Purification and properties of three endo-β-1,4-xylanases produced by Streptomyces sp. strain S38 which differ in their ability to enhance the bleaching of kraft pulps☆

In the presence of xylan, Streptomyces sp. strain S38 secretes three xylanases (Xyl1, Xyl2, and Xyl3) that were purified to protein homogeneity and characterized. When used in bleach boosting tests on kraft hardwood and softwood, Xyl1, a family-11 enzyme, was more effective than Xyl2 and Xyl3 that belonged to family-10. Xyl1 was fully responsible for the biodelignification potential of the culture supernatants with a minimal effective amount of 10 IU per gram of dry pulp for both softwood and hardwood pulp. Complete conventional CEDED bleaching sequences showed that enzymatic pretreatment (20 IU/g dry pulp) could result in active chlorine savings of 8.6 and 4.9 kg/ton of dry pulp with hardwood and softwood, respectively. The purified enzymes were totally devoid of cellulase activity on CM-cellulose and their activities were optimal at about 60°C and pH 6. Moreover, the Vmax value of Xyl1 at 50°C measured on birchwood xylan (5,700 μmoles/min/mg prot.) was significantly higher than those of Xyl2 and Xyl3 whereas their Km values were similar. Their half-lives at 50°C were larger than 16 h but sharply decreased at 60°C where the family-11 Xyl1 was less stable (t1/260°C = 10 min) than both family-10 enzymes Xyl2 (t1/260°C = 30 min) and Xyl3 (t1/260°C = 70 min).