Cellobionic acid conversion to the drop-in biofuel candidate, isobutanol

Converting lignocellulosics into biofuels remains a promising route for biofuel production. To facilitate bacterial strain development for optimal cellulosic biofuel production, the user friendly Escherichia coli host was engineered to produce isobutanol, a drop-in biofuel candidate, from lignocellulosic derived substrates: cellobionic acid and cellobiose. Cellobionic acid is a novel substrate of increasing interest in consolidated bioprocessing systems because it causes less feedback inhibition on cellulases than cellobiose. Additionally, a recently identified class of enzymes, lytic polysaccharide monooygenases (LPMOs), are found to be essential in lignocellulosic degradation and primarily produce aldonic acids such as cellobionic acid. Therefore, microbial conversion of cellobionic acid to renewable chemicals will be desired in future consolidated bioprocessing systems in order to increase product yield from lignocellulosics. In this study, E. coli was given the capacity to convert cellobiose (a common lignocellulosic component) to isobutanol by extracellular expression of a beta-glucosidase via either excretion into the media, or anchoring to the cell membrane. The excretion system allowed E. coli to grow with cellobiose as the sole carbon source. The system was combined with isobutanol production genes to demonstrate successful isobutanol production. Next, we converted the system to utilize cellobionic acid, and isobutanol production from cellobionic acid was also achieved. Results obtained from this study provide an example for renewable isobutanol production from two different lignocellulosic derived substrates that have potential for industrial scale production.