1988

Poutanen, Kaisa

Xylans are heteropolysaccharides with a backbone consisting of 1,4 linked ß-D-xylopyranose units.Xylans are the major hemicellulose of hardwoods and annual plants, where together with cellulose and lignin they form the supporting material of the plant cell walls.In this study the enzymatic hydrolysis of xylans was shown to be influenced by the heterogeneous structure of the substrate.The role of individual xylan-degrading enzymes in the hydrolysis of xylans was studied using enzyme mixtures of different microbial origin and by purifying and characterizing xylanolytic enzymes of the fungus Trichoderma reesei.A steaming process was studied as a means of separating xylan from cellulose and lignin and as a pretreatment method prior to enzymatic hydrolysis of hardwood carbohydrates.Birchwood xylan was partially hydrolyzed during steaming and could subsequently be extracted with water.The steamed xylan was still acetylated and 4-0-methylglucurono-substituted.The high degree of substitution had a significant effect on further biotechnical conversions.Enzymes cleaving side groups from the xylan backbone were shown to act synergistically with endoxylanases and ß-xylosidase in the production of xylose.Hydrolysis of the steamed birchwood xylan was incomplete without esterases and ~-glucuronidase. ~-Arabinosidase was also needed in the hydrolysis of arabinoxylan.The capabilities of different hemicellulolytic microorganisms to produce these side group-cleaving enzymes varied considerably.Trichoderma reesei was superior to the other microorganisms tested in terms of both enzyme spectrum and activity levels.The high cellulolytic activity of T. reesei would be beneficial in applications requiring total hydrolysis, but makes this fungus an unsuitable enzyme producer for applications in which selective hydrolysis of xylan is required. Enzyme preparations produced by Aspergillus awamori, Streptomyces olivochromogenes and Bacillus subtilis were relatively free of cellulolytic activity and would be more suitable for removal or modification of the xylan in cellulose pulps.Preliminary characterization of the purified acetyl esterase and ~-arabinosidase and the partially purified ~-glucuronidase of T. reesei indicated that these side group cleaving enzymes can only act efficiently in synergism with the depolymerizing enzymes, and hence could not be used for modification of polysaccharides.The identi fication and separation of the different xylanolytic enzymes will facilitate further research on the roles of, and cooperation between, cellulolytic, hemicellulolytic and lignin degrading enzymes in the processing of lignocellulosic raw materials.