知的保留型木聚糖酶相反，这种酶以异头构型翻转的方式进行水解。未检测到纤维素酶、壳聚糖酶或地衣酶活性。它在功能上似乎与家族11的木聚糖酶相似。它将木聚糖主要水解为木三糖和木四糖，并且对长链木寡糖最具活性。动力学研究表明，它有一个大的底物结合裂隙，至少包含六个木糖结合亚位点。观察到其具有在低温下高催化活性和低热稳定性的典型嗜冷特性。家族8酶的进化树显示存在六个不同的簇。实际上，在家族8中的分类表明其具有（α/α）6折叠，这与目前已知的其他木聚糖酶不同。
