Researchers at the Georgia Institute of Technology and the Joint BioEnergy Institute engineered a bacterium to synthesise pinene, a hydrocarbon produced by trees that could replace high-energy fuels, such as petroleum-based JP-10.
By inserting enzymes from trees into the bacterium, first author and Georgia Tech graduate student Stephen Sarria, working under the guidance of assistant professor in the School of Chemistry and Biochemistry, Pamela Peralta-Yahya, boosted pinene production six-fold over earlier bioengineering efforts.
"We have made a sustainable precursor to a tactical fuel with a high energy density," said Peralta-Yahya.
"We are concentrating on making a 'drop-in' fuel that looks just like what is being produced from petroleum and can fit into existing distribution systems," said Peralta-Yahya.
Fuels with high energy densities are important in applications where minimising fuel weight is important. The gasoline used to power automobiles and the diesel used mainly in trucks both contain less energy per litre than the JP-10.
The amount of JP-10 that can be extracted from each barrel of oil is limited, and sources of potentially comparable compounds such as trees can't provide much help.
The limited supply drives the price of JP-10 to around USD 25 per gallon.
"If you are trying to make an alternative to gasoline, you are competing against USD 3 per gallon," Peralta-Yahya noted.
"That requires a long optimisation process. Our process will be competitive with USD 25 per gallon in a much shorter time," she said.
Peralta-Yahya and collaborators studied enzymes that could be inserted into the E coli bacterium and chose two classes of enzymes - three pinene synthases (PS) and three geranyl diphosphate synthases (GPPS).
Their results were much better than earlier efforts, but the researchers were puzzled because for a different hydrocarbon, similar enzymes produced more fuel per litre.
So they placed the two enzymes adjacent to one another in the E coli cells, ensuring that molecules produced by one enzyme would immediately contact the other.
That boosted their production to 32 milligrammes per litre - much better than earlier efforts, but still not competitive with petroleum-based JP-10.
"We found that the enzyme was being inhibited by the substrate, and that the inhibition was concentration-dependent," Peralta-Yahya said.
"Now we need either an enzyme that is not inhibited at high substrate concentrations, or we need a pathway that is able to maintain low substrate concentrations throughout the run. Both of these are difficult, but not insurmountable, problems," she said.