Abstract
Seven xylanolytic bacterial strains were isolated from saw-dust dump soil. The bacterial strain X6 was selected on the basis of the highest xylanase activity with no cellulase contamination. It was identified as Stenotrophomonas maltophilia by biochemical tests and 16S rRNA gene sequencing approach. Xylanase production studies by S. maltophilia on different commercial xylans and agro-industrial residues suggested that wheat bran was the best carbon source for xylanase production (26.4 ± 0.6 IU/mL). The studies with inorganic and organic nitrogen sources suggested yeast extract as the best support for xylanase production (25 ± 0.6 IU/mL). Maximum xylanase production was observed at initial medium pH = 8.0 (23.8 ± 0.4 IU/mL) with production at pH = 7.0 and pH = 9.0 being almost comparable. Xylanase produced by S. maltophilia was purified to homogeneity using ammonium sulfate precipitation, gel filtration, and ion exchange chromatography. The final purification was 5.43-fold with recovery of 19.18%. The molecular weight of the purified xylanase protein was ~142 kDa. Both crude and purified xylanase had good stability at pH = 9.0 and 80°C with activity retention greater than 90% after 30 min incubation. The enzyme stability at high temperature and alkaline pH make it potentially effective for industrial applications.
Highlights
Xylan is a biopolymer comprising of D-xylose monomers which are linked through β-1,4-glycosyl bond, and is found abundantly in lignocellulosic biomass
3,5-dinitrosalicylic acid (DNSA), carboxymethyl cellulose (CMC), Sephadex G-100, DEAE-Cellulose, Congo red, and D-xylose were purchased from Sigma
The results indicated that S. maltophilia strain X6 is capable of growing well and producing sufficient xylanase at alkaline pH (8.0-9.0)
Summary
Xylan is a biopolymer comprising of D-xylose monomers which are linked through β-1,4-glycosyl bond, and is found abundantly in lignocellulosic biomass. Due to complex structure of xylan, many different enzymes are needed for its complete degradation, but xylanase (EC 3.2.1.8) is sufficient to break down the xylan backbone. The xylanolytic enzymes of microbial origin are of great significance for biotechnological applications in various industries such as animal feed preparation, food processing, textiles, pharmacy, paper, and pulp industries [4]. One of the potential uses of xylanase is the development of ecofriendly biobleaching method used in paper industry, where it is used as pretreatment prior to bleaching of kraft-cooked pulp to reduce application of chlorine and toxicity of chlorine bleached effluent [6]. The pulp bleaching process requires xylanases which are thermostable (>70∘C) and active in alkaline condition (>8.0 = pH), due to alkaline and hot nature of the kraft-cooked pulp [6]
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