Purification and properties of the Clostridium thermocellum bglB gene product expressed in Escherichia coli.

The Clostridium thermocellum beta-glucosidase B was purified to homogeneity in its recombinant form from Escherichia coli. The purification protocol included ion exchange, hydrophobic interaction and hydroxyapatite chromatography. The polypeptide was found to have a molecular mass of 84,000 daltons and a pI of 4.4. There was a differential effect of temperature on the aryl-beta-glucosidase and cellobiase activities of the purified protein. The cellobiase activity had an optimum of 45 degrees C, and aryl-beta-glucosidase 60 degrees C. Both activities had an optimum pH of 5.6, although the aryl-beta-glucosidase had a secondary peak at 7.0. Both activities were stimulated by divalent cations and DTT, but inhibited by thiol reagents. The enzyme was found to have a broad substrate specificity. Using cellobiose as substrate and a temperature of 45 degrees C, the Km and Vmax values were 1.6 mM and 5.5 U mg-1 respectively. The aryl-beta-glucosidase when assayed against pNP glucopyranoside and a temperature of 60 degrees C had Km and Vmax of 2.9 mM and 1.1 U mg-1 respectively. The enzyme was very stable at 45 degrees C, but rapidly inactivated at 60 degrees C.