Purification and characterization of a cellulase-free, thermostable endo-xylanase from Streptomyces griseorubens LH-3 and its use in biobleaching on eucalyptus kraft pulp

Xylanase is an important enzyme involved in degrading xylan. In this study, an extracellular cellulase-free, thermostable endo-xylanase which was produced by Streptomyces griseorubens LH-3 with bagasse semi-cellulose as a carbon source was purified and characterized. The xylanase was purified 4-fold with a recovery yield of 21.6% by precipitation with 25-55% (NH4)2SO4, Mono Q ion exchange chromatography and sephacryl S-200 HR gel filtration chromatography. It appeared as a monomeric protein on SDS-PAGE gel and had an apparent molecular weight of 45.5 kDa with specific activity of 434 IU/mg. Using birchwood xylan as substrate, the maximum velocity (Vmax) and Michaelis-Menten constant (Km) were found to be 1.44 mg/ml and 2.05 μmol/min mg, respectively. The purified xylanase was active at pH 4.0-8.0 with an optimum pH of 5.0. It was stable at temperatures between 30°C and 50°C, exhibiting maximum activity at 60°C. Hg2+ and Al3+ inhibited the enzyme activity significantly. Enzymatic product analysis indicated that the enzyme was an endo-xylanase, whose hydrolysis products were mainly a series of short-chain xylooligosaccharides. Furthermore, it was used for biobleaching of eucalyptus kraft pulp, and results showed that this purified xylanase increased the brightness of the pulp by 14.5% and reduced the kappa number by 24.5%. All these industrially relevant characteristics made it had potential application in the pulp and paper industry as a biobleaching agent.