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  • 标题:糖質加水分解酵素ファミリー74に属するキシログルカン分解酵素の機能と構造
  • 本地全文:下载
  • 作者:矢追 克郎 ; 近藤 英昌 ; 野呂 奈津子
  • 期刊名称:Journal of Applied Glycoscience
  • 印刷版ISSN:1344-7882
  • 电子版ISSN:1880-7291
  • 出版年度:2005
  • 卷号:52
  • 期号:2
  • 页码:169-176
  • DOI:10.5458/jag.52.169
  • 出版社:The Japanese Society of Applied Glycoscience
  • 摘要:

    Recently, we isolated and cloned two xyloglucan-specific endoglucanases (XEGs) from Geotrichum sp. M 128 and Gram-positive bacterium sp. KM 21, and a xyloglucan-specific exoglucanase, oligoxyloglucan reducing-end-specific cellobiohydrolase (OXG-RCBH) from Geotrichum sp. M 128, all of which belong to glycoside hydrolase family 74. Geotrichum and KM 21 XEGs have endoglucanase activity toward xyloglucan but not cellulose, while Geotrichum OXG-RCBH is a unique exoglucanase that releases two Glc residue segments from the reducing end of the xyloglucan main chain. Further analysis of the substrate specificities of Geotrichum XEG and OXG-RCBH using various oligosaccharides revealed that they recognize specific xylose branching structures. Although both Geotrichum XEG and OXG-RCBH have at least four subsites (-2 to +2), specific recognition of xylose residues occurs at the +1 and +2 sites in the former and at the -1 site in the latter. Moreover, xylose branching at the reducing end (site +2) eliminates OXG-RCBH activity. This enzymatic activity is very different from those of known glycosidases. Recently, OXG-RCBH was assigned a new EC number (EC 3.2.1.150). To elucidate the molecular mechanism of OXG-RCBH, its three-dimensional structure was analyzed. The X-ray crystal structure of OXG-RCBH revealed a unique feature of this enzyme: OXG-RCBH consists of two similar domains, both of which are folded into seven-bladed β-propeller structures. The cleft between the two domains can accommodate the oligosaccharide substrate and thus constitutes a putative catalytic core. Mutation of either Asp 35 or Asp 465, located in the cleft, to Asn resulted in a protein with no detectable catalytic activity, indicating the critical role of these amino acids in catalysis.

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