Beta-xylosidase activity of a GH3 glucosidase/xylosidase from yak rumen metagenome promotes the enzymatic degradation of hemicellulosic xylans

Aims: To characterize the duel activities of a glycosyl hydrolase family 3 beta-glucosidase/xylosidase from rumen bacterial metagenome and to investigate the capabilities of its beta-d-xylosidase activities for saccharification of hemicellulosic xylans. Methods and Results: A beta-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-beta-d-glucopyranoside (pNP-Glc) and p-nitrophenyl- beta-d-xylopyranoside (pNP-Xyl) substrates. Analysis of the kinetic properties indicated that RuBGX1 had a lower affinity for pNP-Glc substrate as the K-m was 0.164 mmol l)(- 1) for pNP-Glc and 0.03 mmol l) 1(-1) for pNP-Xyl at pH 6.0 and 50 degrees C, respectively. The capabilities of RuBGX1 beta-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 chromatographypulsed amperometric detection showed that RuBGX1 could release xyloses from intermediate xylooligosaccharides produced by endoxylanase. Conclusions: The RuBGX1 shows beta-glucosidase activity in hydrolysis of cellooligosaccharides; meanwhile, it has beta-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 beta-xylosidase activity of family 3 beta-glucosidase /xylosidase functioned in the degradation of hemicellulosic xylans. The bifunctional beta-glucosidase /xylosidase property of RuBGX1 can be used in simultaneous saccharification of cellulose and xylan into fermentable glucose and xylose.