Proteins keep cells humming. Some are enzymes that taxi electrons to chemicals outside the cell, to discharge excess energy generated during metabolism. This maintains energy flow in the cell and, in turn, keeps the cell alive.
The process has worked a little like that slogan for a hyper-electrified desert gambling town: What happens in the cellular environs stays there.
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-
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Source:DOE/Pacific Northwest National Laboratory
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