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MEME Report Summary

Project ID: 280518
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Final Report Summary - MEME (Membrane-modified Electrodes to study Membrane Enzymes)

In batteries, chemical reactions at the anode and cathode (the - and + sides of the battery) create the potential (typically 1.5 V) and electrons (to create the current in Amps) that give the battery its function.

Energy generation in biology relies on similar chemistry and, for instance, in bacteria and mitochondria, chemical reactions take place that have a potential of ~1 V, while electrons flow from nutrients to oxygen. In this ERC project ‘MEME’, special ‘bio-friendly’ anodes and cathodes were designed that directly interact with enzymes that catalyse these reactions. For instance, anodes were created that interact with a bacterial enzyme that converts hydrogen, freeing electrons. Furthermore, cathodes were created that interact with a different enzyme that consumes electrons and convert oxygen to water. In principle, these two reactions (conversion of hydrogen and oxygen to create water) can generate up to 1.2 V.

In MEME, new discoveries were made about how these enzymes perform their function. Enzymes known as hydrogenase convert hydrogen, but are known to become inactive in the presence of oxygen, limiting their use as biocatalyst in hydrogen-oxygen fuel cells. Results obtained in MEME show that certain hydrogenases have a special switch that ‘disconnects’ an electron pathway when oxygen is present. We think this molecular switch is required because unwanted side reaction between hydrogenase and oxygen can generate radicals that destroy the enzyme. Importantly, our anodes kept the hydrogenase in a more biological or ‘natural’ environment and under these conditions, the sensitivity to oxygen is much less than for other hydroganes-based anodes. This finding could make our hydrogenase-anodes suitable for hydrogen-oxygen fuel cells.

In mitochondria, energy generation does not create electric currents like those that run through a copper wire. Nonetheless, electrons flow from nutrients to oxygen and energy released during these reactions is used to transport positive ions across a 5 nm thin biomembrane, thereby storing energy like a capacitor. Using our specially designed cathode, we studied this process in an enzyme belonging to the heme-copper oxidise family. By monitoring the transport reaction of single heme-copper oxidases, an unexpected behaviour was observed. We found that when the biological capacitor gets fully charged, enzymes are increasingly likely to dysfunction and ‘short circuit’, allowing the positive ions to rapidly flow backwards. We do not know yet whether this dysfunction is exactly that, a dysfunction, or whether this is an evolutionary designed switch that stops bacteria from overcharging. Besides the enzyme that converts oxygen, other enzymes that transport charges across the thin biomembrane were studied and new findings shed light on how these enzymes are able to use the energy of chemical reactions to ‘pump’ charged ions across the membrane.

Finally, some bacteria have a unique feature when they grow under anaerobic conditions (i.e., without oxygen). Instead of transferring electrons to oxygen, these bacteria have the ability to transport electrons to the outside of the bacterium and donate them, for instance, to iron-containing minerals. Microbial fuel cells exploit these bacteria to create electricity from, for instance, sewage waste water. In microbial fuel cells, the extracellular electrons from the bacteria are captured by the anodes. Using our specially designed anodes, we have studied this process for a particular bacterium called Shewanella oneidensis. Previously it was believed that electrons could flow directly from the bacterium to the anode. However, our results have shown that, instead, iron-containing chemicals are used, which shuttle electrons between bacterium and anode. This finding has important consequences for the way that microbial fuel cells can be designed and the environments they can operate in.

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United Kingdom
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