4-02: Using cellulose without complete hydrolysis: a new biofuel production paradigm

The prevailing biomass technology relies on a complete hydrolysis of cellulose to its monomer sugars before the fermentation step, entailing large amounts of enzymes.  The cost of enzymes and slow hydrolysis step are widely recognized as important barriers for biofuel production. We have developed novel microbial catalysts capable of utilizing cellulose partial hydrolysis products, cellobiose or larger glucose oligomers (collectively known as cellodextrin). These microbial strains transport cellodextrin inside the cells, where they are subsequently metabolized and converted to biofuel. The new production scheme has several important advantages. 1.) Directly use of cellodextrin eliminates the need for complete hydrolysis. As a result, exogenous beta-glucosidase is no longer needed, generating significant cost savings.  2.) Once inside the cells, cellodextrin can be metabolized via a more energetically favorable mechanism, phosphorolysis. 3.)  Transporting sugar oligomers inside the cells for intracellular metabolism allows bypassing the catabolite repression. Consequently, glucose oligomers and xylose can be simultaneously converted biofuel molecules, reducing the fermentation time thereby enhancing productivities. 4.)  The new technology reduces the burden of contamination as extracellular glucose concentration can be kept low.

In this presentation, we present a prototype E. coli strain to demonstrate the key advantages of the new paradigm. Ethanol-producing strains were additionally engineered to express newly discovered cellodextrin transporters and a hydrolase or a phosphosrylase. The resulting strains were able to use cellodextrin in M9 medium as sole carbon source. Simultaneous conversion of cellobiose and xylose to ethanol allowed a much higher productivity than a sequential use of sugar mixture.