Modification of the carbohydrate composition of sulfite pulp by purified and characterized beta-xylanase and beta-xylosidase of Aureobasidium pullulans

Both beta-xylanase and beta-xylosidase were purified to homogeneity from a xylose-grown culture of Aureobasidium pullulans. Cellular distribution studies of enzyme activities revealed that beta-xylanase was an extracellular enzyme, during both the exponential and stationary phases, whereas beta-xylosidase was mostly periplasmic associated. The beta-xylanase exhibited very high specificity for xylan extracted from Eucalyptus grandis dissolving pulp, whereas the beta-xylosidase was only active on p-nitrophenyl xyloside and xylobiose. Comparison of kcat/Km ratios showed that the beta-xylanase hydrolyzed xylan from dissolving pulp 1.3, 2.1, and 2. 3 times more efficiently than Eucalyptus hemicellulose B, Eucalyptus hemicellulose A, and larchwood xylan, respectively. The beta-xylosidase exhibited a transxylosylation reaction during the hydrolysis of xylobiose. When applied on acid sulfite pulp, both enzymes released xylose and hydrolyzed xylan to a different extent. Although beta-xylosidase (0.4 U/g pulp) liberated more xylose from pulp than beta-xylanase (4.7 U/g pulp), it was responsible for only 3% of xylan solubilization. Treatment of pulp with beta-xylanase liberated 51.7 microgram of xylose/g and hydrolyzed 10% of xylan. The two enzymes acted additively on pulp and removed 12% of pulp xylan. A synergistic effect in terms of release of xylose from pulp was observed when the enzyme mixture of beta-xylanase and beta-xylosidase was supplemented with beta-mannanase. However, this did not result in further enzymatic degradation of pulp xylan. Both beta-xylanase and beta-xylosidase altered the carbohydrate composition of sulfite pulp by increasing the relative cellulose content at the expense of reduced hemicellulose content of pulp.