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Rubber from Microbes

A plant enzyme improves the yield of renewable rubber made by bacteria.

Working with Goodyear, biotechnology company Genencor has been engineering bacteria that make isoprene–the chemical used to make tire rubber–from sugars derived from biomass. But ramping up microbial production of isoprene to such a scale that it can compete with petroleum-derived rubber has proven to be a major challenge.

Renewable rubber: This prototype tire was made using bioisoprene, a chemical produced by genetically engineered bacteria.

Yesterday at the American Chemical Society meeting in San Francisco, researchers from a Palo Alto, CA-based research division of Genencor described further modifications to the metabolic pathways of the microbes that improve the yield of bioisoprene. The company will decide on plans for building a bioisoprene pilot plant next year.

Microbes including E. coli naturally make small amounts of isoprene as part of their metabolism, but not nearly enough to be used on an industrial scale. To improve the yield, bioengineers at Genencor, which began working on bacterial systems for producing isoprene in 2007, initially made changes to two metabolic pathways that converge to create an isoprene precursor. But yields were still low because the bacteria’s existing genetic machinery takes a meandering path to create isoprene from this precursor. In the most recent results, the company added to the E. coli a plant gene coding for isoprene synthase, an enzyme that converts the precursor directly into isoprene.

Isoprene, which is a gas at room temperature, bubbles out of the cells without damaging them, then out of the fermentation broth. Genencor senior director of business development Rich Laduca says that with no refinement, this system can produce 99 percent pure isoprene gas. Purity is critical because trace contaminants can foul the catalysts used to polymerize isoprene to make synthetic rubber. Goodyear has used Genencor’s bioisoprene to make synthetic rubber, which it then used to make several prototype tires.

“We’re looking for renewable resources to reduce our dependence on foreign oil,” says Jesse Roeck, director of global materials science at Goodyear. Roeck says the isoprene work is still a research project, but that the chemical may be in tires on the market in three to five years. By weight, isoprene makes up about a quarter of the material used to make a tire. Roeck says the company is on the lookout for renewable sources for the other materials as well.

As Genencor moves toward scaling the process up at a pilot plant, Laduca says they’re still working to further improve the yields. “We’ve determined the crystal structure of the [plant] enzyme and are in the process of modifying it,” says Laduca. Genencor developed the basic bioisoprene system in the well-characterized E. coli but will now experiment with transplanting the metabolic pathways it’s developed into other bacteria that grow faster.

The U.S. market for pure isoprene today is two billion pounds per year; 60 percent of that is used in tires, and the rest is used in adhesives and specialty chemicals. Genencor also hopes to use the chemical as a building block for biofuels. Pairing two isoprene molecules makes a fuel that might be blended with gasoline; grouping three makes a chemical with characteristics suitable for blending with diesel. Isoprene could also be used as a building block for making jet fuels.

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