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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.
Credit: Pacific Northwest National Laboratory
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Now, scientists for the first time have
observed this electricity-shuttling process taking place sans cells,
in purified proteins removed from the outer membrane of the versatile,
metal-altering soil bacterium Shewanella oneidensis. Reporting in the
current advance online edition of the Journal of the American Chemical
Society, they suggest that proteins rendered portable from the
organisms that spawned them could make miniature bioreactor cells
feasible.
"We show 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, senior author and lab fellow at the Department of
Energy's Pacific Northwest National Laboratory.
The feat is the bacterial equivalent of removing lungs and coaxing the
disembodied tissue to breathe.
Squier and principal authors Yijia Xiong and Liang Shi, PNNL staff
scientists, discovered that the proteins, outer membrane c-type
cytochrome A, or OmcA, formed a dense coating on the iron-rich mineral
hematite. The metal in the mineral acts as an "acceptor," or dumping
point, for thousands of trillions of electrons per square centimeter
shuttled by the OmcA-donor. The function is a relic of respiration, in
which the cell depends on the protein to dump electrons to maintain a
steady flow of energy and prevent the organism-damaging accumulation
of electrons.
PNNL staff scientist and co-author Uljana Mayer devised new tagging
methods that enabled the team to isolate sufficient amounts of protein.
The tags also allowed fast measurements of protein-mineral binding.
The researchers supplied the protein with energy--directly as
electrons or in the form of a natural cellular fuel called NADH--and
only during binding detected charge-transfer from protein to mineral,
through a combination of techniques that included FCS, or fluorescent
correlation spectroscopy, and confocal microscopy. These 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.
How bright?
"The peak current, or flux, doesn't run long, just a few seconds,"
Squier said, "but flux is at least as good as what you would find in
the most efficient bioreactors, which rely on living bacteria."
Biological fuel cells, or biofuel cells, are not yet powerful enough
to be commercially viable but they offer the promise of breaking down
sewage and other biological waste while generating electricity
directly from the same process. An example, Squier said, is a
self-powering sewage treatment plant.
Using pure protein opens up the possibility of shrinking biofuel cells
to power small electronic devices, Squier said. 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, the latter being the electrode
that carries the electricity to the device. In purified protein fuel
cells, the seal made by the protein coating on the electrode
effectively acts in place of the membrane.
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