Structure of the xylanase from Penicillium simplicissimum

Despite its relatively low pH and temperature optimum, the xylanase from Penicillium simplicissimum performs exceedingly well under conditions of paper bleaching. We have purified and characterized this enzyme, which belongs to family 10 of glycosyl hydrolases. Its gene was cloned, and the sequence of the protein was deduced from the nucleotide sequence. The xylanase was crystallized from ammonium sulfate at pH 8.4, and X-ray data were collected at cryo-temperature to a crystallographic resolution of 1.75 A. The crystal structure was solved by molecular replacement using the catalytic domain of the Clostridium thermocellum xylanase as a search model, and refined to a residual of R = 20% (R(free) = 23%) for data between 10 and 1.75 A. The xylanase folds in an (alpha/beta)8 barrel (TIM-barrel), with additional helices and loops arranged at the "top" forming the active site cleft. In its overall shape, the P. simplicissimum xylanase structure is similar to other family 10 xylanases, but its active site cleft is much shallower and wider. This probably accounts for the differences in catalysis and in the mode of action of this enzyme. Three glycerol molecules were observed to bind within the active site groove, one of which interacts directly with the catalytic glutamate residues. It appears that they occupy putative xylose binding subsites.