Nature's cellulase diversity: Fungal and bacterial multifunctional enzyme systems compared

The thermophilic CelA from Caldicellulosiruptor bescii is one of the most active cellulose degrading enzyme known. Unlike fungal cellulase systems which typically comprise a complex mix of individually active enzymes for biomass degradation, some bacterial systems utilize an alternative strategy of tethered multi-enzyme complexes to effectively degrade biomass. The enzyme CelA from the thermophile Caldicellulosiruptor bescii is one such example. 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. In the saccharification of a common cellulose standard, Avicel, CelA outperforms mixtures of commercially relevant exo- and endoglucanases.

Transmission electron microscopy studies suggest a clearly different mode of action of CelA compared to fungal enzymes.  We have discovered morphological features that suggest CelA does not act solely via the common surface ablation/fibrillation strategies driven by processivity and propose that due to this, CelA and possibly other multi-functional glycoside hydrolases, act in a novel manner when compared to fungal enzyme systems. We have explored the activity of CelA on a variety of pre-treated substrates in order to better understand how the different bulk components of biomass, such as xylan and lignin impact CelA activity, and how the effect of those bulk components may differ between fungal enzymes and CelA. We have also examined the role that both lignin and cellulose crystallinity plays in multifunctional enzyme systems relative to fungal enzyme systems.