A Fragrant New Candidate For Biofuel

A  class of chemical compounds used for flavor and fragrance may one day become a clean, renewable resource with which to fuel our automobiles. U.S. Department of Energy researchers have modified the E. Coli bacteria to create large quantities of methyl ketone from glucose. First tests of this methyl ketone show very high cetane numbers. Cetane is a fuel rating system for diesel fuel, similar to octane ratings for gasoline. This makes the methyl ketones a viable candidate for production of advanced biofuels, according to researchers.
“Our findings add to the list of naturally occurring chemical compounds that could serve as biofuels, which means more flexibility and options for the biofuels industry,” says Harry Beller, a Joint BioEnergy Institute microbiologist who led this study. “We´re especially encouraged by our finding that it is possible to increase the methyl ketone titer production of E. coli more than 4,000-fold with a relatively small number of genetic modifications.”
Beller is a corresponding author of a paper describing his work and findings. “Engineering of Bacterial Methyl Ketone Synthesis for Biofuels” was published in the journal Applied Environmental Microbiology  and was co-authored by first author Ee-Been Goh, Edward Baidoo, and Jay Keasling.
Scientists are looking very closely at advanced biofuels as a potential replacement for gasoline, diesel, and jet fuel. Advanced biofuels are derived from non-food plants and other forms of agricultural waste, therefore presenting a clean and renewable resource. Synthesizing these biofuels presents another promising candidate as a fuel replacement. In previous research, Beller and his team were able to use E. Coli to synthesize diesel fuel.
“In those studies, we noticed that bacteria engineered to produce unnaturally high levels of fatty acids also produced some methyl ketones,” Beller says. “When we tested the cetane numbers of these ketones and saw that they were quite favorable, we were prompted to look more closely at developing methyl ketones as biofuels.”
Methyl Ketones were discovered more than a century ago in rue, an aromatic evergreen herb. Today the ketones are widely used as a fragrance for essential oils and cheeses. Beller and his colleagues were able to create large enough quantities of these ketones using the same means of synthesis used to engineer fatty acid-producing E. Coli.
“For methyl ketone production, we made two major modifications to E. coli,” Beller says. “First we modified specific steps in beta-oxidation, the metabolic pathway that E. coli uses to break down fatty acids, and then we increased the expression of a native E. coli protein called FadM. These two modifications combined to greatly enhance the production of methyl ketones.”
Beller and his team tested two types of methyl ketone to determine cetane numbers. In the United States, the minimum cetane diesel fuel must have is 40. Using a combination of both types of ketones, Beller´s team was able to achieve a number as low as 58.4. While this number is quite impressive, the team still have concerns that the melting point of these ketones is still too high to be a completely viable option for a cold-temperature fuels.
The next step for Beller and his team is increase production of the ketones as well as improve their fuel properties.

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