The stability of extracellular β-glucosidase fromAspergillus niger is significantly enhanced by non-covalently attached polysaccharides

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Abstract
The removal of noncovalently bound polysaccharide coating from the extracellular enzymes ofAspergillus niger, by the technique of compartmental electrophoresis, had a very dramatic effect on the stability of β-glucosidase. The polysaccharide-β-glucosidase complex was extremely resistant to proteinases and far more stable against urea and temperature as compared with polysaccharide-free β-glucosidase. The β-glucosidase-polysaccharide complex was 18-, 36-, 40-, and 82-fold more stable against chymotrypsin, 3 mol/L urea, total thermal denaturation and irreversible thermal denaturation, respectively, as compared with polysaccharide-free β-glucosidase. The activation energy of polysaccharide-complexed β-glucosidase (55 kJ/mol) was lower than polysaccharide-free enzyme (61 kJ/mol), indicating a slight activation of the enzyme by the polysaccharide. No significant difference could be detected in the specificity constant (V/K m) for 4-nitrophenyl β-d-glucopyranoside between polysaccharide-free and polysaccharide-complexed β-glucosidase. We suggest that the function of these polysaccharides secreted by fungi includingA. niger might be to protect the extracellular enzymes from proteolytic degradation, hence increasing their life span.