New understanding of one of nature’s best biocatalysts for biofuels production
Producing fuels from plants and other renewable sources requires breaking down the chemical cellulose; a major candidate to drive, or catalyze, this stubborn chemical is a ubiquitous microorganism called Clostridium thermocellum that works well in hot environments without oxygen. Researchers found that C. thermocellum uses a previously unknown mechanism to degrade cellulose, in addition to other known degradation mechanisms. This discovery helps explain C. thermocellum’s superior ability to digest biomass and demonstrates the highly diverse strategies evolved in nature for biomass conversion. Researchers are using the study’s findings to develop optimal systems for breaking down plant matter to produce biofuels and biobased chemicals. Lignocellulosic biomass is the largest source of organic matter on Earth, making it a promising renewable feedstock for producing biofuels and chemicals. Currently, however, the main bottleneck in biofuel production is the low efficiency of cellulose conversion, which leads to high production costs. To date, C. thermocellum is the most efficient microorganism known for solubilizing lignocellulosic biomass. Its high cellulose digestion capability has been attributed to the organism’s efficient cellulases consisting of both a free enzyme system and a tethered cellulosomal system, where multiple carbohydrate active enzymes are organized by primary and secondary scaffolding proteins to generate large protein complexes attached to the bacterial cell wall. Recently, U.S. Department of Energy BioEnergy Science Center (BESC) researchers discovered that C. thermocellum also expresses a type of cellulosomal system that is not bound to the cell wall, a “cell‐free” cellulosomal system. Researchers believe the cell‐free cellulosome complex functions as a “long-range” cellulosome because it can diffuse away from the cell and degrade polysaccharide substrates distant from the bacterial cells. This discovery reveals that C. thermocellum utilizes not only all the previously known cellulase degradation mechanisms (cellulosomes and free enzymes), but also a new category of scaffolded enzymes not attached to the cell. This unexpected finding explains C. thermocellum’s superior performance on biomass, demonstrating that nature’s strategies for biomass conversion are not yet fully understood and could provide further opportunities for microbial enzyme discovery and engineering efforts. This work was supported by the BioEnergy Science Center (BESC). BESC is a U.S. Department of Energy (DOE) Bioenergy Research Center supported by the Office of Biological and Environmental Research within DOE’s Office of Science. A portion of this work also was supported by the United States – Israel Binational Science Foundation, Jerusalem, Israel; Israel Science Foundation (ISF; grant number 1349), Israeli Center of Research Excellence (I – CORE; number 152/11); European Union NMP.2013.1.1 – 2: CellulosomePlus Project 8 number 604530; and the ERA – IB Consortium (EIB.12.022) FiberFuel.
Xu, Q., et al. Dramatic performance of Clostridium thermocellum explained by its wide range of cellulase modalities. Science Advances, 2016DOI: 10.1126/sciadv.1501254 Source: U.S. Department of Energy