Abstract
The understanding of microbial cellulose degradation systems is a crucial prerequisite to designing an effective operating process for the bioconversion of lignocellulosic biomass into sustainable biofuels. Relevant in this context are members of the extremely thermophilic Gram-positive bacteria genus Caldicellulosiruptor that have been shown to efficiently degrade cellulose, as well as hemicellulose. Although individual representatives from this genus have been closely examined in bioenergy related studies and single components of their cellulolytic enzyme systems have been described, an overall characterization of the cellulose degradation system is still lacking. To this end, a comparative systems level investigation of two closely related species, Caldicellulosiruptor bescii and Caldicellulosiruptor obsidiansis, based on label free quantitative proteomics was conducted to determine the protein composition in the organisms’ secretome over the course of crystalline cellulose fermentations. Mass spectrometric characterizations together with cellulase activity measurements revealed a substantial abundance increase of a few bifunctional multidomain glycosidases that were composed of the domain families 5, 9, 10 and 48, that appear to be important elements for the cellulose degradation process in Caldicellulosiruptor. However, the number and arrangement of these domains varied in the two organisms, and C. bescii enzymes also contained an additional family 44 and 74, indicating significant differences at the species level. Investigation of a glycosidase solution enriched via affinity digestion revealed the presence of highly thermostable enzymes with optimum cellulase activity at 85 ºC and pH 5 in both organisms. The C. obsidiansis preparation, however, displayed twice the CMCase and Avicelase activity as the C. bescii preparation.
