Relationship between Carbohydrate Moiety and Thermostability of β-Glucosidase from Mucor miehei YH-10

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Two forms of thermostable β-glucosidase (a more thermostable G-a and less thermostable G-b) were purified to homogeneity from the culture filtrate of the thermophilic fungus Mucor miehei YH-10. G-a and the G-b were glycoenzymes, and they contained 23.4 % and 13.0 % carbohydrate residues, respectively. The carbohydrate residues were mannose and N-acetylglucosamine. The carbohydrate residues in G-a had a high content of N-acetylglucosamine. In the culture filtrate, a high activity of β-N-acetylglucosaminidase was observed. The carbohydrate content of native G-a decreased to 13.0 % when sufficiently digested with a commercial preparation of β-N-acetylglucosaminidase instead of β-N-acetylglucosaminidase produced by this fungus. The digested enzyme was identical with native G-b in carbohydrate content, molecular weight, amino acid composition, isoelectric point, hydrolysis curve for cellobiose, Km value for β-glucosides, and in both thermo- and pH-dependency and stability. Therefore, the liberation of carbohydrates from more thermostable G-a by β-N-acetylglucosaminidase in culture filtrate led to the formation of less thermostable G-b.
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