Purification and characterization of a cellulase (CMCase) from a newly isolated thermophilic aerobic bacterium Caldibacillus cellulovorans gen. nov., sp nov

One thermostable endoglucanase (CMCase) was purified to homogeneity from the culture supernatant of a new isolated thermophilic bacterium Caldibacillus cellulovorans. The molecular weight of the enzyme was 85.1 kDa as determined by SDS Polyacrylamide gel electrophoresis ( PAGE) and 174 kDa by size-exclusion chromatography. The isoelectric point of the enzyme was at pH 4.12. The temperature for maximum activity was 80 degreesC, with half-lives of 32 min at 80 degreesC, and 2 min at 85 degreesC, and 83% activity remaining after 3 h at 70 degreesC. Thermostability of the enzyme was increased twofold by the addition of bovine serum albumin. Maximal activity was observed between pH 6.5 and 7.0. The enzyme activity was significantly inhibited by Zn2+, Hg2+, and p-chloromercuribenzenesulphonic acid. The enzyme showed high activity on carboxymethylcellulose (CMC) with much lower activity on Avicel; a low level of activity was also found against xylan. Cellobiose was the major product of hydrolysis of amorphous cellulose and CMC. Viscometric analysis indicated that the enzyme hydrolysed CMC in an exo-acting fashion. Cellotriose and cellobiose were not degraded and at least four contiguous glucosyl residues were necessary for degradation by the enzyme. The K-m and V-max of the enzyme for CMC were 3.4 mg ml(-1) and 44.7 mumol min(-1) (mg protein)(-1), respectively.