3-DIMENSIONAL STRUCTURE OF ENDO-1,4-BETA-XYLANASE-II FROM TRICHODERMA-REESEI - 2 CONFORMATIONAL STATES IN THE ACTIVE-SITE

The three-dimensional structure of endo-1,4-beta-xylanase II (XYNII) f rom Trichoderma reesei has been determined by X-ray diffraction techni ques and refined to a conventional R-factor of 18.3% at 1.8 Angstrom r esolution. The 190 amino acid length protein was found to exist as a s ingle domain where the main chain folds to form two mostly antiparalle l beta-sheets, which are packed against each other in parallel. The be ta-sheet structure is twisted, forming a large cleft on one side of th e molecule. The structure of XYNII resembles that of Bacillus 1,3 - 1, 4-beta-glucanase. The cleft is an obvious suggestion for an active sit e, which has putative binding sites for at least four xylose residues. The catalytic residues are apparently the two glutamic acid residues (Glu86 and Glu177) in the middle of the cleft. One structure was deter mined at pH 5.0, corresponding to the pH optimum of XYNII. The second structure was determined at pH 6.5, where enzyme activity is reduced c onsiderably. A clear structural change was observed, especially in the position of the side chain of Glu177. The observed conformational cha nge is probably important for the mechanism of catalysis in XYNII.