Engineering of a synthetic electron conduit in living cells. Proc Natl Acad Sci USA

Department of Chemistry and Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 10/2010; 107(45):19213-8. DOI: 10.1073/pnas.1009645107
Source: PubMed


Engineering efficient, directional electronic communication between living and nonliving systems has the potential to combine the unique characteristics of both materials for advanced biotechnological applications. However, the cell membrane is designed by nature to be an insulator, restricting the flow of charged species; therefore, introducing a biocompatible pathway for transferring electrons across the membrane without disrupting the cell is a significant challenge. Here we describe a genetic strategy to move intracellular electrons to an inorganic extracellular acceptor along a molecularly defined route. To do so, we reconstitute a portion of the extracellular electron transfer chain of Shewanella oneidensis MR-1 into the model microbe Escherichia coli. This engineered E. coli can reduce metal ions and solid metal oxides ∼8× and ∼4× faster than its parental strain. We also find that metal oxide reduction is more efficient when the extracellular electron acceptor has nanoscale dimensions. This work demonstrates that a genetic cassette can create a conduit for electronic communication from living cells to inorganic materials, and it highlights the importance of matching the size scale of the protein donors to inorganic acceptors.

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    • "MtrC copurifies with MtrAB from S. oneidensis, and the functionality of the MtrABC complex in transmembrane electron transfer was demonstrated in sealed proteoliposomes (Hartshorne et al., 2009). Further support for a role for MtrCAB in trans-membrane electron transfer comes from expression of mtrCAB in E. coli that confers a capacity for hematite reduction on the recipient organism (Jensen et al., 2010). It should be noted though that the iron-reducing activity reported was very low com- Microbe-to-mineral electron transfer 203 pared to S. oneidensis and so the question of whether MtrCAB on its own is sufficient to confer high rates on mineral iron respiration remains. "
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    ABSTRACT: Many species of bacteria can couple anaerobic growth to the respiratory reduction of insoluble minerals containing Fe(III) or Mn(III/IV). It has been suggested that in Shewanella species electrons cross the outer membrane to extracellular substrates via 'porin-cytochrome' electron transport modules. The molecular structure of an outer-membrane extracellular-facing deca-haem terminus for such a module has recently been resolved. It is debated how, once outside the cells, electrons are transferred from outer-membrane cytochromes to insoluble electron sinks. This may occur directly or by assemblies of cytochromes, perhaps functioning as 'nanowires', or via electron shuttles. Here we review recent work in this field and explore whether it allows for unification of the electron transport mechanisms supporting extracellular mineral respiration in Shewanella that may extend into other genera of Gram-negative bacteria.
    Molecular Microbiology 05/2012; 85(2):201-12. DOI:10.1111/j.1365-2958.2012.08088.x · 4.42 Impact Factor
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    • "The purified MtrABC complex can transfer electrons across a lipid bilayer following incorporation into proteoliposomes, providing direct evidence that together, MtrABC serve as an electron conduit between the periplasm of S. oneidensis MR-1 cells and its extracellular environments (Hartshorne et al., 2009). Consistent with these results, heterologous co-expression of MtrABC enables E. coli to reduce solid-phase Fe(III) oxides (Jensen et al., 2010). Furthermore, while MtrAB can form a stable complex in the absence of MtrC, an MtrBC complex cannot be isolated in the absence of MtrA. "
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    ABSTRACT: In the absence of O2 and other electron acceptors, the Gram-negative bacterium Shewanella oneidensis MR-1 can use ferric [Fe(III)] (oxy)(hydr)oxide minerals as the terminal electron acceptors for anaerobic respiration. At circumneutral pH and in the absence of strong complexing ligands, Fe(III) oxides are relatively insoluble and thus are external to the bacterial cells. S. oneidensis MR-1 has evolved the machinery (i.e., metal-reducing or Mtr pathway) for transferring electrons across the entire cell envelope to the surface of extracellular Fe(III) oxides. The protein components identified to date for the Mtr pathway include CymA, MtrA, MtrB, MtrC and OmcA. CymA is an inner-membrane tetraheme c-type cytochrome (c-Cyt) that is proposed to oxidize the quinol in the inner-membrane and transfers the released electrons to redox proteins in the periplasm. Although the periplasmic proteins receiving electrons from CymA during Fe(III) oxidation have not been identified, they are believed to relay the electrons to MtrA. A decaheme c-Cyt, MtrA is thought to be embedded in the trans outer-membrane and porin-like protein MtrB. Together, MtrAB deliver the electrons across the outer-membrane to the MtrC and OmcA on the outmost bacterial surface. Functioning as terminal reductases, the outer membrane and decaheme c-Cyts MtrC and OmcA can bind the surface of Fe(III) oxides and transfer electrons directly to these minerals. To increase their reaction rates, MtrC and OmcA can use the flavins secreted by S. oneidensis MR-1 cells as diffusible co-factors for reduction of Fe(III) oxides. MtrC and OmcA can also serve as the terminal reductases for soluble forms of Fe(III). Although our understanding of the Mtr pathway is still far from complete, it is the best characterized microbial pathway used for extracellular electron exchange. Characterizations of the Mtr pathway have made significant contributions to the molecular understanding of microbial reduction of Fe(III) oxides.
    Frontiers in Microbiology 02/2012; 3:50. DOI:10.3389/fmicb.2012.00050 · 3.99 Impact Factor
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    • "Low-cost synthesis will undoubtedly drive more " synthetic metagenomics " studies, in which libraries of homologous enzymes are synthesized and evaluated (Bayer et al., 2009). Demands for renewable energy have led synthetic biologists to construct more spatially complex pathways, such as the expression of the electron transfer apparatus in E. coli (Jensen et al., 2010). "
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    ABSTRACT: Synthetic biologists combine modular biological "parts" to create higher-order devices. Metabolic engineers construct biological "pipes" by optimizing the microbial conversion of basic substrates to desired compounds. Many scientists work at the intersection of these two philosophies, employing synthetic devices to enhance metabolic engineering efforts. These integrated approaches promise to do more than simply improve product yields; they can expand the array of products that are tractable to produce biologically. In this review, we explore the application of synthetic biology techniques to next-generation metabolic engineering challenges, as well as the emerging engineering principles for biological design.
    Metabolic Engineering 10/2011; 14(3):223-32. DOI:10.1016/j.ymben.2011.10.003 · 6.77 Impact Factor
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