THERMOPHILIC XYLANASE FROM THERMOMYCES-LANUGINOSUS - HIGH-RESOLUTION X-RAY STRUCTURE AND MODELING STUDIES

The crystal structure of the thermostable xylanase from Thermomyces la nuginosus was determined by single-crystal X-ray diffraction. The prot ein crystallizes in space group P2(1), a = 40.96(4) Angstrom, b = 52.5 7(5) Angstrom, c = 50.47 (5) Angstrom, beta = 100.43(5)degrees, Z = 2. Diffraction data were collected at room temperature for a resolution range of 25-1.55 Angstrom, and the structure was solved by molecular r eplacement with the coordinates of xylanase II from Trichoderma reesei as a search model and refined to a crystallographic R-factor of 0.155 for all observed reflections. The enzyme belongs to the family 11 of glycosyl hydrolases [Henrissat, B., and Bairoch, A. (1993) Biochem. J. 293, 781-788]. pK(a) calculations were performed to assess the proton ation state of residues relevant for catalysis and enzyme stability, a nd a heptaxylan was fitted into the active-site groove by homology mod eling, using the published crystal structure of a complex between the Bacillus circulans xylanase and a xylotetraose. Molecular dynamics ind icated the central three sugar rings to be tightly bound, whereas the peripheral ones can assume different orientations and conformations, s uggesting that the enzyme might also accept xylan chains which are bra nched at these positions. The reasons for the thermostability of the T . lanuginosus xylanase were analyzed by comparing its crystal structur e with known structures of mesophilic family 11 xylanases. It appears that the thermostability is due to the presence of an extra disulfide bridge, as well as to an increase in the density of charged residues t hroughout the protein.