Optimizing microbial fuel cells
Oeiras, 14.06.11
Bruno Fonseca, PhD student at the Inorganic Biochemistry and NMR lab, was awarded the prize for the best poster at the 3rd International Microbial Fuel Cell Conference in Leeuwarden, The Netherlands.
The poster communication was entitled Characterization of multiheme cytochromes from Sheanella oneidensis MR-1: A key step for the optimization of Microbial Fuel Cells.
Poster abstract
Characterization of multi-heme cytochromes from Shewanella oneidensis MR-1: A key step for the optimization of Microbial Fuel Cells
Bruno M. Fonseca, Catarina M. Paquete, Alexandra S. Alves & Ricardo O. Louro
Instituto de Tecnologia Química e Biológica – Universidade Nova de Lisboa
Shewanella oneidensis MR-1 gained prominence by being the first organism shown to be capable of powering a Microbial Fuel Cell (MFC) without the need of mediators. Direct electron transfer between Shewanella cells and the extracellular substrates, such as metal oxides and anodes of the MFCs, involves c-type multi-heme cytochromes. These cytochromes allow the electron transfer through the periplasmic space and outer-membrane onto the anodo of the MFC.
Several of the 42 putative cytochromes c encoded in the genome, have been implicated in this process. The inner-membrane tetraheme cytochrome c known as CymA works as a redox hub receiving electrons from the menaquinone pool and distributing them to various other cytochromes in the periplasmic space. In the periplasm there are various candidates involved in the electron transfer to the electrode. The small tetraheme cytochrome c (STC) is highly abundant in the periplasm and has been connected to extracellular respiration via MtrDEF. The decaheme cytochromes MtrA and MtrD, both periplasmic, seem to receive electrons from CymA and transfer them to the outer-membrane cytochromes, through a channel in the membrane made by β-barrel proteins called MtrB and MtrE, respectively. These β-barrel proteins are essential for the correct localization of the outer-membrane associated decaheme cytochromes, MtrC and OmcA, and MtrF. All of the three outer-membrane cytochromes MtrC, OmcA and MtrF have been shown to be exposed to the exterior, which has made them of extreme interest as plausible terminal reductases, for the MFCs anodes. Recent studies have additionally shown that MtrC is responsible for most of the electron transfer to carbon electrodes, while OmcA is involved in cellular attachment to solid surfaces and plays a smaller role in electron transfer. MtrF, like MtrC, also displays the ability to reduce the MFC anodes with a high rate.
Thermodynamic and kinetic characterization of the STC from Shewanella has been performed over a wide pH range (5.5 – 9.0), using NMR, Stopped-flow and UV-Visible spectroscopy. The results demonstrate that the thermodynamic and the kinetic properties of the redox centers in STC lead to functional specificity and directional electron transfer within the protein. Also in order to scrutinize the functional properties of the more complex decaheme cytochromes, such as MtrA, novel methods using site-directed mutagenesis and isotopic labeling of the protein have been developed to complement the previous techniques.
The biochemical characterization of these multi-heme cytochromes c will allow, not only a better understanding on how these proteins function and interact in the cell but also enhance the way MFCs involving Shewanella species are presently built.
