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.