SACCHARIDE METABOLISM OF CLOSTRIDIUM THERMOCELLUM

Growth, cellular and saccharolytic features of several Clostridium thermocellum strains obtained from various sources were examined. All strains produced an O(,2) insensitive, extracellular cellulase that hydrolyzed carboxymethylcellulose, microcrystalline cellulose or xylan, but they did not ferment pentoses. The ability to produce cellulose during growth on cellobiose and the capacity to ferment glucose were strain-variable properties. A stable co-culture comprised of C. thermocellum and C. thermohydrosulfuricum, a non-cellulolytic species, was developed that fermented cellulosic biomass components to ethanol. Co-culture cellulose fermentations produced higher ethanol yields than mono-cultures because of more efficient mono- and disaccharide utilization inherent to C. thermohydrosulfuricum. The molar growth yield of C. thermocellum was higher on cellobiose than glucose, and correlated with constitutive cellobiose phosphorylase and phosphoglucomutase but inducible glucose permease and hexokinase activities. The growth preference for glucose over cellobiose displayed by C. thermohydrosulfuricum correlated with constitutive glucose permease and hexokinase but inducible cellobiose permease and cellobiase activities. The cellulase of C. thermocellum differed from that of Trichoderma reesei enzyme in: containing a higher activity ratio of endoglucanase to exoglucanase; lacking cellobiase or xylosidase activity; producing mainly long chain oligosaccharides as initial cellulose hydrolysis products; and, forming cellobiose or xylobiose as the major long term cellulose hydrolysis products. The major endoglucanase component of C. thermocellum cellulase was purified to ultracentrifugational homogeneity and displayed a molecular weight of (TURN)90,000. The glycoprotein was more active on carboxymethylcellulose than Avicel, and both substrate affinity and enzyme activity increased with increasing cellodextrin size above cellotetraose.