Revealing nature's cellulase diversity: The hyperactive CelA from Caldicellulosiruptor bescii

The thermal tolerant CelA from Caldicellulosiruptor bescii is one of the most active cellulose degrading enzyme we have tested to date. In the saccharification of a common cellulose standard, Avicel, CelA outperforms mixtures of commercially relevant exo- and endoglucanases. The modular structure of CelA is defined as: Cel48-Cbm3-Cbm3-Cbm3-Cel9 and the enzyme is extremely thermostable and highly active at elevated temperatures. We have solved the crystal structures of the two catalytic domains of CelA and modeled the solution structures of the unbound enzyme. From transmission electron microscopy studies of cellulose following incubation with CelA, we discovered morphological features that suggest CelA is capable of not only the common surface ablation/fibrillation strategies driven by processivity, but also of forming extensive cavities of roughly the size of the enzyme. It is proposed that CelA, and possibly other multi-functional glycoside hydrolases, act in this completely novel manner and thus, when combined with traditional fungla cellulases, may result in significant improvements in mixed cocktails. These results suggest that Nature’s repertoire of cellulose digestion paradigms remain only partially understood. Unlike fungal cellulase systems which typically comprise multiple single enzymes for biomass degradation such as family 7 and family 5 glycoside hydrolases, some bacterial systems utilize an alternative strategy that utilizes tethered multi-enzyme complexes to effectively degrade biomass. The enzyme CelA from the thermophile Caldicellulosiruptor bescii is one such example.