Fuel synthesis: Capturing the energy content of sugars
Sugars derived from starch-based biomass such as corn are readily fermented to ethanol, as they are present in nearly pure solutions. In contrast, lignocellulose deconstruction results in both five- and six-carbon sugars together with a number of inhibitory compounds, including organic acids, furan derivatives, phenolics and inorganics. Hardwoods and agricultural residues contain 5-25% pentose sugars, primarily xylose and arabinose. These are not fermented to ethanol by the most commonly used yeast Saccharomyces cerevisiae. Anaerobic bacteria ferment pentose sugars, but are typically inhibited by low concentrations of ethanol and other by-products from deconstruction. Filamentous fungi are able to tolerate these inhibitors, but grow and produce ethanol too slowly to be commercially attractive. The challenge in the biofuels production division of JBEI is thus to convert all the monomer sugars (hexoses and pentoses) released from depolymerization of lignocellulosic biomass into transportation fuels and other chemicals. Accomplishing this objective will first rely on developing and improving fuel production systems in selected model microorganisms: the bacterium Escherichia coli, the yeast Saccharomyces cerevisiae, and the thermoacidophilic archaeon Sulfolobus solfartaricus. JBEI is initially employing E. coli and S. cerevisiae strains that have been previously engineered to produce ethanol from five- and six-carbon sugars. All three hosts will be engineered to improve their tolerance to by-products formed during biomass processing and to high concentrations of ethanol and other fuel products. JBEI will develop biochemical synthesis pathways for production of a range of other candidate fuel molecules and chemicals that are currently based on petroleum feedstocks. We will construct and validate these pathways in E. coli and then, when functional, introduce theminto S. cerevisiae and S. solfataricus. JBEI is initially targeting five existing or proposed fuel molecules: ethanol, butanol, isopentanol, hexadecane, and geranyl decanoate ester.
