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

The removal of noncovalently bound polysaccharide coating from the extracellular enzymes of Aspergillus niger, by the technique of compartmental electrophoresis, had a very dramatic effect on the stability of beta-glucosidase. The polysaccharide-beta-glucosidase complex was extremely resistant to proteinases and far more stable against urea and temperature as compared with polysaccharide-free beta-glucosidase. The beta-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 beta-glucosidase. The activation energy of polysaccharide-complexed beta-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 A-nitrophenyl beta-D-glucopyranoside between polysaccharide-free and polysaccharide-complexed beta-glucosidase. We suggest that the function of these polysaccharides secreted by fungi including A. niger might be to protect the extracellular enzymes from proteolytic degradation, hence increasing their life span.