Exo1 | exoglucanase isoenzyme 1

Group 3 beta-D-glucan glucose hydrolases are widely distributed in higher plants. These enzymes catalyze the hydrolysis of β-D-glucoside residues from a series of non-reducing ends of β-D-glucan and β-D-oligosaccharide. Their extensive specificity can be reasonably explained from X-ray crystallography data of complexes of barley β-D-glucan glucose hydrolase with non-hydrolyzed S-glycoside substrate analogues, as well as molecular models of enzyme-substrate complexes. The glucose-based residue occupying the binding subsite-1 is tightly locked to a fixed position with the six amino acid residues near the bottom of the active site pocket by extensive hydrogen bonding. In contrast, the glucose residue on subsite +1 is located between two tryptophan residues at the pocket entrance, where it is less constrained. The relative flexibility of subsite +1 binding, coupled with the projection of the remaining portion of the binding substrate away from the enzyme surface, means that the entire active site can accommodate a range of substrates with a variable spatial distribution of adjacent β-D-glucose-based residues. Broad specificity for glycoside linkage types will enable enzymes to perform multiple functions during plant development.