Molecular characterization of SCO0765 as a cellotriose releasing endo-β-1,4-cellulase from Streptomyces coelicolor A(3).

The sco0765 gene was annotated as a glycosyl hydrolase family 5 endoglucanase from the genomic sequence of Streptomyces coelicolor A3(2) and consisted of 2,241 bp encoding a polypeptide of 747 amino acids (molecular weight of 80.5 kDa) with a 29-amino acid signal peptide for secretion. The SCO0765 recombinant protein was heterogeneously over-expressed in Streptomyces lividans TK24 under the control of a strong ermE* promoter. The purified SCO0765 protein showed the expected molecular weight of the mature form (718 aa, 77.6 kDa) on sodium dodecyl sulfate-polyacryl amide gel electrophoresis. SCO0765 showed high activity toward β-glucan and carboxymethyl cellulose (CMC) and negligible activity to Avicel, xylan, and xyloglucan. The SCO0765 cellulase had a maximum activity at pH 6.0 and 40°C toward CMC and at pH 9.0 and 50-60°C toward β-glucan. Thin layer chromatography of the hydrolyzed products of CMC and β-glucan by SCO0765 gave cellotriose as the major product and cellotetraose, cellopentaose, and longer oligosaccharides as the minor products. These results clearly demonstrate that SCO0765 is an endo-β-1,4-cellulase, hydrolyzing the β-1,4 glycosidic bond of cellulose into cellotriose.