3-DIMENSIONAL STRUCTURE OF A THERMOSTABLE BACTERIAL CELLULASE

CELLULOSIC biomass is recycled by a variety of microorganisms occupying different habitats 1. Studies of their cellulase systems have included the purification of enzyme components, the determination of their enzymological properties 2 and the cloning and characterization of their structural genes 3. Sequence analysis of more than 70 cellulases permits grouping into seven families corresponding to distinct structural types 4,5. The three-dimensional structure of the catalytic core of cellobiohydrolase CBHII from the fungus Trichoderma reesei has been reported 6. Here we show that endoglucanase CelD from Clostridium thermocellum, which is representative of a different family of cellulose-degrading enzymes consisting of at least 11 bacterial, fungal and plant endoglucanases 5,7, has a globular structure, with an amino-terminal immunoglobulin-like domain tightly packed against a larger catalytic domain. The latter shows a novel protein fold, shaped like an alpha-barrel of 12 helices connected by loops that form the active site. The structure of a complex CelD with a substrate analogue suggests a mechanism for substrate hydrolysis.