MtrF crystals held in suspension. Crystals have approximate dimensions of 135 micrometres square.
Credit: PNAS
LONDON: The molecular structure of proteins that allow bacteria to release an electrical charge has been revealed for the first time by researchers in the UK.
These results, published in the Proceedings of the National Academy of Sciences, could lead to the development of new energy technologies, such as microbial fuel cells - or 'bio-batteries' - and microbe-based agents that can clean up toxic pollution.
"Identifying the precise molecular structure of the key proteins involved in this process is a crucial step towards tapping into microbes as a viable source of electricity," said lead author Tom Clarke of the University of East Anglia's School of Biological Sciences in Norwich.
Rock-breathing bacteria
Like other cells, bacteria use electrons taken in from food to generate energy, but must ensure that these electrons are subsequently removed.
Shewanella oneidensis is a bacterium that can utilise oxygen to respire aerobically, but if in an environment where oxygen is scarce can release electrons by attaching them to nearby minerals, such as Iron and Manganese oxides found in rocks.
"Three different possible mechanisms for this electron transfer between bacteria and mineral substrates have previously been suggested; direct physical transfer, an indirect mediated system using 'electron shuttles', or by transfer along conductive filaments," said Clarke.
However prior to this study, little was known about the molecular structure of the membrane proteins, or 'cytochromes,' which carry out this electron transfer.
The devil is in the detail
In this latest research, Clarke and his colleagues used a technique called x-ray crystallography to reveal the molecular structure of one particular cytochrome - called MtrF - which sits on the outer surface of Shewanella oneidensis and is part of a larger electron transfer complex.
"This particular protein represents the electric terminal on the outer surface of a bacterial cell, and by learning more about it we can improve our understanding of how the bacteria connect to minerals and transfer electrical charge," said Clarke.
The crystal structure of the MtrF cytochrome was solved to a resolution high enough for the researchers to confidently reveal its molecular arrangement and identify mechanisms for both direct and 'electron shuttle' based electron transfer.
"This proved quite a difficult protein to solve, and it has taken over three years of work to finally reveal its molecular structure," said Clarke.
'Bio-batteries' on the cards
The discovery means that scientists can now start developing ways to 'tether' bacteria directly to electrodes - creating efficient microbial fuel cells or 'bio-batteries'. The advance could also speed up the development of microbe-based agents that can clean up oil or uranium pollution, and fuel cells powered by human or animal waste.
"I'm excited about the implications our bacterial research could have in these wider areas," added Clarke.
"For years, studies have suggested Shewanella use both direct contact and electron shuttle strategies to transfer their electrons," said Daniel Bond, associate professor in the Department of Microbiology at the University of Minnesota, who was not involved in the study.
"Remarkably, this structure provides a molecular basis for both these mechanisms…. It's no surprise that an opportunist always ready to take advantage of multiple electron acceptors has a cytochrome that is equally versatile."
