Structural Basis for Broad Substrate Specificity in Higher Plant β-d-Glucan Glucohydrolases

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Abstract
Family 3 β-d-glucan glucohydrolases are distributed widely in higher plants. The enzymes catalyze the hydrolytic removal of β-d-glucosyl residues from nonreducing termini of a range of β-d-glucans and β-d-oligoglucosides. Their broad specificity can be explained by x-ray crystallographic data obtained from a barley β-d-glucan glucohydrolase in complex with nonhydrolyzable S-glycoside substrate analogs and by molecular modeling of enzyme/substrate complexes. The glucosyl residue that occupies binding subsite −1 is locked tightly 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 Trp residues at the entrance of the pocket, where it is constrained less tightly. 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 β-d-glucosyl residues. The broad specificity for glycosidic linkage type enables the enzyme to perform diverse functions during plant development.

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