Characterization of a novel β-glucosidase from a Stachybotrys strain

An improved mutant was isolated from the cellulolytic fungus Stachybotrys sp. after nitrous acid mutagenesis. It was fed-batch cultivated on cellulose and its extracellular cellulases (mainly the endoglucanases and β-glucosidases) were analyzed. One β-glucosidase was purified to homogeneity after two steps, MonoQ and gel filtration and shown to be a dimeric protein. The molecular weight of each monomer is 85 kDa. Besides its aryl β-glucosidase activity towards salicin, methyl-umbellypheryl-β-d-glucoside (MUG) and p-nitrophenyl-β-d-glucoside (pNPG), it showed a true β-glucosidase activity since it splits cellobiose into two glucose monomers. The Vmax and the Km kinetics parameters with pNPG as substrate were 78 U/mg and 0.27 mM, respectively. The enzyme shows more affinity to pNPG than cellobiose and salicin whose apparent values of Km were, respectively, 2.22 and 37.14 mM. This enzyme exhibits its optimal activity at pH 5 and at 50 °C. Interestingly, this activity is not affected by denaturing gel conditions (SDS and β-mercaptoethanol) as long as it is not pre-heated. The N-terminal sequence of the purified enzyme showed a significant homology with the family 1 β-glucosidases of Trichoderma reesei and Humicola isolens even though these two enzymes are much smaller in size.