The impact of Trichoderma reesei Cel7A carbohydrate binding domain mutations on its binding to a cellulose surface: a molecular dynamics free energy study

CelS-Catalyzed Processive Cellulose Degradation and Cellobiose Extraction for the Production of Bioethanol

Bacterial cellulase enzymes are potent candidates for the efficient production of bioethanol, a promising alternative to fossil fuels, from cellulosic biomass. These enzymes catalyze the breakdown of cellulose in plant biomass into simple sugars and then to bioethanol. In the absence of the enzyme, the cellulosic biomass is recalcitrant to decomposition due to fermentation-resistant lignin and pectin coatings on the cellulose surface, which make them inaccessible for hydrolysis. Cellobiohydrolase CelS is a microbial enzyme that binds to cellulose fiber and efficiently cleaves it into a simple sugar (cellobiose) by a repeated processive chopping mechanism. The two contributing factors to the catalytic reaction rate and the yield of cellobiose are the efficient product expulsion from the product binding site of CelS and the movement of the substrate or cellulose chain into the active site. Despite progress in understanding product expulsion in other cellulases, much remains to be understood about the molecular mechanism of processive action of these enzymes. Here, nonequilibrium molecular dynamics simulations using suitable reaction coordinates are carried out to investigate the energetics and mechanism of the substrate dynamics and product expulsion in CelS. The calculated free energy barrier for the product expulsion is three times lower than that for the processive action indicating that product removal is relatively easier and faster than the sliding of the substrate to the catalytic active site. The water traffic near the active site in response to the product expulsion and the processive action is also explored.

Microbial cellulases - An update towards its surface chemistry, genetic engineering and recovery for its biotechnological potential

The inherent resistance of lignocellulosic biomass makes it impervious for industrially important enzymes such as cellulases to hydrolyze cellulose. Further, the competitive absorption behavior of lignin and hemicellulose for cellulases, due to their electron-rich surfaces augments the inappropriate utilization of these enzymes. Hence, modification of the surface charge of the cellulases to reduce its non-specific binding to lignin and enhance its affinity for cellulose is an urgent necessity. Further, maintaining the stability of cellulases by the preservation of their secondary structures using immobilization techniques will also play an integral role in its industrial production. In silico approaches for increasing the catalytic activity of cellulase enzymes is also significant along with a range of substrate specificity. In addition, enhanced productivity of cellulases by tailoring the related genes through the process of genetic engineering and higher cellulase recovery after saccharification seems to be promising areas for efficient and large-scale enzyme production concepts.

Nanoscale Engineering of Designer Cellulosomes

Biocatalysts showcase the upper limit obtainable for high-speed molecular processing and transformation. Efforts to engineer functionality in synthetic nanostructured materials are guided by the increasing knowledge of evolving architectures, which enable controlled molecular motion and precise molecular recognition. The cellulosome is a biological nanomachine, which, as a fundamental component of the plant-digestion machinery from bacterial cells, has a key potential role in the successful development of environmentally-friendly processes to produce biofuels and fine chemicals from the breakdown of biomass waste. Here, the progress toward so-called "designer cellulosomes", which provide an elegant alternative to enzyme cocktails for lignocellulose breakdown, is reviewed. Particular attention is paid to rational design via computational modeling coupled with nanoscale characterization and engineering tools. Remaining challenges and potential routes to industrial application are put forward.

Measurements of single molecular affinity interactions between carbohydrate-binding modules and crystalline cellulose fibrils

Combining atomic force microscopy (AFM) recognition imaging and single molecule dynamic force spectroscopy (SMDFS), we studied the single molecule affinity interactions between the carbohydrate-binding module (CBM) and plant cell wall cellulose using the CBM3a (from Clostridium thermocellum) and CBM2a (from Cellvibrio japonicus) functionalized AFM tips. The binding efficiencies of the CBMs to the cellulose were determined by the binding areas on the crystalline cellulose fibrils surface using the recognition imaging. Several dynamic and kinetic parameters, such as the reconstructed free energy change, energy barrier and bond lifetime constant, were also obtained based on the measured single molecule unbinding forces, which are used to illuminate the affinity of the CBMs binding to the natural and single cellulose surface from a totally different aspect. It was found that CBM3a has a little higher binding efficiency and affinity than CBM2a to both natural and extracted cellulose surfaces and both the CBMs have higher affinities to the natural cell wall cellulose compared to the extracted single cellulose. The in-depth understanding of the binding mechanisms of the CBM-cellulose interactions of this study may pave the way for more efficient plant cell wall degradation and eventually facilitate biofuel production.