β‐Glucosidase, β‐galactosidase, family A cellulases, family F xylanases and two barley glycanases form a superfamily of enzymes wit 8‐fold β/α architecture and with two conserved glutamates near the carboxy‐terminal ends of β‐strands four and seven

Abstract

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.