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Bacteria Make Better Alcohol Fuels

Modified E. coli produce long-chain alcohol fuels that have advantages over ethanol and butanol.
December 9, 2008

By engineering the metabolic process of the common E. coli bacteria, researchers at the University of California, Los Angeles (UCLA), have coaxed the microorganism into churning out useful long-chain alcohols that have potential as new biofuels. The bacteria-produced biofuels have between five and eight carbon atoms, compared with ethanol, which has two carbons.

Protein boost: Genes inserted into E. coli bacteria encode proteins (ribbon- and string-shaped structures) that convert an amino acid precursor (green and red molecule) into alcohols that could be used as biofuels.

The higher number of carbon atoms gives the biofuels as much energy per gallon as gasoline; by comparison, ethanol has 30 percent less energy than gasoline. And unlike ethanol, the new biofuels are compatible with today’s gasoline infrastructure, says James Liao, a UCLA chemical- and biomolecular-engineering professor, who headed the research. Since the long-chain alcohols do not absorb water as easily as ethanol, they could be transported around the country in existing petroleum pipelines.

The longer-chain alcohols also have an advantage over butanol, another alcohol-based biofuel, Liao says. The long-chain alcohols separate from water much more readily than butanol does, so they would not need energy-intensive distillation. Many companies, including DuPont and BP, are trying to commercialize a process to make the four-carbon alcohol butanol using microbes. Liao’s group has also engineered bugs that make butanol, and its technology has been licensed by Pasadena, CA, startup Gevo.

Liao and his colleagues use synthetic-biology tools to tinker with the amino acid metabolism of E. coli. All organisms produce a large number of amino acids, which are the building blocks of proteins. The researchers reengineer this metabolic pathway so that toward the end, the precursor compounds that would normally get converted into amino acids instead turn into long-chain alcohols.

To do this, the researchers insert genes into the bacteria that make them produce unnaturally long amino acid precursor molecules that have more than six carbon atoms. They also engineer two genes–one from a type of yeast, one from a cheese-making bacterium–into the microbe. These modified genes produce two new proteins that can convert the precursors into five-to-eight-carbon alcohols.

Startups LS9 and Amyris Biotechnologies are already reengineering microbes to produce hydrocarbon fuels. Both plan to begin commercial production of their fuels by 2010.

As is the case with the new work, both LS9 and Amyris use synthetic biology, rewiring the metabolic systems of microbes by inserting genes from other organisms, redesigning known genes, and altering the expressions of proteins. But the approaches of Liao, LS9, and Amyris all target a different type of metabolic pathway. LS9 researchers have reengineered the fatty acid metabolism of E. coli, while Amyris is tinkering with the pathways that produce natural compounds known as isoprenoids.

Liao says that the amino acid pathway could have a slight advantage. It is naturally more active in bacteria, so toying with it could be more productive. “We think this is intrinsically a more efficient way to make these compounds,” he says. “So potentially, we’ll have a higher yield.”

The new long-chain alcohol fuel has grabbed the interest of companies, according to Liao. But there is still a long road ahead. One big challenge to overcome might be the long-chain alcohols’ toxicity to the bacteria, says Chris Somerville, director of the Energy Biosciences Institute at the University of California, Berkeley. Ethanol is deadly to microbes at a concentration of around 14 percent. Butanol is even more toxic, killing microbes at about 2 percent concentration. This toxicity is one of the major problems facing butanol processes. Making a product that is relatively nontoxic to the culture, says Somerville, “is really important in getting the yield up.”

Liao does not think that toxicity will be a show stopper. He says that the bacteria could be engineered to make them more alcohol tolerant. But, he says, increasing the yield will be in the hands of the company that licenses the new technology.

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