Several thermophilic fungi are exceptionally high producers of xylanases with the application in the bio-bleaching of wood pulp for paper manufacture. They are generally single-chain glycoproteins, ranging from 6 to 80 kDa, active between a pH of 4.5 to 6.5, and at temperatures between 55 to 65°C. Xylanases are co-induced with cellulases by natural substrates or inferior quality filter-paper containing hemicelluloses. Thermoascus aurantiacus and Paecilomyces varioti are exceptionally high producers of xylanase—the enzyme simply crystallized out in concentrated protein solution facilitating structural analysis by x-ray (Maheshwari et al., 2000). The thermostability of xylanases from thermophiles is postulated to be due to an extra disulfide bond and the preponderance of salt bridges holding the secondary structure of protein.
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