Cellulose degradation by the cellulosome

Sustainable production fuels, chemicals and materials from renewable plant biomass represents a major effort of global importance. The potential advantages, whether for diversifying the energy portfolios, decreasing emissions or supporting rural developments, are important for society in the long run. The capability of biotechnology-based biorefineries to play a significant role depends on finding sustainable and cost effective ways of degrading the complex lignocellulosic composite material in abundant biomass feedstocks, like agricultural residues and forestry wastes. Most of the current biorefinery designs involve advanced biofuels, predominantly ethanol but also many others, produced from sugars released from the feedstock by enzymatic saccharification. The high resistance of biomass polysaccharides, most notably that of cellulose, constitutes a main hurdle any viable process design must overcome. Besides effective pretreatment, enzyme efficiency in breaking down the cellulose is key. There are two natural paradigms of enzymatic cellulose degradation: cellulase systems and the cellulosome. The term "cellulase" stands broadly for a consortium of polysaccharide hydrolases, differing in specificity and mode of action, that act synergistically in the degradation of lignocellulosic substrates. Enzymes of cellulase systems do not typically assemble into larger protein complexes. The cellulosome is fundamentally different to cellulases. It is a biological nanomachine. The cellulosome presents a unique scaffold-assisted supramolecular assembly of enzymes. It is a megadalton-size protein complex built from multiple enzymatic subunits anchored on a noncatalytic scaffold protein. Interestingly, the polysaccharide hydrolases found in cellulosomes appear to be essentially the same as the ones present in cellulase systems. The current project`s main aim therefore is to advance fundamental understanding of the cellulosome (complexed-enzymes) paradigm of enzymatic cellulose deconstruction, in relation to the free-enzyme paradigm. The underlying research hypothesis is that the degradative action of the cellulosome at nano- and meso-scale differs fundamentally from the action of an ensemble of free enzymes. Single-molecule experimental studies by high-resolution atomic force microscopy are designed to visualize the process of cellulose surface deconstruction by the cellulosome. The full-fledged native cellulosome is investigated and smaller variants of the cellulosome, so-called mini- or designer cellulosomes, are also studied. The dynamics of enzyme behavior on the cellulose surface is elucidated at single molecule resolution. The degradation of substrate occurring concomitantly is observed directly. Besides its broad biological importance, the cellulosome also has considerable industrial significance. It shows particular efficiency in the degradation of cellulosic substrates and so could play a central role in the development of advanced biorefinery applications. The project therefore addresses a topic of urgent interest and immediate relevance.

The plant component cellulose is an exceptionally resilient, water-insoluble polymer made from glucose and a potentially promising source for sustainable fuels, chemicals, and materials from plant biomass. However, breaking down the cellulose structure into easily usable monomers poses a significant technical and economic challenge. In nature, biological cellulose degradation occurs either through cellulases or the cellulosome. Cellulases are enzymes that differ in specificity and mode of action, working synergistically in the breakdown of cellulose from woody plants such as trees or shrubs. Although individual cellulases can be in close spatial proximity, they are individual, physically independent units. In contrast, the cellulosome is a protein complex, an organized and physically connected assembly of enzymes necessary for cellulose degradation. In the course of this project, the cellulosome of Clostridium thermocellum was better understood in its fundamental basics as a cellulose-degrading biological nanomachine and compared with the archetypal cellulases of Trichoderma reesei. This comparison took place on both the micrometer level and the single-molecule level during cellulose degradation using atomic force microscopy. During the investigation of highly crystalline cellulose, concrete insights into significantly different degradation patterns on the micrometer level of the two analyzed systems were obtained. Subsequently, both cellulase systems were examined with high temporal resolution. The results of these analyses not only allowed the establishment of a link between the form and movement of cellulosomes but also revealed a hitherto undescribed degradation mode of free cellulases. This mode involves a loose complex of various cellulases, similar to the cellulosome. This has created a new understanding of the mechanisms of cellulose degradation that could potentially pave the way for the development of innovative strategies, including the possibility of cellulase/cellulosome hybrids. In summary, the insights gained in the project into the fundamental workings of the two archetypal cellulase systems could contribute to the development of new strategies for the depolymerization of cellulose, such as the development of cellulase/cellulosome hybrids.