Three-dimensional structure of a thermostable bacterial cellulase
Résumé
CELLULOSIC biomass is recycled by a variety of microorganisms occupying different habitats1. Studies of their cellulase systems have included the purification of enzyme components, the determination of their enzymological properties2 and the cloning and characterization of their structural genes3. Sequence analysis of more than 70 cellulases permits grouping into seven families corresponding to distinct structural types4,5. The three-dimensional structure of the catalytic core of cellobiohydrolase CBHII from the fungus Trichoderma reesei has been reported6. 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 endoglucanases5,7, 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 α-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.