Purification and properties of three endo-beta-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 xylana ses (Xy11, Xy12, and Xy13) th:lt were purified to protein homogeneity and c haracterized. When used in bleach boosting tests on kraft hardwood and soft wood, Xy11, a family-11 enzyme, was more effective than Xy12 and Xy13 that belonged to family-10. Xy11 was fully responsible for the biodelignificatio n 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 co nventional 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/t on of dry pulp with hardwood and softwood, respectively. The purified enzym es were totally devoid of cellulase activity on CM-cellulose and their acti vities were optimal at about 60 degrees C and pH 6. Moreover, the V-max val ue of Xy11 at 50 degrees C measured on birchwood xylan (5,700 mu moles/min/ mg prot.) was significantly higher than those of Xy12 and Xy13 whereas thei r k(m) values were similar. Their half-lives at 50 degrees C were larger th an 16 h but sharply decreased at 60 degrees C where the family-11 Xy11 was less stable (t(1/2)(60 degrees C) = 10 min) than both family-10 enzymes Xy1 2 (t(1/2) = 30 min) and Xy13 (t(1/2)(60 degrees C) = 70 min). (C) 2000 Else vier Science Inc. All rights reserved.