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 (mu)g 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)-xylpsidase 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.