Characterization of an extremely thermo-active archaeal β-glucosidase and its activity towards glucan and mannan in concert with an endoglucanase
A metagenome from an enrichment culture of a hydrothermal vent sample taken at Vulcano Island (Italy) was sequenced and an endoglucanase-encoding gene (vulcel5A) was identified in a previous work. VulCel5A with maximal activity at 115 °C was characterized as the most heat-active endoglucanase to date. Based on metagenome sequences, genomes were binned and bin4 included vulcel5A as well as a putative GH1 β-glycosidase-encoding gene (vulbgl1A) with highest identities to sequences from the archaeal genus Thermococcus. The recombinant β-glucosidase VulBgl1A produced in E. coli BL21 pQE-80L exhibited highest activity at 105 °C and pH 7.0 (76.12 ± 5.4 U/mg, 100%) using 4NP β-D-glucopyranoside as substrate and 61% relative activity at 120 °C. Accordingly, VulBgl1A represents one of the most heat-active β-glucosidases to date. The enzyme has a broad substrate specificity with 155% activity towards 4NP β-D-mannopyranoside in comparison with 4NP β-D-glucopyranoside. Moreover, nearly complete hydrolysis of cellobiose was demonstrated. The enzyme exhibited a high glucose tolerance with 26% residual activity in presence of 2 M glucose and was furthermore activated at glucose concentrations of up to 0.5 M. When the endoglucanase VulCel5A and the β-glucosidase VulBgl1A were applied simultaneously at 99 °C, 158% activity towards barley β-glucan and 215% towards mannan were achieved compared with the activity of VulCel5A alone (100%). Consequently, a significant increase in glucose formation was observed when both enzymes were incubated with β-glucan and mannan suggesting a synergistic effect. Hence, the two archaeal extremozymes are ideal candidates for complete glucan and mannan saccharification at temperatures above the boiling point of water.