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Three-dimensional structures of two major endo-1,4-xylanases, XYNI and XYNII from Trichoderma reesei, have been determined by X-ray crystallography. The amino acid sequences of both enzymes are highly homologous (identity approximately 50%), and both XYNI and XYNII exist as a single domain that contains two mostly antiparallel beta-sheets which are packed against each other. The beta-sheet structure is twisted, forming a cleft where the active site is situated. Two glutamic acids in the cleft, Glu75 and Glu164 in XYNI as well as Glu86 and Glu177 in XYNII, are most likely involved in catalysis. Inspection of the structures reveals that the width of the active site cleft and the number of subsites are different in XYNI and XYNII. The active site is narrower in XYNI and probably contains only three subsites, whereas the number of subsites in XYNII is most likely five. Variations in the surroundings of catalytic residue Glu164XYNI/Glu177XYNII are thought to explain the pH optimum differences observed in XYNI and XYNII.
Törrönen et al. (Sun,) studied this question.