Powerful Pinene Credit: ACS Synth. Biol.

To create renewable energy sources for transportation, some scientists have engineered microbes to produce biofuels that can power cars. But commercial or military aircraft require fuels with a higher energy density. Now researchers have developed bacteria that produce pinene, which, when converted to a dimer, could fuel airplanes, rockets, and missiles (ACS Synth. Biol. 2014, DOI: 10.1021/sb4001382).

Conventional jet fuels, such as the formulation known as JP-10, contain hydrocarbons with strained ring systems that pack a high-energy punch per volume, allowing aircraft to have small fuel tanks, says Pamela Peralta-Yahya of the Georgia Institute of Technology. Recently, researchers discovered that dimers of pinene, which includes a strained four-member ring, have fuel properties similar to JP-10. Conifers and other plants produce this pine-scented molecule. “There’s no way we could grow enough trees to make the tactical fuel,” Peralta-Yahya says. “What we need is a new source of pinene.”

Plants make pinene with two key enzymes—geranyl diphosphate synthase and pinene synthase. Peralta-Yahya and her team added genes for the most active pair of proteins from six plants to Escherichia coli so that the bacteria could take their own metabolites and convert them into pinene. The engineered bacteria produced 28 mg/L of pinene, far below the yield necessary to make the process commercially viable, Peralta-Yahya says. The yield would have to be 26-times larger to reach that level.

She and her colleagues thought the reason for the low yield was that the intermediate, geranyl diphosphate, binds to another site on pinene synthase and inhibits it. To try to reduce inhibition, the researchers linked the enzymes together on a single gene. This connected the two enzymes so that geranyl diphosphate could move directly into the active site of pinene synthase and not escape to inhibit the enzyme. The linked gene increased the pinene yield to 32 mg/L—an improvement, but still below the goal.

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