Higher Yields of Advanced Biofuels from Genetically Engineered Yeast
The development of renewable substitutes for fuels and chemicals supplied by petroleum is an important aspect of achieving energy security. Currently, the United States annually produces more than 10 billion gallons of the biofuel ethanol from microbial fermentation of corn sugars using yeast. As this industry has matured, it has become clear that ethanol is not an ideal gasoline replacement due to its low-energy density, handling challenges, and limited compatibility with the current transportation fleet. Focus therefore has shifted to the production of advanced biofuels, designed to be “drop-in” fuels, having the same properties as gasoline, diesel, or jet fuel. Researchers at the Joint BioEnergy Institute (JBEI) recently achieved the highest ever reported yields of drop-in fuel precursors in yeast. Diesel fuels are composed mainly of long-chain hydrocarbon esters, similar to the fatty acids produced by yeast and other microorganisms for construction of their cell membranes. Overproduction of fatty acids in yeast is no easy task as elaborate regulatory and feedback systems exist to prevent excessive accumulation of these building blocks. To overcome this hurdle, the JBEI researchers replaced the highly-regulated native promoters for fatty acid production machinery with new high-intensity promoters. These promoters are effectively always “on,” directing the cell to make more fatty acid assembly machinery. The researchers also engineered cellular machinery to reroute fatty acids from cell membrane manufacture to free fatty acids that can be transformed through industrial processes to drop-in biofuels. These engineering changes led to an over 500-fold increase in production of free fatty acids when compared to the native strain. Strains also were engineered to produce drop-in biofuels directly, rerouting fatty acids into fatty alcohols and fatty acid ethyl esters that can be used in diesel engines. With these increased yields of fatty alcohols and fatty acid ethyl esters, this work represents a major advance toward production of next generation drop-in biofuels.
Runguphan, W., and J. D. Keasling. 2013. “Metabolic Engineering of Sacchromyces cerevisiae for Production of Fatty Acid-Derived Biofuels and Chemicals,” Metabolic Engineering 21, 103–13. DOI: 10.1016/j.ymben.2013.07.003.