Characterization of a novel thermostable β-glucosidase from a metagenomic library of termite gut

A novel β-glucosidase-encoding gene, bgl-gs1, which was identified from a positive fosmid clone in a metagenomic library of the gut of Globitermes brachycerastes, [corrected] encodes a 455 amino acid polypeptide that contains a catalytic domain belonging to glycoside hydrolase family 1 (GH1). It was expressed in Escherichia coli BL21 (DE3) and the expression product showed a molecular mass of ∼51.7 kDa by SDS-PAGE. The optimal temperature and pH for the activity of the purified recombinant enzyme Bgl-gs1 with p-nitrophenyl-β-D-glucoside (pNPG) were 90°C and 6.0, respectively. The specific activities of Bgl-gs1 on pNPG and salicin were 110 and 14U/mg of protein, respectively, and its K(m) values were 0.18 and 2.59 mM, respectively. The residual activity of Bgl-gs1 was maintained above 70% after the recombinant enzyme was incubated at 75°C and pH 6.0 for 2h, and its half-life at 90°C was approximately 1h in the presence of 4mM pNPG. Bgl-gs1 showed synergistic effect with either a crude enzyme mixture of the fungal strain Trichoderma reesei Rut-C30 or a fusion protein (TcE1) created from the cellobiohydrolase cbh1 gene of T. reesei and endoglucanase from Acidothermus cellulolyticus; 87 and 137% increases in hydrolytic efficiency were noted on microcrystalline cellulose, respectively. These results suggest that the thermostable β-glucosidase Bgl-gs1 is a likely candidate for industrial applications.