3-DIMENSIONAL STRUCTURE OF ENDO-1,4-BETA-XYLANASE-I FROM ASPERGILLUS-NIGER - MOLECULAR-BASIS FOR ITS LOW PH OPTIMUM

The crystal structure of endo-1,4-beta-xylanase I from Aspergillus nig er has been solved by molecular replacement and was refined to 2.4 Ang strom resolution. The final R-factor for all data from 6 to 2.4 Angstr om is 17.9%. The A. niger xylanase has a characteristic fold which is unique for family G xylanases (root-mean-square deviation = 1.1 Angstr om to Trichoderma reesei xylanase I, which has 53% sequence identity). It consists of a single domain composed predominantly of beta-strands . Two beta-sheets are twisted around a deep, long cleft, which is line d with many aromatic amino acid residues and is large enough to accomm odate at least four xylose residues. The two conserved glutamate resid ues, Glu79 and Glu170, which are likely to be involved in catalysis, r each into this cleft from opposite sides. A. niger xylanase I is of pa rticular commercial interest because of its low pH optimum. A model is proposed which explains this low pH optimum compared to other members of xylanase family G. (C) 1996 Academic Press Limited