PROPERTIES OF A XYLANASE FROM Aureobasidium pullulans PBU109 AND ITS APPLICATION IN PREBLEACHING OF RICE STRAW PULP

Abstract

A xylanase from Aureobasidium pullulans PBU 109 was produced, purified, and studied for its properties including the potential use as a prebleaching agent of rice straw pulp. The 72 kDa xylanase was produced from A. pullulans PBU 109 at a maximum yield of 10.09 ± 0.27 U/ml after cultivated in an optimized production medium containing 3.9 % (w/v) corncob for 72 hours. The xylanase was purified 17.3-fold to apparent homogeneity with a recovery yield of 13.6%. The purified xylanase was active over a broad pH range from 4 to 10 and at the temperature between 50 and 80 ºC. The optimal condition for activity of the purified enzyme was at pH 6.0 and 70 ºC. However, the enzyme was relatively unstable at 70 ºC, losing more than half of its original activity after 1-h incubation. Thermostability of the enzyme was improved by the addition of 0.75 mM sorbitol that prolonged its half-life up to 10-fold at 70 ºC. At 10 mM, the metal ions from CaCl2, MgCl2, and CoCl2 enhanced the xylanase activity, while the ions from FeSO4 and CuCl2 displayed an inhibitory effect. Both crude and purified xylanases were active on different xylans but not on α-cellulose, filter paper, CMC and p-Nitrophenyl-β-D-xylopyranoside. The Km and Vmax of the purified enzyme were comparable among the reactions on xylans from beech wood, oat spelt and rice straw. When the crude enzyme was used to pretreat rice straw pulp, the greatest efficiency was obtained from a mixture containing xylanase and 0.75 mM sorbitol. Pretreatment of the rice straw pulp with the enzyme prior to H2O2 bleaching increased the ISO sheet brightness and the tensile and tear indexes up to 13.5%, 1.16- and 1.71-fold, respectively. This xylanase from A. pullulans PBU 109 has a high potential to be used as a pretreatment agent of rice straw pulp, especially in the presence of sorbitol.