Characterization and pH-dependent substrate specificity of alkalophilic xylanase from Bacillus alcalophilus

The gene of endo-beta-1-4 xylanase, xynT, was cloned from Bacillus alcalophilus AX2000 and expressed in Escherichia coli. This XynT, which belongs to glycoside hydrolase (GH) family 10, was found to have a molecular weight of approximately 37 kDa and exhibit optimal activity at pH 7-9 and 50 °C. It exhibits a high activity towards birchwood xylan and has the ability to bind avicel. Under optimal conditions, XynT hydrolyzes all xylooligomers into xylobiose as an end product with a preference for cleavage sites at the second or third glycosidic bond from the reducing end. XynT has a different substrate affinity on xylooligomers at pH 5.0, which contributes to its low activity toward xylotriose and its derived intermediate products. This low activity may be due to an unstable interaction with the amino acids that constitute subsites of the active site. Interestingly, the addition of Co(2+) and Mn(2+) led to a significant increase in activity by up to 40 and 50 %, respectively. XynT possesses a high binding affinity and hydrolytic activity toward the insoluble xylan, for which it exhibits high activity at pH 7-9, giving rise to its efficient biobleaching effect on Pinus densiflora kraft pulp.