Bacterial Nanowires of Shewanella Oneidensis MR-1 are Outer Membrane and Periplasmic Extensions of the Extracellular Electron Transport Components

AbstractExtracellular electron transport (EET) occurs in environmental iron-reducing bacteria and is mediated by an outer membrane multi-heme cytochrome complex (Cyts). It has critical implications for global mineral cycling and electrochemical microbial catalysis. The rate of EET mediated by multiple heme redox centers significantly increases in the presence of flavins and quinones. Their electron free energy does not entirely account for the fact that differential effects on EET rate enhancement vary significantly by factors ≥100. Here, we report on whole-cell electrochemical analysis ofShewanella oneidensisMR-1 using six flavin analogs and four quinones. We demonstrated that protonation of the nitrogen atom at position 5 (N5) of the isoalloxazine ring is essential for electron outflow acceleration as a bound non-covalent cofactor of Cyts. EET mediated by Cyts was accelerated at a rate dependent on pKa(N5). The EET rate largely decreased in response to the addition of deuterated water (D2O), while low concentration of D2O (4 %) had little impact on electron free energy difference of the heme and non-covalent bound cofactors, strongly suggesting that the protonation of N5 limits the rate of EET. Our findings directly link EET kinetics to proton transport reaction via N5 and provide a basis for the development of novel strategies for controlling EET-associated biological reactions.Significance statementThe potential of various small molecules such as flavins and quinones to enhance the rate of extracellular electron transport (EET) has been exploited to develop environmental energy conversion systems. Flavins and quinones have similar molecular structures but their abilities to enhance EET vary by >100× inShewanella oneidensisMR-1. These large differences are inconsistent with conventional models, which rely on redox potentials or diffusion constant of shuttling electron mediators. In this study, we demonstrated that the basicity of the nitrogen atom of the isoalloxazine ring (N5) enhances the rate of electron outflow when a flavin or quinone is a non-covalent cofactor ofS. oneidensisMR-1 outer membranec-type cytochromes.

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Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1410551111 ◽

2014 ◽

Vol 111 (35) ◽

pp. 12883-12888 ◽

Cited By ~ 327

Author(s):

S. Pirbadian ◽

S. E. Barchinger ◽

K. M. Leung ◽

H. S. Byun ◽

Y. Jangir ◽

...

Keyword(s):

Electron Transport ◽

Outer Membrane ◽

Shewanella Oneidensis ◽

Extracellular Electron Transport

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Outer membrane compositions enhance the rate of extracellular electron transport via cell-surface MtrC protein in Shewanella oneidensis MR-1

Bioresource Technology ◽

10.1016/j.biortech.2020.124290 ◽

2021 ◽

Vol 320 ◽

pp. 124290 ◽

Cited By ~ 1

Author(s):

Xizi Long ◽

Akihiro Okamoto

Keyword(s):

Electron Transport ◽

Cell Surface ◽

Outer Membrane ◽

Shewanella Oneidensis ◽

Extracellular Electron Transport

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Ultrastructure of Shewanella oneidensis MR-1 nanowires revealed by electron cryotomography

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1718810115 ◽

2018 ◽

Vol 115 (14) ◽

pp. E3246-E3255 ◽

Cited By ~ 70

Author(s):

Poorna Subramanian ◽

Sahand Pirbadian ◽

Mohamed Y. El-Naggar ◽

Grant J. Jensen

Keyword(s):

Electron Microscopy ◽

Electron Transport ◽

Outer Membrane ◽

Outer Membrane Proteins ◽

Shewanella Oneidensis ◽

Extracellular Electron Transfer ◽

Field Calculation ◽

Microscopy Imaging ◽

Electron Cryotomography ◽

Bacterial Nanowires

Bacterial nanowires have garnered recent interest as a proposed extracellular electron transfer (EET) pathway that links the bacterial electron transport chain to solid-phase electron acceptors away from the cell. Recent studies showed that Shewanella oneidensis MR-1 produces outer membrane (OM) and periplasmic extensions that contain EET components and hinted at their possible role as bacterial nanowires. However, their fine structure and distribution of cytochrome electron carriers under native conditions remained unclear, making it difficult to evaluate the potential electron transport (ET) mechanism along OM extensions. Here, we report high-resolution images of S. oneidensis OM extensions, using electron cryotomography (ECT). We developed a robust method for fluorescence light microscopy imaging of OM extension growth on electron microscopy grids and used correlative light and electron microscopy to identify and image the same structures by ECT. Our results reveal that S. oneidensis OM extensions are dynamic chains of interconnected outer membrane vesicles (OMVs) with variable dimensions, curvature, and extent of tubulation. Junction densities that potentially stabilize OMV chains are seen between neighboring vesicles in cryotomograms. By comparing wild type and a cytochrome gene deletion mutant, our ECT results provide the likely positions and packing of periplasmic and outer membrane proteins consistent with cytochromes. Based on the observed cytochrome packing density, we propose a plausible ET path along the OM extensions involving a combination of direct hopping and cytochrome diffusion. A mean-field calculation, informed by the observed ECT cytochrome density, supports this proposal by revealing ET rates on par with a fully packed cytochrome network.

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