Shewanella oneidensis and other anaerobic gram-negative bacterial species depend on several protein types to connect them to external and in soluble minerals containing iron or manganese.
Those proteins act as wiring and deliver electrons to those minerals, which serve as extracellular respiratory electron acceptors, while the cells generate metabolic energy, according to biochemists Robert S. Hartshorne and David Richardson of the University of East Anglia, in Norwich, United Kingdom, and their collaborators.
Typically for living cells, oxygen, nitrate, sulfate, and carbon dioxide serve as electron acceptors. All those molecules are soluble, and they ordinarily diffuse within cells to wherever oxidation-reduction reactions are situated. In the case of S. oneidenis, however, part of the reaction takes place outside the cell itself. Thus, these bacteria produce two kinds of deca-heme cytochrome proteins and a third, porin-type transmembrane protein. One of the cytochrome proteins, MtrA, forms a nano-sized electrical wire inside the cell, while the other, MtrC, runs from outside the membrane to the mineral, which serves as an electron sink. The porin protein (MtrB) serves as an insulating sleeve for the other two sets of heme proteins as they bridge the cell membrane and provide a conduit for electrons. Details appear in the December 29, 2009 Proceedings of the National Academy of Sciences (106:22169-22174). S. oneidensis and other bacterial species like it are prized for their potential usefulness in bioremediation- for instance, because they can immobilize radioactive contaminants such as uranium.More recently, because of their electron-carrying wires, they are being eyed for other possible applications, including as batteries or fuel cells (see Microbe, November 2009, p. 506).
However, the "wires" that Hartshorne, Richardson, and their collaborators are studying are different from the "pili nanowires" that, according to Gemma Reguera of Michigan State University (MSU), appear critical for cell-cell communication. Although both types of microbial wires transmit electrons, those moving along pili travel relatively longer distances, which is consistent with their role in communication. How those pili proteins transfer electrons is not understood, Richardson says.
"We are addressing how electrons get out of cells to the extracellular environment," he continues, referring to the other protein wiring system. "How do electrons flow from organic carbon being catabolized inside the cell to an extracellular electron sink-a mineral?"
One part of that process depends on two proteins that "bind electrontransferring hemes, MtrA and MtrC, into an outer membrane protein porin, MtrB," Richardson says. These proteins are arranged as "a small wire containing 20 hemes" that is "around 20 nm in length . . . Electrons hop from heme to heme in this wire." Further experiments involving liposomes show that the complex composed of those three types of protein "can move electrons across a membrane," he adds. "Once outside the cell, the electrons can flow from the hemes to minerals with which the bacteria can form a physical interaction. This transfer of electrons between the cytochrome and a solid surface is . . . very fast." "We used different model systems, but insights from one can help studies of the other," says Reguera of MSU.
"We need to first study each component, cytochromes and nanowires, separately, [and] now we are in a better position to investigate potential interactions between the cytochrome complex and other extracellular components- for example, the nanowires."
The three MtrCAB proteins and the pili proteins are not related in evolutionary terms, according to Richardson. How they might relate physiologically is not yet known. "One system may be more important under one growth condition and . . . it is also possible that they interact under some conditions," he says. "For example, the MtrCAB system could potentially pass electrons to an extracellular pili nanowire that could then facilitate longer-range electron transfer, but this is not proven."
Barry E. DiGregorio
Barry E. DiGregorio is a freelance writer in Middleport, N.Y. |
Current Topics 
Facebook
del.icio.us
Digg
Furl 
Yahoo!My Web
StumbleUpon
Google
Technorati 
reddit
Twitter
