Characterization of a thermostable family 10 endo-xylanase (XynB) from Thermotoga maritima that cleaves p-nitrophenyl-β-D-xyloside

Thermotoga maritima MSB8 possesses two xylanase genes, xynA and xynB. The xynB gene was isolated from the genomic DNA of T. maritima, cloned, and expressed in Escherichia coli. XynB was purified to homogeneity by heat treatment, affinity chromatography and ion-exchange column chromatography. The purified enzyme produced a single band upon SDS-PAGE corresponding to a molecular mass of 42 kDa. At 70°C, the enzyme was stable between pH 5.0 and pH 11.4, and it was stable at temperatures of up to 100°C from pH 7.0 to pH 8.5. At 50°C, XynB displayed an optimum pH of 6.14 and at this pH the temperature for optimal enzyme activity was 90°C. XynB exhibited broad substrate specificity and was highly active towards p-nitrophenyl-β-D-xylobioside with K_m and k_cat values of 0.0077 mM and 5.5 s^-1, respectively, at 30°C. It was also active towards p-nitrophenyl-β-D-xyloside. The initial product of the cleavage of p-nitrophenyl-β-D-xyloside was xylobiose, indicating that the major reaction in the cleavage was transglycosylation, not hydrolysis.