The temperature and pH properties of the extracellular hemicellulose-Degrading enzymes of aureobasidium pullulans NRRL Y 2311-1

Production and properties of hemicellulases by a Thermomyces lanuginosus strain

Thermophilic fungi producing extremely high beta-xylanase and their associated hemicellulases have attracted considerable attention because of potential industrial applications. Thermomyces lanuginosus strain SSBP isolated from soil, produced beta-xylanase activity of 59 600 nkat ml-1 when cultivated on a medium containing corn cobs as substrate and yeast extract as nitrogen source. Lower beta-xylanase activities were produced after growth on other xylan substrates, sugars and soluble starch. Other hemicellulases were produced extracellularly at significantly lower levels than the beta-xylanase activity produced on corn cobs. No cellulase activity was observed. The optimal conditions for beta-xylanase production were 50 degrees C and pH 6.5, whereas 70 degrees C and between pH 5. 5 and 9.5 were optimal for beta-xylanase activity. The temperature optima for other hemicellulases were less than the xylanase with the exception of beta-mannosidase. The pH optima of the other hemicellulases were between 5.0 and 6.5. Xylanase was stable up to 70 degrees C and between pH 5.5 and 9.0 for 30 min whereas the other hemicellulase were less stable. These results suggest that the most suitable conditions for hydrolysis of hemicellulose by these enzymes would be at 50 degrees C and pH 6.0.

Purification and characterization of two thermostable acetyl xylan esterases from Thermoanaerobacterium sp. strain JW/SL-YS485

Two acetyl esterases (EC 3.1.1.6) were purified to gel electrophoretic homogeneity from Thermoanaerobacterium sp. strain JW/SL-YS485, an anaerobic, thermophilic endospore former which is able to utilize various substituted xylans for growth. Both enzymes released acetic acid from chemically acetylated larch xylan. Acetyl xylan esterases I and II had molecular masses of 195 and 106 kDa, respectively, with subunits of 32 kDa (esterase I) and 26 kDa (esterase II). The isoelectric points were 4.2 and 4.3, respectively. As determined by a 2-min assay with 4-methylumbelliferyl acetate as the substrate, the optimal activity of acetyl xylan esterases I and II occurred at pH 7.0 and 80 degrees C and at pH 7.5 and 84 degrees C, respectively. Km values of 0.45 and 0.52 mM 4-methylumbelliferyl acetate were observed for acetyl xylan esterases I and II, respectively. At pH 7.0, the temperatures for the 1-h half-lives for acetyl xylan esterases I and II were 75 degrees and slightly above 100 degrees C, respectively.