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)‐β‐glucanase and (1–3,1–4)‐β‐glucanase and the catalytic domain of E. coli β‐galactosidase reveals that they belong to a superfamily of 8‐fold β/α‐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 β‐strand seven. In addition all three enzymes have the sequence asparagine‐glutamate close to the carboxy‐terminus of β‐strand four. This glutamate has been identified as the acid/base in the family F xylanases and is essential for catalysis in β‐galactosidase. We suggest that the equivalent residue in the barley glucanases is the acid/base. Analysis of the sequences of family 1 β‐glucosidases and family 5 cellulases shows that these enzymes also belong to this superfamily which we call the superfamily.