Gonzalez and his colleagues outlined their new approach in a paper published online in the journal Nature. The researchers tapped into a pathway that microbes use to break down fatty acids, which are hydrocarbon molecules, to generate energy. They modified about a dozen genes in E. coli to reverse this beta-oxidation pathway so that the microbes build fatty acids.
The method is more efficient than others because it adds two carbon atoms at a time, rather than one, to the hydrocarbon molecules being formed. “What makes it really efficient is that the mechanism by which those two carbon atoms are added to the chain doesn’t require [energy],” Gonzalez says.
By selectively manipulating genes, the researchers can program the microbes to synthesize many different fuels and chemicals. In addition to butanol, the bacteria can produce various useful fatty acids that existing processes derive from plant and animal oils.
Because the beta-oxidation pathway is found in nearly all organisms, it could also be engineered in yeast and algae, which might make it easier for many different companies to adopt the technology, Gonzalez says. He is looking at modifying various organisms with the goal of making the process even cheaper and more efficient. Yeast, for instance, are more tolerant to ethanol and butanol, he says—but E. coli grow faster.
However, in the end, cautions Mielenz, all biobutanol processes face the crucial challenge of requiring a switch from food sources such as corn, sugarcane or beets for their sugar feedstock to cellulosic biomass, which is more expensive to convert to fuels.