Cloning and characterization of low-temperature adapted GH5-CBM3 endo-cellulase from Bacillus subtilis 1AJ3 and their application in the saccharification of switchgrass and coffee grounds

Endocellulase is a key cellulase for cellulosic material pretreatment in the industry by hydrolyzing long cellulose chains into short chains. To investigate the endocellulase characteristics from Bacillus subtilis 1AJ3, and increase its production yield, this paper cloned an endocellulase gene denoted CEL-5A from strain 1AJ3 and expressed in E. coli BL21 (DE3). The CEL-5A gene was sequenced with a full-length of 1500 bp, encoding a totally of 500 amino acids, and containing two domains: the GH5 family catalytic domain (CD) and the CBM3 family cellulose-binding domain (CBD). Recombinant endocellulase Cel-5A with a His-tag was purified of the Ni-NTA column, and SDS-PAGE results demonstrated that Cel-5A exhibited a molecular weight of 56.4 kDa. The maximum enzyme activity of Cel-5A was observed at pH 4.5 and 50 °C. Moreover, it was active over the broad temperature region of 30–60 °C, and stable within the pH range of 4.5–10.0. In addition, Co(2+) was able to increase enzyme activity, while the majority of metal ions demonstrated stable enzyme activity under low- concentration. The substrate specificity of Cel-5A exhibited a high specific activity on the β-1,3-1,4 glucan linkage from barley. The Michaelis–Menten constant and the maximum velocity of the recombinant Cel-5A for CMC-Na were determined as 14.87 mg/mL and 19.19 μmol/min/mg, respectively. When Cel-5A was applied to the switchgrass and coffee grounds, its color became lighter and the biomass was observed to loosen following hydrolyzation. The saccharification rate reached 12% of the total weight of switchgrass in 20 h. These properties highlight the potential application of Cel-5A as an endocellulase in the pretreatment of biomass, for example, in the coffee grounds/waste, and related industries.