Family 3 beta-d-glucan glucohydrolases are widely distributed in higher plants. The enzymes catalyse the hydrolytic removal of beta-d-glucosyl residues from non-reducing termini of a range of beta-d-glucans and beta-d-oligoglucosides. Their broad specificity can be rationalized from X-ray crystallographic data obtained from a barley beta-d-glucan glucohydrolase in complex with non-hydrolysable S-glycoside substrate analogues, and from molecular modelling of enzyme-substrate complexes. The glucosyl residue occupying binding subsite -1 is tightly locked into a fixed position through extensive hydrogen bonding with six amino acid residues near the bottom of an active site pocket. In contrast, the glucosyl residue at subsite +1 is located between two tryptophan residues at the entrance of the pocket, where it is less tightly constrained. The relative flexibility of binding at subsite +1, coupled with the projection of the remainder of bound substrate away from the enzyme's surface, means that the overall active site can accommodate a range of substrates with variable spatial dispositions of adjacent beta-d-glucosyl residues. The broad specificity for glycosidic linkage type would enable the enzyme to perform diverse functions during plant development.
