High-octane biofuels — produced with the aid of newly created lines of engineered bacteria — may be in the near future thanks to new research from the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Department of Systems Biology at Harvard Medical School.
These new lines of engineered bacteria can custom-produce a variety of key precursors to high-octane biofuels, as well as precursors to commonly used pharmaceuticals, plastics, detergents, etc.
“The big contribution is that we were able to program cells to make specific fuel precursors,” said Pamela Silver, PhD, a Wyss Institute Core Faculty member, Professor of Systems Biology at Harvard Medical School, and senior author of the study.
The researchers argue that new, higher-energy biofuels are a necessity because of the inbuilt limitations of ethanol and other widely-used biofuels — ethanol only possesses roughly 2/3 of the energy that gasoline does, and is rather corrosive, degrading storage and transportation infrastructure over time.
That’s where the new research comes in — research with the goal of creating “gasoline-like biofuels that could be stored at gas stations and used to fuel the cars we already have.” In order to create such fuels, the researchers have been working with the infamous bacterium E. coli, engineering the bacterium to produce specific fatty acids that can function effectively as precursors to gasoline — those with chains between 4 and 12 carbons long.
Currently, the medium-chain-length compounds that are used for gasoline are refined from crude oil. But why not get the compounds elsewhere? “Instead of using petroleum products, you can have microbes or other living organisms do it for you,” said Silver.
The Wyss Institute gets into the specifics of how that is done:
To accomplish that, the researchers tweaked an E. coli metabolic pathway that produces fatty acids. Specifically, they mass-produced an eight-carbon fatty acid called octanoate that can be converted into octane.
In this pathway, carbon from sugar — which the bacterium eats — flows through the pathway like a river, growing longer as it flows. Downstream, it exits as a long-chain fatty acid. The researchers first partially dammed the river and built an irrigation ditch using a drug that blocks enzymes that extend fatty-acid chains. This caused medium-chain fatty acids to pool behind the dam, while still allowing enough of the river to flow by for the bacteria to build their membranes and stay alive. The strategy increased octanoate yields, but the drug is too expensive for the process to be scaled up.
For that reason, the scientists tried a second strategy that could be scaled up more readily. They let the cells grow up, then dammed the river using a genetic trick. They also genetically altered a second enzyme that normally builds long-chain fatty acids such that it extends fatty acids to eight carbons and no longer.
By using the ‘two-pronged’ second strategy — along with a couple of other ‘genetic nips and tucks’ — the researchers achieved the highest yields yet.
“We found if we stop up the river — if we slow fatty acid elongation — we encouraged the creation of medium-chain fatty acids,” stated researcher Joe Torella, a co-author of the new study that details the research.
“Sustainability is one of the biggest problems we face today, and developing potent biofuels to replace gasoline is a major challenge in the field,” stated Don Ingber, MD, PhD, Wyss Institute Founding Director. “Using ingenious synthetic-biology strategies to engineer microbes so that they can produce octane, Pam’s team has taken a giant step toward meeting this challenge.”
The researchers state that they will now be working to engineer E. coli to “convert octanoate and other fatty acids into alcohols, potential fuel molecules themselves, and just one chemical step away from octane.”
The new research was just published in the online edition of the Proceedings of the National Academy of Sciences.
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