Characterization of a thermostable and alkaline xylanase from Bacillus sp. and its bleaching impact on wheat straw pulp

Abstract

Delignification efficacy of xylanases to facilitate the consequent chemical bleaching of Kraft pulps has been studied widely. In this work, an alkaline and thermally stable cellulase-less xylanase, derived from a xylanolytic Bacillus subtilis, has been purified by a combination of gel filtration and Q-Sepharose chromatography to its homogeneity. Molecular weight of the purified xylanase was 61kDa by SDS-PAGE. The purified enzyme revealed an optimum assay temperature and pH of 60°C and 8.0, respectively. Xylanase was active in the pH range of 6.0-9.0 and stable up to 70°C. Divalent ions like Ca(2+), Mg(2+) and Zn(2+) enhanced xylanase activity, whereas Hg(2+), Fe(2+), and Cu(2+) were inhibitory to xylanase at 2mM concentration. It showed K ( m ) and V ( max ) values of 9.5mg/ml and 53.6μmol/ml/min, respectively, using birchwood xylan as a substrate. Xylanase exhibited higher values of turn over number (K (cat)) and catalytic efficiency (K (cat)/K (m)) with birchwood xylan than oat spelt xylan. Bleach-boosting enzyme activity at 30U/g dry pulp displayed the optimum bio-delignification of Kraft pulp resulting in 26.5% reduction in kappa number and 18.5% ISO induction in brightness at 55°C after 3h treatment. The same treatment improved the pulp properties including tensile strength and burst index, demonstrating its potential application in pre-bleaching of Kraft pulp.