Structure of XynB, a highly thermostable beta-1,4-xylanase from Dictyoglomus thermophilum Rt46B.1, at 1.8 angstrom resolution

Microorganisms employ a large array of enzymes to break down the cellulose and hemicelluloses of plant biomass. These enzymes, especially those with h igh thermal stability, have many uses in biotechnology. We have solved the crystal structure of a beta -1,4-xylanase, XynB, from the extremely thermop hilic bacterium Dictyoglomus thermophilum, isolate Rt46B.1. The protein cry stallized from 1.6 M ammonium sulfate, 0.2 M HEPES pH 7.2 and 10% glycerol, with unit-cell parameters a = b = 91.3, c = 44.9 Angstrom and space group P4(3). The structure was solved at high resolution (1.8 Angstrom) by X-ray crystallography, using the method of isomorphous replacement with a single mercury derivative, and rerned to a final R factor of 18.3% (R-free = 22.1% ). XynB has the single-domain fold typical of family 11 xylanases, comprisi ng a jelly roll of two highly twisted fi-sheets that create a deep substrat e-binding cleft. The two catalytic residues, Glu90 and Glu180, occupy this cleft. Compared with other family 11 xylanases, XynB has a greater proporti on of polar surface and has a slightly extended C-terminus that, combined w ith the extension of beta -strand A5, gives additional hydrogen bonding and hydrophobic packing. These factors may account for the enhanced thermal st ability of the enzyme.