Structure of the catalytic domain of the Clostridium thermocellum cellulase CelT

Cellulases hydrolyze cellulose, a major component of plant cell walls, to oligosaccharides and monosaccharides. Several Clostridium species secrete multi-enzyme complexes (cellulosomes) containing cellulases. C. thermocellum CelT, a family 9 cellulase, lacks the accessory module(s) necessary for activity, unlike most other family 9 cellulases. Therefore, characterization of the CelT structure is essential in order to understand its catalytic mechanism. Here, the crystal structure of free CelT Delta doc, the catalytic domain of CelT, is reported at 2.1 angstrom resolution. Its structure differs in several aspects from those of other family 9 cellulases. CelT Delta doc contains an additional alpha-helix, alpha-helices of increased length and two additional surface-exposed beta-strands. It also contains three calcium ions instead of one as found in C. cellulolyticum Cel9M. CelT Delta doc also has two flexible loops at the open end of its active-site cleft. Movement of these loops probably allows the substrate to access the active site. CelT is stable over a wide range of pH and temperature conditions, suggesting that CelT could be used to convert cellulose biomass into biofuel.