New research revealed the secrets of electron travel in bacteria; these findings could help scientists gain insight into how these bacteria "do chemistry in the ground." The research could also help them better-utilize bacteria in functions such as batteries and microbial fuel cells.
Bacteria contain protein-based wires, which in turn contain molecular groups dubbed "hemes"; the hemes interact with each other, which allows electrons to "hop along the chain like stepping stones," a Pacific Northwest National Laboratories news release reported.
The team found that when the electron's "drive to hop" was high the hemes were farther apart, but when it's low the hemes are more closely connected; this allows the electrons to hop down the chain.
"We were perplexed at how weak the thermodynamic driving force was between some of these hemes," geochemist Kevin Rosso of the Department of Energy's Pacific Northwest National Laboratory, said. "But it turns out those pairs of hemes are essentially hugging each other. When the driving force is strong between hemes, they are only shaking hands. We've never seen this compensation scheme before, but it seems that the purpose is to allow the protein to transfer electrons with a steady flow along heme wires."
Bacteria use these "living wires" to steal electrons from carbon, ultimately moving them into minerals or metals in the ground. They accomplish this by sending electricity across the wires that are built into these proteins which moves "internal electrons to the outside of their cells," the news release reported.
In modern power lines electrons shoot down the wire on a stable path; it's much more complicated for electrons in living wires. The electrons must move through complicated heme groups that vary in composition.
"Some hemes hold onto electrons tightly and others let electrons slip away easily. Depending on how the hemes are lined up, this can create energetic hills that electrons have a hard time climbing over, or energetic valleys that electrons easily march across," the news release reported.
Some hemes have been well-studied in the past, but researchers only recently discovered what a protein called MtrF that makes up molecular wires looks like.
The team used "high-powered computers" to simulate heme movement in MtrF and how they work to transfer electrons down the chain.
The team found hemes arrange themselves in three different types of pairs: "perpendicular to each other, side-by-side, or stacked on top of each other," the news release reported.
The team looked at the position of 10 hemes within MtrF. They found eight of the hemes ran down the center of the protein while the other two branched off.
The team found the electrons did not move smoothly through the hemes but instead "lueched" down the chain. "Sometimes the driving force makes the electrons march across a valley and the electrons move quickly. In other pairs the electrons face a hill, and electron travel gets delayed," the news release reported.
The team found the hemes were farther apart when the electrons wished to walk hop through the valley and closer together when it was time for them to tackle the hill.
"We think the variation in driving force between the hills and the valleys helps the protein interact with other components in the environment," said Rosso. The tops of the hills could be exit points to higher energy electron acceptors in the environment, such as molecules that shuttle electrons elsewhere.
