As part of a larger $2.2-million ARPA-E funded project on direct solar hydrocarbon fuels (earlier post), University of Minnesota graduate student Janice Frias (who earned her doctorate in January) has determined the biosynthetic mechanism by which a protein (OleA) transforms fatty acids produced by bacteria into ketones, which can be cracked to make hydrocarbon fuels. The university is filing patents on the process.

Direct solar fuel technologies typically utilize sunlight as an ingredient in chemical reactions, alongside microorganisms, to produce fuels such as liquid hydrocarbon. The University of Minnesota has already shown that naturally-occurring Shewanella bacteria produce hydrocarbons and are tolerant to the same. The ARPA-E project, led by professor Lawrence Wackett, aims to engineer Shewanella bacteria to produce higher levels of hydrocarbons from carbon dioxide.

The research is also exploring innovative bio-production methodologies to allow continuous harvesting of hydrocarbons, which would generate significant cost savings compared to traditional batch fermentation. The hydrocarbon feedstock generated by this approach will be chemically processed using knowledge obtained from petroleum refining.

The enzyme Frias studied normally works in concert with other enzymes to make hydrocarbons, although the mechanism was unclear. In a paper published in the Journal of Biological Chemistry, Frias et al. (Dr. Wackett is senior author) presents data supporting a theory of how the mechanism for that olefin biosynthetic pathway works.

OleA catalyzes the condensation of fatty acyl groups in the first step of bacterial long-chain olefin biosynthesis but the mechanism of the condensation reaction is controversial. In this study, OleA from Xanthomonas campestris was expressed in Escherichia coli and purified to homogeneity.

…In total, these data are consistent with OleA catalyzing a non-decarboxylative Claisen condensation reaction in the first step of the olefin biosynthetic pathway previously found to be present in at least 70 different bacterial strains.

—Frias et al.

In the study, Frias found that engineered in bacteria by itself, OleA makes ketones (more precisely, β-keto acids are the initial products and those decarboxylate quantitatively to the corresponding ketone.) Ketones are mostly hydrogen and carbon with one oxygen; they are compounds with the structure RC(=O)R’, where R and R’ can be a variety of atoms and groups of atoms, and feature a carbonyl group (C=O) bonded to two other carbon atoms. Some ketones could be used directly as fuels. Others can be cracked by catalysts to make a fuel.

Aditya Bhan and Lanny Schmidt, chemical engineering professors in the University of Minnesota College of Science and Engineering, are turning the ketones into diesel fuel using catalytic technology they have developed. (Bhan and Schmidt are also part of the ARPA-E project, investigating the conversion of the bio-hydrocarbons into gasoline and diesel fuels.)

There is enormous interest in using carbon dioxide to make hydrocarbon fuels. CO2 is the major greenhouse gas mediating global climate change, so removing it from the atmosphere is good for the environment. It’s also free. And we can use the same infrastructure to process and transport this new hydrocarbon fuel that we use for fossil fuels.

—Lawrence Wackett

Resources

  • Janice A. Frias, Jack E. Richman, Jasmine S. Erickson and Lawrence P. Wackett (2011) Purification and characterization of OleA from Xanthomonas campestris and demonstration of a non-decarboxylative Claisen condensation reaction. Journal of Biological Chemistry doi: 10.1074/jbc.M110.216127


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