High molecular weight cellulase-free xylanase from alkali-tolerant Aspergillus fumigatus AR1

Fungi producing xylanases are numerous but alkali-tolerant fungi producing cellulase-free xylanases are rare. Presently, an alkali-tolerant Aspergillus fumigatus AR1 producing xylanases was isolated from a paper mill effluent. A. fumigatus produced xylanases in a wide range of growth pH (30 U/ml at pH 9 and 135 U/ml at pH 5) and cellulase was detected at negligible level (<0.4 U/ml) at all the growth conditions. It is strictly non-cellulolytic since it did not grow on carboxymethyl cellulose (CMC) and avicel substrates. The pH-dependent induction of A. fumigatus xylanase by xylan (substrate for xylanase catalysis) and xylose (product of xylanase catalysis) is contrasting; the ratio of xylanase expression at pH 9 to that at pH 5 was 2.1 in xylose medium and 0.38 in xylan medium. The xylanase exhibited optimum pH of 6.0–6.5 and temperature of 60 °C for activity. It was stable in the pH range 5–9 and up to 50 °C. At 5 mM concentration, HgCl2 inhibited xylanase activity (32%) while MnSO4 and CuSO4 enhanced xylanase activity (17–33%). Activity staining indicated that A. fumigatus AR1 produced multiple xylanases of high molecular weight ranging from 212 to 253 kDa contrasting to other two alkali-tolerant fungal xylanases, which are less than 35 kDa.