*5.B.3 The Geobacter* Nanowire Electron Transfer (G-NET) Family** Bacteria have the capacity to transfer electrons from cytoplasmic electron donors to extracellular substances. Geobacter and other bacteria from diverse bacterial kingdoms have this capacity. Geobacter utilizes 'nanowires' distantly related to type IV pili (fimbriae) to transfer electrons to iron oxide (Fe₂O₃; rust) to generate soluble Fe²⁺ and solid magnetite. The entire pathway of electron transfer has been elucidated (see Lovley, 2006). An electron pair is transferred from NADH to menaquinone, and then single electrons are transferred through the chain to iron oxide via an inner membrane (IM), periplasm, outer membrane (OM) pathway as follows: NADH → NADHDH (3.D.1.4.1) → Menaquinone → MacA (IM) → PpcA (periplasm) → OmcB (a trans OM protein) → OmcE (outer surface of the OM) → OmcS (outer surface of the OM) → pilin → Fe₂O₃. These proteins are all c-type cytochromes (Lovley, 2006). Extracellular reduction of uranium via Geobacter conductive pili provides a protective cellular mechanism (Cologgi et al., 2011). Shewanella nanowires display a surprising non-linear electrical transport behavior, where the voltage dependence of the conductance reveals peaks indicating discrete energy levels with higher electronic density of states (El Naggar et al., 2008). The molecular constitutes along Shewanella nanowires apparently possess intricate electronic structures that play roles in mediating transport. El-Naggar et al. (2010) have quantitated electrical transport along bacterial nanowires from S. oneidensis and showed that the process is absolutely dependent on two c-type decaheme cytochromes, MtrC and OmcA.
This family belongs to the Transmembrane One Electron Transfer Cytochrome (TM-Cyt) Superfamily.
References:
Cologgi, D.L., S. Lampa-Pastirk, A.M. Speers, S.D. Kelly, and G. Reguera. (2011). Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism. Proc. Natl. Acad. Sci. USA 108: 15248-15252.
El-Naggar, M.Y., Y.A. Gorby, W. Xia, and K.H. Nealson. (2008). The molecular density of states in bacterial nanowires. Biophys. J. 95: L10-12.
Gorby, Y.A., S. Yanina, J.S. McLean, K.M. Rosso, D. Moyles, A. Dohnalkova, T.J. Beveridge, I.S. Chang, B.H. Kim, K.S. Kim, D.E. Culley, S.B. Reed, M.F. Romine, D.A. Saffarini, E.A. Hill, L. Shi, D.A. Elias, D.W. Kennedy, G. Pinchuk, K. Watanabe, S. Ishii, B. Logan, K.H. Nealson, and J.K. Fredrickson. (2006). Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms. Proc. Natl. Acad. Sci. USA 103: 11358-11363.
Jones, S. (2006). Microbial physiology: new electricigens get wired. Nature Rev. Microbiol. 4: 642.
Londer, Y.Y., P.R. Pokkuluri, V. Orshonsky, L. Orshonsky, and M. Schiffer. (2006). Heterologous expression of dodecaheme “nanowires” cytochromes c from Geobacter sulfurreducens. Protein Expr. Purif. 47: 241-248.
Lovley, D.R. (2006). Bug juice: harvesting electricity with microorganisms. Nature Rev. Microbiol. 4: 497-508.
Reguera, G., K.D. McCarthy, T. Mehta, J.S. Nicoll, M.T. Tuominen, and D.R. Lovley. (2005). Extracellular electron transfer via microbial nanowires. Nature 435: 1098-1101.
Reguera, G., K.P. Nevin, J.S. Nicoll, S.F. Covalla, T.L. Woodard, and D.R. Lovley. (2006). Biofilm and nanowire production leads to increased current in Geobacter sulfurreducens fuel cells. Appl. Environ. Microbiol. 72: 7345-7348.
Examples:
TC#	Name	Organismal Type	Example
5.B.3.1.1	The electron transfer chain for reduction of extracellular Fe₂O₃, G-NET	Bacteria	G-NET of Geobacter sulfurreducens MacA (Q74FY6) PpcA (Q8GGK7) OmcB (Q749L1) OmcE (Q74AE4) OmcS (Q74A86)