Purification and properties of an extracellular ?-glucosidase from the cellulolytic thermophile Clostridium stercorarium

Abstract

Clostridium stercorarium cultures grown on cellobiose contain both an extracellular and a cell-bound β-glucosidase activity. A substantial portion of the cell-bound enzyme could be extracted by osmotic shock, suggesting a periplasmic localization. The β-glucosidase present in culture supernatants was purified to homogeneity. It was found to be identical in all aspects tested with the cell-bound β-glucosidase. The enzyme exists as a monomer with an apparent molecular weight of 85.000 (SDS-PAGE) and a pI of 4.8. It shows optimal activity as pH 5.5 and 65° C. Thiol groups are essential for enzyme activity. In the presence of reducing agents and divalent cations the half-life of the purified enzyme was more than 5 h at 60°C. The enzyme hydrolyses at different rates a wide range of substrates including aryl-β-glucosides, cellobiose, and disordered cellulose. K m values were determined as 0.8 mM for p-nitrophenyl-β-glucoside (PNPG) and 33 mM for cellobiose. The cellular localization and the substrate specificity pattern are consistent with a dual role of the C. stercorarium β-glucosidase in cellulose saccharification: (1) Cleavage of cellobiose formed by exoglucanase and (2) degradation of cellodextrins produced by endoglucanase action.