BETA-GLUCOSIDASE, BETA-GALACTOSIDASE, FAMILY-A CELLULASES, FAMILY-F XYLANASES AND 2 BARLEY GLYCANASES FORM A SUPERFAMILY OF ENZYMES WITH 8-FOLD BETA/ALPHA-ARCHITECTURE AND WITH 2 CONSERVED GLUTAMATES NEAR THE CARBOXY-TERMINAL ENDS OF BETA-STRAND-4 AND BETA-STRAND-7

Comparison of the recently determined crystal structures Pseudomonas fluorescens subsp. cellulosa family F xylanase, (1-3)-beta-glucanase and (1-3,1-4)-beta-glucanase and the catalytic domain of E. coli beta-galactosidase reveals that they belong to a superfamily of 8-fold beta/alpha-barrels with similar amino acid residues at their active sites, In the three families that these enzymes represent, the nucleophile is a glutamate, which is located close to the carboxy-terminus of beta-strand seven, In addition all three enzymes have the sequence asparagine-glutamate close to the carboxy-terminus of beta-strand four. This glutamate has been identified as the acid/base in the family F xylanases and is essential for catalysis in beta-galactosidase. We suggest that the equivalent residue in the barley glucanases is the acid/base. Analysis of the sequences of family 1 beta-glucosidases and family 5 cellulases shows that these enzymes also belong to this superfamily which we call the 4/7 superfamily.