title: Structural basis for the specificity of the reducing end xylose-releasing exo-oligoxylanase from Bacillus halodurans C-125.
abstract: Reducing end xylose-releasing exo-oligoxylanase from Bacillus halodurans C-125 (Rex) hydrolyzes xylooligosaccharides whose degree of polymerization is greater than or equal to 3, releasing the xylose unit at the reducing end. It is a unique exo-type glycoside hydrolase that recognizes the xylose unit at the reducing end in a very strict manner, even discriminating the beta-anomeric hydroxyl configuration from the alpha-anomer or 1-deoxyxylose. We have determined the crystal structures of Rex in unliganded and complex forms at 1.35-2.20-A resolution and revealed the structural aspects of its three subsites ranging from -2 to +1. The structure of Rex was compared with those of endo-type enzymes in glycoside hydrolase subfamily 8a (GH-8a). The catalytic machinery of Rex is basically conserved with other GH-8a enzymes. However, subsite +2 is blocked by a barrier formed by a kink in the loop before helix alpha10. His-319 in this loop forms a direct hydrogen bond with the beta-hydroxyl of xylose at subsite +1, contributing to the specific recognition of anomers at the reducing end.
authors.affiliation: Department of Biotechnology, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. asfushi@mail.ecc.u-tokyo.ac.jp
authors.forename: Shinya
authors.initials: S
authors.lastname: Fushinobu
authors.forename: Masafumi
authors.initials: M
authors.lastname: Hidaka
authors.forename: Yuji
authors.initials: Y
authors.lastname: Honda
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Wakagi
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Shoun
authors.forename: Motomitsu
authors.initials: M
authors.lastname: Kitaoka
mesh_terms: D001407:Bacillus / Q000201:enzymology; D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D004187:Disaccharides / Q000737:chemistry; D004583:Electrons; D006026:Glycoside Hydrolases / Q000737:chemistry; D006860:Hydrogen Bonding; D006868:Hydrolysis; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009844:Oligosaccharides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D013379:Substrate Specificity; D013997:Time Factors; D014994:Xylose / Q000737:chemistry*; D014995:Xylosidases / Q000737:chemistry*
chemical_list: D004187:Disaccharides; D008024:Ligands; D009844:Oligosaccharides; D014994:Xylose; D006026:Glycoside Hydrolases; D014995:Xylosidases; C004173:xylobiose