Purification and biochemical properties of a thermostable, haloalkaline cellulase from Bacillus licheniformis AMF-07 and its application for hydrolysis of different cellulosic substrates to bioethanol production

Abstract

A thermophilic strain AMF-07, hydrolyzing carboxymethylcellulose (CMC) was isolated from Kerman hot spring and was identified as Bacillus licheniformis based on 16S rRNA sequence homology. The carboxymethylcellulase (CMCase) enzyme produced by the B. licheniformis was purified by (NH4)2SO4 precipitation, ion exchange and gel filtration chromatography. The purified enzyme gave a single band on SDS-PAGE with a molecular weight of 37 kDa. The CMCase enzyme was highly active and stable over broad ranges of temperature (40-80 ºC), pH (6.0-10.0) and NaCl concentration (10-25%) with an optimum at 70 ºC, pH 9.0 and 20% NaCl, which showed excellent thermostable, alkali-stable and halostable properties. Moreover, it displayed high activity in the presence of cyclohexane (134%) and chloroform (120%). Saccharification of rice bran and wheat bran by the CMCase enzyme resulted in respective yields of 24 and 32 g L-1 reducing sugars. The enzymatic hydrolysates of rice bran were then used as the substrate for ethanol production by Saccharomyces cerevisiae. Fermentation of cellulosic hydrolysate using S. cerevisiae, reached maximum ethanol production about 0.125 g g-1 dry substrate (pretreated wheat bran). Thus, the purified cellulase from B. licheniformis AMF-07 utilizing lignocellulosic biomass could be greatly useful to develop industrial processes.