Heterologous expression and characterization of a glucose-stimulated β-glucosidase from the termite Neotermes koshunensis in Aspergillus oryzae

Neotermes koshunensis is a lower termite that secretes endogenous beta-glucosidase in the salivary glands. This beta-glucosidase (G1NkBG) was successfully expressed in Aspergillus oryzae. G1NkBG was purified to homogeneity from the culture supernatant through ammonium sulfate precipitation and anion exchange, hydrophobic, and gel filtration chromatographies with a 48-fold increase in purity. The molecular mass of the native enzyme appeared as a single band at 60 kDa after gel filtration analysis, indicating that G1NkBG is a monomeric protein. Maximum activity was observed at 50 degrees C with an optimum pH at 5.0. G1NkBG retained 80% of its maximum activity at temperatures up to 45 degrees C and lost its activity at temperatures above 55 degrees C. The enzyme was stable from pH 5.0 to 9.0. G1NkBG was most active towards laminaribiose and p-nitrophenyl-beta-D-fucopyranoside. Cellobiose, as well as cello-oligosaccharides, was also well hydrolyzed. The enzyme activity was slightly stimulated by Mn2+ and glycerol. The K-m and V-max values were 0.77 mM and 16 U/mg, respectively, against p-nitrophenyl-beta-D-glucopyranoside. An unusual finding was that G1NkBG was stimulated by 1.3-fold when glucose was present in the reaction mixture at a concentration of 200 mM. These characteristics, particularly the stimulation of enzyme activity by glucose, make G1NkBG of great interest for biotechnological applications, especially for bioethanol production.