Biochemical Studies of Purified Extracellular Xylanases from Aspergillus versicolor

Fungal xylanases from alkalo-thermophilic Aspergillus versicolor that hydrolyse xylan, had been purified and characterized. The purification procedure included ammonium sulphate precipitation and chromatography on gel filteration (Sephadex G-200 and G-100) combined with ion exchange chromatography on gel filteration (DEAE-sephadex). Three forms of extracellular xylanases XYI, XYII and XYIII are obtained. Their specific activities were 95.9, 83.33 and 106.7 unit g-1. protein which represented 16.31, 14.17 and 18.15 fold purification over the crude extract, respectively. Characterization was carried out for all forms of xylanases. The Km values are 0.66, 0.50 and 0.22 mg mL-1. For XYI, XYII and XYIII, respectively. The optimum temperature for all xylanases was at 70°C except for XYII at 50°C. The optimum pH for XYII and XYIII were 10 and for XYI at pH 7. The effect of metal ions was examined. In conclusion, the present study indicate that the low molecular weight isoenzyme XYIII (31.6 kDa) produced from the alkalo-thermophilic Aspergillus versicolor is the most preferable isoenzyme due to its high pH and temperature optima (10 and 70°C) alkali-thermostable) accompanied with the highest affinity (Km = 0.22 mg mL-1) and specific activity (106.7).