Automated docking of α-(1,4)- and α-(1,6)-linked glycosyl trisaccharides in the glucoamylase active site

Low-energy conformers of five alpha-(1,4)- and alpha-(1,6)-linked glucosyl trisaccharides were flexibly docked into the glucoamylase active site using AutoDock 2.2. To ensure that all significant conformational space was searched, the starting trisaccharide conformers for docking were all possible combinations of the corresponding disaccharide low-energy conformers. All docked trisaccharides occupied subsites -1 and +1 in very similar modes to those of corresponding nonreducing-end disaccharides. For linear substrates, full binding at subsite +2 occurred only when the substrate reducing end was alpha-(1,4)-linked, with hydrogen-bonding with the hydroxymethyl group being the only polar interaction there. Given the absence of other important interactions at this subsite, multiple substrate conformations are allowed. For the one docked branched substrate, steric hindrance in the alpha-(1,6)-glycosidic oxygen suggests that the active-site residues have to change position for hydrolysis to occur. Subsite +1 of the glucoamylase active site allows flexibility in binding but, at least in Aspergillus glucoamylases, subsite +2 selectively binds substrates alpha-(1,4)-linked between subsites +1 and +2. Enzyme engineering to limit substrate flexibility at subsite +2 could improve glucoamylase industrial properties.