Improved lignocellulose degradation efficiency by fusion of β-glucosidase, exoglucanase, and carbohydrate-binding module from Caldicellulosiruptor saccharolyticus

Abstract

Bifunctional cellulases with β-glucosidase (Bgl1), exoglucanase (Exo5), and carbohydrate-binding modules (CBMs) from Caldicellulosiruptor saccharolyticus were fused to yield several recombinant plasmids, Bgl1-CBM-Exo5, Bgl1-2CBM-Exo5, and Bgl1-3CBM-Exo5. The fused enzymes possessed both β-glucosidase and exoglucanase activities and were used to improve the degradation efficiency of lignocellulosic biomass. The optimal temperature of Bgl1-3CBM-Exo5 was 70 °C, which was the same as Bgl1, and the optimal temperature of the other two enzymes was 80 °C, which was the same as Exo5. The optimal pH of fused enzymes was 4 to 5, the same as Exo5, but the optimal pH of Bgl1 was 5.5. Compared with Bgl1-CBM-Exo5 and Bgl1-2CBM-Exo5, the hydrolysis efficiency of Bgl1-3CBM-Exo5 on sodium carboxymethyl cellulose (CMC-Na) was increased by 67% and 50%, respectively. The activities of these enzymes on CMC-Na were increased by 128 to 192% when 10 mM MnCl2 was added. Filter paper, microcrystalline cellulose (MCC), steam-pretreated rice straw, rice straw, and wheat straw were efficiently degraded by these fused enzymes. Specific activities of the fusion enzymes on MCC reached 34.4 to 76.4 U/μmol. The results indicated that bifunctional cellulases fused with CBMs were functional on cellulosic biomass, and CBMs contributed to further deconstruction of MCC and other natural substrates.