Biochemical properties of xylanases from a thermophilic fungus, Melanocarpus albomyces, and their action on plant cell walls

Melanocarpus albomyces, a thermophilic fungus isolated from compost by enri chment culture in a liquid medium containing sugarcane bagasse, produced ce llulase-free xylanase in culture medium. The fungus was unusual in that xyl anase activity was inducible not only by hemicellulosic material but also b y the monomeric pentosan unit of xylan but not by glucose. Concentration of bagasse-grown culture filtrate protein followed by size-exclusion and anio n-exchange chromatography separated four xylanase activities. Under identic al conditions of protein purification, xylanase I was absent in the xylose- grown culture filtrate. Two xylanase activities, a minor xylanase IA and a major xylanase ILIA, were purified to apparent homogeneity from bagasse-gro wn cultures. Both xylanases were specific for beta-1,4 xylose-rich polymer, optimally active, respectively, at pH 6.6 and 5.6, and at 65 degrees C. Th e xylanases were stable between pH 5 to 10 at 50 degrees C for 24 h. Xylana ses released xylobiose, xylotriose and higher oligomers from xylans from di fferent sources. Xylanase IA had a M-r of 38 kDa and contained 7% carbohydr ate whereas xylanase IIIA had a M-r of 24 kDa and no detectable carbohydrat e. The K-m for larchwood xylan (mg ml(-1)) and V-max (mu mol xylose min(-1) mg(-1) protein) of xylanase IA were 0.33 and 311, and of xylanase IIIA 1.6 9 and 500, respectively. Xylanases IA, II and ILIA showed no synergism in t he hydrolysis of larchwood glucuronoxylan or oat spelt and sugarcane bagass e arabinoxylans. They had different reactivity on untreated and delignified bagasse. The xylanases were mole reactive than cellulase on delignified ba gasse. Simultaneous treatment of delignified bagasse by xylanase and cellul ase released more sugar than individual enzyme treatments. By contrast, the primary cell walls of a plant, particularly from the legion of elongation, were more susceptible to the action of cellulase than xylanase. The effect s of xylanase and cellulase on pl;int cell walls were consistent with the v iew that hemicellulose surrounds cellulose in plant cell walls.