Cytochrome studies provide biofuel cell potential

Artist’s depiction of purified, electrified bacterial cell outer membrane protein binding with and passing electrons to the iron-rich mineral hematite. In this purified-protein fuel cell, the seal made by the protein coating on the electrode effectively acts in place of a membrane necessary in whole-organism biofuel cells. Eliminating the membrane could aid the design of bioreactors to power small electronic devices.

Researchers from Pacific Northwest National Laboratory and collaborators have purified the protein called outer membrane cytochrome A (OmcA) from Shewanella oneidensis, a bacterium with promise for bioremediation of contaminants and the design of microbial fuel cells. They have measured its ability to bind and transfer electrons to mineral hematite, a solid ferric oxide. The team has shown that purified OmcA can directly reduce solid metals and that purified proteins are a next step in biofuel cell development.

The work appeared in the November 1, 2006, issue of the Journal of the American Chemical Society. “We showed that you can directly transfer electrons to a mineral using a purified protein, and I don’t think anyone had shown that before,” said Thomas Squier, PNNL senior author and Laboratory Fellow. The feat is the bacterial equivalent of removing lungs and coaxing the disembodied tissue to breathe.

Bio-cells use enzymes to oxidize reduced materials in the cells and release electrons. An optimal enzyme system would directly transfer electrons captured from a chemical reaction to an electrode surface— ideally, to use enzymes immobilized on the surface of an electrode. Lack of a membrane permits miniaturization for use in biomedicine or other remote applications where self-sustaining systems are needed.

PNNL staff scientist Liang Shi devised a new protein expression system that enabled the team to isolate sufficient amounts of protein. The researchers used techniques including fluorescent correlation spectroscopy and confocal microscopy to measure protein-mineral binding. These measurements yielded a “fluorescence intensity trace” whose brightness depended entirely on whether hematite was available to bind with OmcA in solution. No hematite, dim; hematite, bright. In addition to binding, they also detected direct electron transfer from OmcA to the hematite.

Using pure protein opens up the possibility of shrinking biofuel cells to power small electronic devices, according to Squier. Whole-organism biofuel cells require engineers to design a space-adding membrane that prevents unwanted reactions between fuel, the charge-transporting agent and the electron-accepting metal that carries electricity to the device. In purified protein fuel cells, the seal made by the protein coating on the electrode acts in place of the membrane.

The work was performed at the Environmental Molecular Sciences Laboratory as part of the Department of Energy’s Genomics: GTL and Biogeochemistry Grand Challenge programs.