ORIGINAL PAPER Characterization of a thermostable and alkaline xylanase from Bacillus sp. and its bleaching impact on wheat straw pulp Mahjabeen Saleem • Farheen Aslam • Muhammad Saleem Akhtar • Mohsin Tariq • Muhammad Ibrahim Rajoka Received: 23 June 2010 / Accepted: 8 July 2011 / Published online: 17 July 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Delignification efficacy of xylanases to facili- tate the consequent chemical bleaching of Kraft pulps has been studied widely. In this work, an alkaline and ther- mally stable cellulase-less xylanase, derived from a xylanolytic Bacillus subtilis, has been purified by a com- bination of gel filtration and Q-Sepharose chromatography to its homogeneity. Molecular weight of the purified xy- lanase was 61 kDa 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 2 mM concentration. It showed K m and V max values of 9.5 mg/ml and 53.6 lmol/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 30 U/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 3 h treatment. The same treatment improved the pulp properties including tensile strength and burst index, demonstrating its potential application in pre- bleaching of Kraft pulp. Keywords Bacillus subtilis Á Cellulase-less xylanase Á Characterization Á Bleaching Á Wheat straw pulp Introduction Lignocellulose is a principal component of the terrestrial biomass and is mainly composed of cellulose, hemicellu- lose and lignin. Since it is a renewable resource, it is an inexhaustible source of energy and raw materials. Hemi- celluloses are amorphous branched hetero-polymers that mainly consist of pentoses. These hetero-polymers consti- tute about 20-40% of plant biomass. Xylan is one of these polymers with a backbone consisting of b-1,4-linked D-xylose residues (Bhat and Hazlewood 2001). Complete degradation of xylan requires the concerted action of sev- eral enzymes (Wong and Mans 2001). Endo-1,4-b-xylan- ases (EC 3.2.1.8) hydrolyse the xylan backbone, and b-xylosidases (EC 3.2.1.37) cleave the resulting xylo- oligomers into xylose monomers. Xylanases also find applications in the production of ethanol, aroma, fruit jui- ces, animal feed, baking, textile, leather, paper and pulp industries (Bhat and Hazlewood 2001). Some important bacterial species producing xylanases include Bacillus subtilis (Kiddinamoorthy et al. 2008; Sa- Pereira et al. 2002; Sanghi 2008; Shrinivas et al. 2010), Bacillus pumilus (Duarte et al. 2003; Nagar et al. 2010; Yasinoka et al. 2010). Clostridium cellulovorans (Kosugi et al. 2001), Staphylococcus sp. (Gupta et al. 2000), M. Saleem (&) Á F. Aslam Institute of Biochemistry and Biotechnology, University of the Punjab, Lahore 54590, Pakistan e-mail: mahjabeensaleem1@hotmail.com M. S. Akhtar Government Islamia College, Civil Lines, Lahore 54590, Pakistan M. Tariq Á M. I. Rajoka National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad 38000, Pakistan M. Tariq Á M. I. Rajoka Government College University, Allama Iqbal Road, Faisalabad 38000, Pakistan 123 World J Microbiol Biotechnol (2012) 28:513–522 DOI 10.1007/s11274-011-0842-z