Aims: To characterize the duel activities of a glycosyl hydrolase family 3 β-glucosidase/xylosidase from rumen bacterial metagenome and to investigate the capabilities of its β-d-xylosidase activities for saccharification of hemicellulosic xylans.
Methods and Results: A β-glucosidase/xylosidase gene RuBGX1 was cloned from yak (Bos grunniens) rumen using the metagenomic technology. Recombinant RuBGX1, expressed in Escherichia coli, demonstrated high hydrolytic activities on both p-nitrophenyl-β-d-glucopyranoside (pNP-Glc) and p-nitrophenyl-β-d-xylopyranoside (pNP-Xyl) substrates. Analysis of the kinetic properties indicated that RuBGX1 had a lower affinity for pNP-Glc substrate as the Km was 0·164 mmol l−1 for pNP-Glc and 0·03 mmol l−1 for pNP-Xyl at pH 6·0 and 50°C, respectively. The capabilities of RuBGX1 β-xylosidase for hydrolysis of xylooligosaccharide substrates were further investigated using an endoxylanase-coupled assay. Hydrolysis time courses illustrated that a significant increase (about 50%) in the reducing sugars, including xylobiose, xylotriose and xylotetraose, was achieved by supplementing endoxylanase with RuBGX1. Enzymatic product analysis using high-performance anion-exchange chromatography-pulsed amperometric detection showed that RuBGX1 could release xyloses from intermediate xylooligosaccharides produced by endoxylanase.
Conclusions: The RuBGX1 shows β-glucosidase activity in hydrolysis of cello-oligosaccharides; meanwhile, it has β-xylosidase activity and functions synergistically with endoxylanase to promote the degradation of hemicellulosic xylans.
Significance and Impact of the study: This was the first to report the β-xylosidase activity of family 3 β-glucosidase/xylosidase functioned in the degradation of hemicellulosic xylans. The bifunctional β-glucosidase/xylosidase property of RuBGX1 can be used in simultaneous saccharification of cellulose and xylan into fermentable glucose and xylose.