High resolution structure and sequence of T-aurantiacus Xylanase I: Implications for the evolution of thermostability in family 10 xylanases and enzymes with beta alpha-barrel architecture

Xylanase I is a thermostable xylanase from the fungus Thermoascus aurantiac us, which belongs to family 10 in the current classification of glycosyl hy drolases, We have determined the three-dimensional X-ray structure of this enzyme to near atomic resolution (1.14 Angstrom) by molecular replacement, and thereby corrected the chemically determined sequence previously publish ed, Among the five members of family 10 enzymes for which the structure has been determined, Xylanase I from I: aurantiacus and Xylanase Z from C. the rmocellum are from thermophilic organisms. A comparison with the three othe r available structures of the family 10 xylanases from mesophilic organisms suggests that thermostability is effected mainly by improvement of the hyd rophobic packing, favorable interactions of charged side chains with the he lix dipoles and introduction of prolines at the N-terminus of helices, In c ontrast to other classes of proteins, there is very little evidence for a c ontribution of salt bridges to thermostability in the family 10 xylanases f rom thermophiles, Further analysis of the structures of other proteins from thermophiles with eight-fold beta alpha-barrel architecture suggests that favorable interactions of charged side chains with the helix dipoles may be a common way in which thermophilic proteins with this fold are stabilized. As this is the most common type of protein architecture, this finding may provide a useful guide for site-directed mutagenesis aimed to improve the t hermostability of beta alpha-barrel proteins, (C) 1999Wiley-Liss Inc.