Cloning, expression and characterization of glycoside hydrolases from the thermophilic cellulolytic anaerobic bacterium Caldicellulosiruptor kristjanssonii

Abstract

Lignocellulosic biomass has great potential as a substrate for ethanol production as it is a renewable and rather abundant energy source. However, the rigid and complex structure of lignocellulose is a major bottleneck preventing the development of cost-effective production methods. By the use of thermostable cellulolytic enzymes, hydrolysis of cellulose and fermentation of glucose to ethanol could be performed at high temperatures and this would lower the production cost of ethanol significantly. Hence, it is important to find thermostable cellulolytic enzymes. Here, three putative cellulolytic enzymes from the thermophilic cellulolytic anaerobic bacterium Caldicellulosiruptor kristjanssonii were cloned, expressed and characterized. Cel19, a Glycoside hydrolase family (GH) 9 protein with two carbohydrate binding modules (CBMs) on the C-terminal, turned out to be a β-1,3-1,4 endoglucanase active on β-glucan (barley) and lichenan with an optimal temperature of 85Ë C and pH of 5.5 to 6.5. Cel19 was also cloned in two truncated versions, with one or both of the CBMs removed. Both truncated versions were less stable than the wild-type enzyme, showing that CBMs are important for the structure of the whole enzyme and do not work solely to bring the substrate in proximity to the active hydrolyzing domain. Bgl3, a GH 1 protein was a β-glucosidase active on cellobiose and cellooligosaccharides with an optimum temperature of 80Ë C, pH 6.5 and retained more than 60% activity after 48h at 60Ë C. Cel14, a GH 5 protein, showed no activity towards any substrate tested. The results from this study indicate that Bgl13 could be used in the process of producing ethanol, Cel19 might have some industrial uses in food processing and finally it shows that C. kristjanssonii can be used in the search for thermostable enzymes.