The crystal structure of endoglucanase CelA, a family 8 glycosyl hydrolase from Clostridium thermocellum

Background: Cellulases, which catalyze the hydrolysis of glycosidic bonds in cellulose, can be classified into several different protein families. Endoglucanase CelA is a member of glycosyl hydrolase family 8, a family for which no structural information was previously available. Results The crystal structure of CelA was determined by multiple isomorphous replacement and refined to 1.65 å resolution. The protein folds into a regular ( α/ α) 6 barrel formed by six inner and six outer α helices. Cello-oligosaccharides bind to an acidic cleft containing at least five D -glucosyl-binding subsites (A–E) such that the scissile glycosidic linkage lies between subsites C and D. The strictly conserved residue Glu95, which occupies the center of the substrate-binding cleft and is hydrogen bonded to the glycosidic oxygen, has been assigned the catalytic role of proton donor. Conclusion The present analysis provides a basis for modeling homologous family 8 cellulases. The architecture of the active-site cleft, presenting at least five glucosyl-binding subsites, explains why family 8 cellulases cleave cello-oligosaccharide polymers that are at least five D -glycosyl subunits long. Furthermore, the structure of CelA allows comparison with ( α/ α) 6 barrel glycosidases that are not related in sequence, suggesting a possible, albeit distant, evolutionary relationship between different families of glycosyl hydrolases.