Biochemical and Structural Characterization of a Highly Glucose-Tolerant β-Glucosidase from the Termite Reticulitermes perilucifugus

The enzymatic hydrolysis of lignocellulose is often hindered by the glucose-mediated inhibition of β-glucosidases, a major bottleneck in industrial cellulose degradation. Identifying novel glucose-tolerant β-glucosidases is essential for enhancing saccharification efficiency. In this study, we cloned and heterologously expressed a novel β-glucosidase, RpBgl8, from the termite <i>Reticulitermes perilucifugus</i> in <i>Escherichia coli</i>. Sequence and structural analyses classified RpBgl8 as a glycoside hydrolase family 1 enzyme. The purified enzyme exhibited optimal activity at 45 °C and pH 7.0, with broad stability across pH 4.0-8.0. Notably, RpBgl8 demonstrated high tolerance to lignocellulose-derived inhibitors and organic solvents, maintaining 100% activity in 15% ethanol. Furthermore, RpBgl8 exhibited outstanding glucose tolerance, retaining 100% activity at 2.5 M glucose and 82% activity at 4.0 M glucose-outperforming most previously reported β-glucosidases. A structural analysis revealed a narrow, hydrophobic substrate pocket, with residue F124 at the glycone-binding site critical for minimizing glucose accumulation. The F124W mutation significantly reduced glucose tolerance, confirming that hydrophobic interactions at the active site mitigate inhibition. These findings establish RpBgl8 as a promising candidate for high-solid biomass processing and simultaneous saccharification and fermentation applications, highlighting termites as underexplored sources of biocatalysts with unique industrial potential.