Biological wires carry electricity thanks to special amino acids

March 12, 2013
The bacterium Geobacter sulfurreducens conducts electricity very effectively along metallic-like pili dubbed "microbial nanowires" by discoverers at UMass Amherst. They recently determined the impact of this conductivity on the biology of the bacterium, confirming an assumption in their earlier work that real metallic-like conductivity is taking place. Credit: UMass Amherst

Slender bacterial nanowires require certain key amino acids in order to conduct electricity, according to a study to be published in mBio, the online open-access journal of the American Society for Microbiology, on Tuesday, March 12.

In nature, the Geobacter sulfurreducens uses these , called pili, to transport electrons to remote or other , but the benefits of these wires can also be harnessed by humans for use in fuel cells or bioelectronics. The study in mBio reveals that a core of aromatic amino acids are required to turn these hair-like into functioning electron-carrying biological wires.

"It's the aromatic amino acids that make it a wire," says lead author Derek Lovley of the University of Massachusetts, Amherst. Lovley and his colleagues removed the pivotal amino acids from the pili and replaced them with smaller, non-aromatic amino acids. Without these key components, Lovley says, the pili are nothing more than protein strings. "We showed it's not good enough to just make the string - you've got to make a wire," says Lovley.

G. sulfurreducens "breathes" by removing electrons from organic materials and funneling them to iron oxides or to other microorganisms, much the way humans pull electrons out of in food and dump them on oxygen. The bacteria use their pili to reach out to iron or other microbes, transferring the "waste" electrons along the structure to the destination. Geobacter's pili are only 3-5 nanometers wide, but they can be 20 micrometers long, many times longer than the cell itself.

Trafficking in electrons is how all living things breathe, but it is normally carried out by discrete proteins or other molecules that act like containers for shuttling electrons from one place to another. Lovley says earlier results showed the pili in G. sulfurreducens possess metallic-like conductivity, the ability to carry electrons along a continuous structure, a controversial finding in biology.

To investigate how pili accomplish this singular feat, Lovley says they looked to non-biological organic materials that can conduct electricity. "In those synthetic materials, it's aromatic compounds that are responsible for the conductivity. We hypothesized that maybe it's similar in the Geobacter pili. In this case, it would be aromatic amino acids." Aromatic compounds have a highly stable ring-shaped structure made of carbon atoms.

Turning to the pili, Lovley says his group looked for aromatic amino acids in the parts of the pili proteins that would most likely contribute to the . Using genetic techniques, they developed a strain of Geobacter that makes pili that lack aromatic amino acids in these key regions, then they tested whether these pili could still conduct electricity. They could not. Removing the aromatic amino acids was a bit like taking the copper out of a plastic-covered electrical wire: no copper means no current, and all you're left with is a string.

Removing aromatic amino acids from the pili prevents the bacteria from reducing iron, too, says Lovley, an important point because it adds further proof that Geobacter uses its pili as nanowires for carrying electrons to support respiration.

Metal reducers like Geobacter show a lot of promise for use in fuel cells, says Lovley, and by feeding electrons to the microbes that produce the methane, they're an important component of anaerobic digesters that produce methane gas from waste products. Understanding how they shuttle their electrons around and how to optimize the way the pili function could lead to better technologies.

Moving forward, Lovley says his own lab plans to explore the possibilities of biological nanowires, exploring how to make them more or less conductive.

Explore further: Researchers Develop New Geobacter Microbe Strain to Produce More Electricity, Open New Applications

Related Stories

Making more efficient fuel cells

September 7, 2009

Bacteria that generate significant amounts of electricity could be used in microbial fuel cells to provide power in remote environments or to convert waste to electricity. Professor Derek Lovley from the University of Massachusetts, ...

Researchers discover how bacteria can immobilize uranium

September 6, 2011

(PhysOrg.com) -- For several years, researchers have known that certain kinds of bacteria are able to "feed" off certain metals by either adding or removing electrons from their structure, but until now, haven’t really ...

Recommended for you

Genomes uncover life's early history

August 24, 2015

A University of Manchester scientist is part of a team which has carried out one of the biggest ever analyses of genomes on life of all forms.

Rare nautilus sighted for the first time in three decades

August 25, 2015

In early August, biologist Peter Ward returned from the South Pacific with news that he encountered an old friend, one he hadn't seen in over three decades. The University of Washington professor had seen what he considers ...

Why a mutant rice called Big Grain1 yields such big grains

August 24, 2015

(Phys.org)—Rice is one of the most important staple crops grown by humans—very possibly the most important in history. With 4.3 billion inhabitants, Asia is home to 60 percent of the world's population, so it's unsurprising ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.