Highly thermostable GH39 β-xylosidase from a Geobacillus sp strain WSUCF1

Background: Complete enzymatic hydrolysis of xylan to xylose requires the action of endoxylanase and beta-xylosidase. beta-xylosidases play an important part in hydrolyzing xylo-oligosaccharides to xylose. Thermostable beta-xylosidases have been a focus of attention as industrially important enzymes due to their long shelf life and role in the relief of end-product inhibition of xylanases caused by xylo-oligosaccharides. Therefore, a highly thermostable beta-xylosidase with high specific activity has significant potential in lignocellulose bioconversion. Results: A gene encoding a highly thermostable GH39 beta-xylosidase was cloned from Geobacillus sp. strain WSUCF1 and expressed in Escherichia coli. Recombinant beta-xylosidase was active over a wide range of temperatures and pH with optimum temperature of 70 degrees C and pH 6.5. It exhibited very high thermostability, retaining 50% activity at 70 degrees C after 9 days. WSUCF1 beta-xylosidase is more thermostable than beta-xylosidases reported from other thermophiles (growth temperature <= 70 degrees C). Specific activity was 133 U/mg when incubated with p-nitrophenyl xylopyranoside, with K-m and V-max values of 2.38 mM and 147 U/mg, respectively. SDS-PAGE analysis indicated that the recombinant enzyme had a mass of 58 kDa, but omitting heating prior to electrophoresis increased the apparent mass to 230 kDa, suggesting the enzyme exists as a tetramer. Enzyme exhibited high tolerance to xylose, retained approximately 70% of relative activity at 210 mM xylose concentration. Thin layer chromatography showed that the enzyme had potential to convert xylo-oligomers (xylobiose, triose, tetraose, and pentaose) into fermentable xylose. WSUCF1 beta-xylosidase along with WSUCF1 endo-xylanase synergistically converted the xylan into fermentable xylose with more than 90% conversion. Conclusions: Properties of the WSUCF1 beta-xylosidase i.e. high tolerance to elevated temperatures, high specific activity, conversion of xylo-oligomers to xylose, and resistance to inhibition from xylose, make this enzyme potentially suitable for various biotechnological applications.