Biochemical properties of xylanases from a thermophilic fungus,Melanocarpus albomyces, and their action on plant cell walls |
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Authors: | K Ashok Prabhu Ramesh Maheshwari |
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Institution: | (1) Department of Biochemistry, Indian Institute of Science, 560 012 Bangalore, India;(2) Present address: Department of Biochemistry, Kasturba Medical College, 575 001 Mangalore, India |
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Abstract: | Melanocarpus albomyces, a thermophilic fungus isolated from compost by enrichment culture in a liquid medium containing sugarcane bagasse, produced
cellulase-free xylanase in culture medium. The fungus was unusual in that xylanase activity was inducible not only by hemicellulosic
material but also by the monomeric pentosan unit of xylan but not by glucose. Concentration of bagasse-grown culture filtrate
protein followed by size-exclusion and anion-exchange chromatography separated four xylanase activities. Under identical 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 IIIA, were purified to apparent homogeneity from bagasse-grown cultures. Both xylanases were specific
forβ-1,4 xylose-rich polymer, optimally active, respectively, at pH 6.6 and 5.6, and at 65°C. The xylanases were stable between
pH 5 to 10 at 50°C for 24 h. Xylanases released xylobiose, xylotriose and higher oligomers from xylans from different sources.
Xylanase IA had a Mr of 38 kDa and contained 7% carbohydrate whereas xylanase IIIA had a Mr of 24 kDa and no detectable carbohydrate. The Km for larchwood xylan (mg ml−1) and Vmax (μmol xylose min−1 mg−1 protein) of xylanase IA were 0.33 and 311, and of xylanase IIIA 1.69 and 500, respectively. Xylanases IA, II and IIIA showed
no synergism in the hydrolysis of larchwood glucuronoxylan or oat spelt and sugarcane bagasse arabinoxylans. They had different
reactivity on untreated and delignified bagasse. The xylanases were more reactive than cellulase on delignified bagasse. Simultaneous
treatment of delignified bagasse by xylanase and cellulase released more sugar than individual enzyme treatments. By contrast,
the primary cell walls of a plant, particularly from the region of elongation, were more susceptible to the action of cellulase
than xylanase. The effects of xylanase and cellulase on plant cell walls were consistent with the view that hemicellulose
surrounds cellulose in plant cell walls. |
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Keywords: | Cellulase free-xylanase Melanocarpus albomyces thermophilic fungus hemicellulase biodegradation sugarcane bagasse plant cell wall |
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