Abstract
β-Glucosidases catalyze the hydrolysis of cellobiose to glucose during lignocellulosic biomass depolymerization. A significant limitation of many β-glucosidases is product inhibition by glucose, leading to reduced conversion efficiency. However, certain β-glucosidases exhibit tolerance or even stimulation by glucose. The mechanisms underlying this remarkable feature remain poorly elucidated. Here, we employ molecular dynamics simulations to investigate the molecular basis of glucose tolerance and stimulation within the family 1 β-glucosidase from Humicola insolens (HiBgl). Potential of mean force calculations reveal a substantial difference in binding free energies between cellobiose (-12.5 kcal/mol) and glucose (-4.3 kcal/mol) at the HiBgl active site, indicating that the glucose product is a considerably weaker ligand than the cellobiose substrate. These findings are consistent with our observations that HiBgl undergoes conformational changes in its substrate binding site, specifically involving the Trp349 side chain, in the presence of glucose, potentially facilitating glucose expulsion and mitigating product inhibition. Simulations of HiBgl solvated in a 200 mM aqueous glucose environment show that glucose molecules from the bulk solution are capable of penetrating and widening the substrate binding pocket, forming direct interactions with cellobiose in the active site, which may contribute to catalytic stimulation. Additionally, we identify seven distinct secondary glucose binding sites located on the HiBgl surface, spatially distant from the active site, implying a potential role in allosteric regulation. Finally, we demonstrate that glucose at subsite +1 can adopt multiple orientations relative to glucose at subsite -1, a prerequisite for transglycosylation reactions in HiBgl. Our findings elucidate the molecular mechanisms governing HiBgl's glucose tolerance and stimulation, thereby enabling the design of site-directed mutagenesis experiments to improve enzyme efficiency for industrial applications, particularly in biofuel production and oligosaccharide synthesis.