By rerouting the metabolic pathway that makes fatty acids in E. coli bacteria, researchers at the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Department of Systems Biology at Harvard Medical School have devised a new way to make targeted precursors of high-octane biofuels. A paper on their work is published online in the Proceedings of the National Academy of Sciences.
Lines of bacteria engineered using the same strategy can also produce precursors of pharmaceuticals, bioplastics, herbicides, detergents, and more.
The big contribution is that we were able to program cells to make specific fuel precursors.—Pamela Silver, Ph.D., senior author
Silver and her team are focusing on medium-chain fatty acids (MCFA)—those with chains between four and 12 carbons long. Fatty acids with shorter chains do not store enough energy to be good fuels and they tend to vaporize easily, while those with chains longer than 12 carbons are too waxy.
Medium-chain fatty acids (MCFAs, 4–12 carbons) are valuable as precursors to industrial chemicals and biofuels, but are not canonical products of microbial fatty acid synthesis. We engineered microbial production of the full range of even- and odd-chain–length MCFAs and found that MCFA production is limited by rapid, irreversible elongation of their acyl-ACP precursors.
To address this limitation, we programmed an essential ketoacyl synthase to degrade in response to a chemical inducer, thereby slowing acyl-ACP elongation and redirecting flux from phospholipid synthesis to MCFA production. Our results show that induced protein degradation can be used to dynamically alter metabolic flux, and thereby increase the yield of a desired compound.
The strategy reported herein should be widely useful in a range of metabolic engineering applications in which essential enzymes divert flux away from a desired product, as well as in the production of polyketides, bioplastics, and other recursively synthesized hydrocarbons for which chain-length control is desired.—Torella et al.
Joe Torella, Ph.D., and Tyler Ford, Harvard Medical School Systems Biology graduate students in Silver’s laboratory and the paper’s lead coauthors, used the strategy of slowing elongation of fatty acids to tweak an E. coli metabolic pathway to mass-produce an eight-carbon fatty acid called octanoate that can be converted into octane.
Next, the scientists plan 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.
This work was funded by the Department of Energy’s Advanced Research Project Agency-Energy (ARPA-E) and by the National Science Foundation. The ARPA-E project is part of the agency’s Electrofuels program (earlier post), and received a $4,194,125 award from the agency.
The goal of Dr. Silver’s project—“Engineering a Bacterial Reverse Fuel Cell”—is to engineer a self-contained, scalable Electrofuels production system that can directly generate liquid fuels from bacteria, carbon dioxide, water, and sunlight.
As part of the project, the team is engineering bacteria to produce fuel molecules that have properties similar to gasoline or diesel fuel—making them easier to incorporate into the existing fuel infrastructure. These molecules are designed to spontaneously separate from the water-based culture that the bacteria live in and to be used directly as fuel without further chemical processing once they’re pumped out of the tank.
In addition to Silver, Torella, and Ford, the research team included Scott Kim and Amanda Chen, students on Silver’s team, and Jeffrey Way, Ph.D., a Senior Staff Scientist at the Wyss Institute.
Joseph P. Torella, Tyler J. Ford, Scott N. Kim, Amanda M. Chen, Jeffrey C. Way, and Pamela A. Silver (2013) Tailored fatty acid synthesis via dynamic control of fatty acid elongation PNAS doi: 10.1073/pnas.1307129110