RESUMO
The cooperation between cellobiohydrolase (CBHI) and endoglucanase (EG) is necessary for biodegradation of native cellulose, but its mechanism is still poorly understood. The present paper report at the first time that an isolated component, the cellulose binding domain with its linker sequence of cellobiohydrolase I from Penicillium janthinellum (CBD(CBHI)), plays an important role in the synergism between CBHI and EGI during cellulose biodegradation. A recombinantplasmid (pUC18C), containing the gene fragment encoding CBD(CBHI) from P.janthinellum was derived from pUC18-181. In pUC 18C, the catalytic domain region of cbhI gene was deleted by in vitro DNA manipulations and then E.coli JM 109 was transformed for the production of LacZ-CBD fusion protein. The active LacZ-CBD fusion protein was digested by papain and then purified by re-exclusion chromatography. The purified peptide sequence of CBD(CBHI) had the ability of binding crystalline cellulose. The detailed morphological and structural changes of cotton fibers after binding CBD(CBHI) were investigated by using scanning electron microscopy, calorimetric activity and X-ray diffraction. The results demonstrated that the CBD(CBHI) not only has a high binding capacity to cellulose, but also causes non-hydrolytic disruption of crystalline cellulose, which leads to the release of short fibers. IR spectroscopy and X-ray diffraction show that destabilization is caused by the non-hydrolytic disruption of cellulose and the disruption of hydrogen bonds in crystalline cellulose. The efficiency of crystalline cellulose degradation was enhanced by synergistic action of CBD(CBHI) with EGI. These results suggest that the cellulose-binding domain with its linker plays an important role in crystalline cellulose degradation.