Comparison of Extracellular Cellulase Activities of Clostridium thermocellum LQRI and Trichoderma reesei QM9414

The crude extracellular cellulase ofClostridium thermocellum LQRI (virgin strain) was very active and solubilized microcrystalline cellulose at one-half the rate observed for the extracellular cellulase ofTrichoderma reesei QM9414 (mutant strain).C. thermocellum cellulase activity differed considerably from that ofT. reesei as follows: higher endoglucanase/exoglucanase activity ratio; absence of extracellular cellobiase orβ -xylosidase activity; long-chain oligosaccharides instead of short-chain oligosaccharides as initial (15-min) hydrolytic products on microcrystalline cellulose; mainly cellobiose or xylobiose as long-term (24-h) hydrolysis products of Avicel and MN300 or xylan; and high activity and stability at 60 to 70°C. Under optimized reaction conditions, the kinetic properties (V max , 0.4 μmol/min per mg of protein; energy of activation, 33 kJ; temperature coefficient, 1.8) ofC. thermocellum cellulose-solubilizing activity were comparable to those reported forT. reesei , except that the dyed Avicel concentration at half-maximal velocity was twofold higher (182 μM). The cellulose-solubilizing activity of the two crude cellulases differed considerably in response to various enzyme inhibitors. Most notably, Ag2+ and Hg2+ effectively inhibitedC. thermocellum but notT. reesei cellulase at <20 μM, whereas Ca2+ , Mg2+ , and Mn2+ inhibitedT. reesei but notC. thermocellum cellulase at >10 mM. Both enzymes were inhibited by Cu2+ (>20 mM), Zn2+ (>1.0 mM), and ethylene glycol-bis(β -aminoethyl ether)-N,N -tetraacetic acid (>10 mM).T. reesei but notC. thermocellum cellulose-solubilizing activity was 20% inhibited by glucose (73 mM) and cellobiose (29 mM). Both cellulases preferentially cleaved the internal glycosidic bonds of cellooligosaccharides. The overall rates of cellooligosaccharide degradation were higher forT. reesei than forC. thermocellum cellulase, except that the rates of conversion of cellohexaose to cellotriose were equivalent.