title: The substrate/product-binding modes of a novel GH120 β-xylosidase (XylC) from Thermoanaerobacterium saccharolyticum JW/SL-YS485.
abstract: Xylan-1,4-β-xylosidase (β-xylosidase) hydrolyses xylo-oligomers at their non-reducing ends into individual xylose units. Recently, XylC, a β-xylosidase from Thermoanaerobacterium saccharolyticum JW/SL-YS485, was found to be structurally different from corresponding glycosyl hydrolases in the CAZy database (http://www.cazy.org/), and was subsequently classified as the first member of a novel family of glycoside hydrolases (GH120). In the present paper, we report three crystal structures of XylC in complex with Tris, xylobiose and xylose at 1.48-2.05 Å (1 Å=0.1 nm) resolution. XylC assembles into a tetramer, and each monomer comprises two distinct domains. The core domain is a right-handed parallel β-helix (residues 1-75 and 201-638) and the flanking region (residues 76-200) folds into a β-sandwich domain. The enzyme contains an open carbohydrate-binding cleft, allowing accommodation of longer xylo-oligosaccharides. On the basis of the crystal structures and in agreement with previous kinetic data, we propose that XylC cleaves the glycosidic bond by the retaining mechanism using two acidic residues Asp382 (nucleophile) and Glu405 (general acid/base). In addition to the active site, nine other xylose-binding sites were consistently observed in each of the four monomers, providing a possible reason for the high tolerance of product inhibition.
authors.affiliation: Industrial Enzymes National Engineering Laboratory, Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, Tianjin 300308, China.
authors.forename: Chun-Hsiang
authors.initials: CH
authors.lastname: Huang
authors.forename: Yu
authors.initials: Y
authors.lastname: Sun
authors.forename: Tzu-Ping
authors.initials: TP
authors.lastname: Ko
authors.forename: Chun-Chi
authors.initials: CC
authors.lastname: Chen
authors.forename: Yingying
authors.initials: Y
authors.lastname: Zheng
authors.forename: Hsiu-Chien
authors.initials: HC
authors.lastname: Chan
authors.forename: Xuefei
authors.initials: X
authors.lastname: Pang
authors.forename: Juergen
authors.initials: J
authors.lastname: Wiegel
authors.forename: Weilan
authors.initials: W
authors.lastname: Shao
authors.forename: Rey-Ting
authors.initials: RT
authors.lastname: Guo
mesh_terms: D000595:Amino Acid Sequence; D003460:Crystallization; D004187:Disaccharides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D011485:Protein Binding / Q000235:genetics; D017434:Protein Structure, Tertiary / Q000235:genetics; D013379:Substrate Specificity / Q000235:genetics; D046950:Thermoanaerobacterium / Q000201:enzymology* / Q000235:genetics; D014995:Xylosidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D004187:Disaccharides; D014995:Xylosidases; C026579:exo-1,4-beta-D-xylosidase; C004173:xylobiose