Purification and characterization of the Bacillus sp. KK-1 β-xylosidase from a recombinant Escherichia coli

beta-Xylosidase was purified from the recombinant Escherichia coli carrying the Bacillus sp. KK-1 beta-xylosidase gene (xylB). The molecular mass of the purified enzyme was estimated to be 62 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, the apparent molecular mass of the beta-xylosidase was 140 kDa, indicating that the native beta-xylosidase has an oligomeric structure composed of two identical subunits. The isoelectric point was determined to be pH 5.5. The enzyme was highly active on p-nitrophenyl-beta-D-xylopyranoside but it barely hydrolyzed xylan substrates, and did not exhibit activity towards carboxymethylcellulose and p-nitrophenyl-beta-D-glucopyranoside. The enzyme had a pH optimum for its activity at pH 6.5 and a temperature optimum at 40 degrees C. The enzyme activity was completely inhibited by the presence of Hg++, and also markedly inhibited by D-xylose and D-glucose.