Biomethane as alternative fuel for the EU road sector: analysis of existing and planned infrastructure

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Mechanisms of Bacterial Extracellular Electron Exchange.

The biochemical mechanisms by which microbes interact with extracellular soluble metal ions and insoluble redox-active minerals have been the focus of intense research over the last three decades. The process presents two challenges to the microorganism; firstly electrons have to be transported at the cell surface, which in Gram negative bacteria presents an additional problem of electron transfer across the ~ 6 nm of the outer membrane. Secondly the electrons must be transferred to or from the terminal electron acceptors or donors. This review covers the known mechanisms that bacteria use to transport electrons across the cell envelope to external electron donors/acceptors. In Gram negative bacteria electron transfer across the outer membrane involves the use of an outer membrane β-barrel and cytochrome. These can be in the form of a porin-cytochrome protein, such as Cyc2 of Acidothiobacillus ferrioxydans, or a multiprotein porin-cytochrome complex like MtrCAB of Shewanella oneidensis MR-1. For mineral respiring organisms there is the additional challenge of transferring the electrons from the cell to mineral surface. For the strict anaerobe Geobacter sulfurreducens this requires electron transfer through conductive pili to associated cytochrome OmcS that directly reduces Fe(III)oxides, while the facultative anaerobe S. oneidensis MR-1 accomplishes mineral reduction through direct membrane contact, contact through filamentous extentions and soluble flavin shuttles, all of which require the outer membrane cytochromes MtrC and OmcA in addition to secreted flavin

The Crystal Structure of the Extracellular 11-heme Cytochrome UndA Reveals a Conserved 10-heme Motif and Defined Binding Site for Soluble Iron Chelates

Members of the genus Shewanella translocate deca- or undeca-heme cytochromes to the external cell surface thus enabling respiration using extracellular minerals and polynuclear Fe(III) chelates. The high resolution structure of the first undeca-heme outer membrane cytochrome, UndA, reveals a crossed heme chain with four potential electron ingress/egress sites arranged within four domains. Sequence and structural alignment of UndA and the deca-heme MtrF reveals the extra heme of UndA is inserted between MtrF hemes 6 and 7. The remaining UndA hemes can be superposed over the heme chain of the decaheme MtrF, suggesting that a ten heme core is conserved between outer membrane cytochromes. The UndA structure has also been crystallographically resolved in complex with substrates, an Fe(III)-nitrilotriacetate dimer or an Fe(III)-citrate trimer. The structural resolution of these UndA-Fe(III)-chelate complexes provides a rationale for previous kinetic measurements on UndA and other outer membrane cytochromes

Comparative structure-potentio-spectroscopy of the Shewanella outer membrane multiheme cytochromes

Many species of bacteria can generate energy in the anoxic subsurface by directly coupling intracellular oxidative reactions to the reduction of extracellular metal oxides. Coupling these processes requires electron transfer networks that extend from the inside of the cell, across the outer membrane to the extracellular terminal electron acceptors. The best described of these networks is from Shewanella oneidensis MR-1, where four structures of outer membrane multiheme cytochromes (OMMCs) have been determined. These OMMCs contain 10-11 bis-histidine ligated c-type hemes and are directly involved in the reduction of iron and manganese oxides at the cell surface. The heme ligands for some of these structures have been characterised using electron paramagnetic resonance (EPR), the redox-properties have been mapped by protein film electrochemistry (PFE) and more recently molecular dynamic simulations have been used to obtain microscopic redox potentials for individual heme groups. This review maps these different experimental techniques onto the structures, providing insight into the intramolecular electron transfer pathways of OMMCs, revealing future directions for study

Redox-active ferrocene-modified Cowpea mosaic virus nanoparticles

A naturally occurring nanoparticle, the plant virus Cowpea mosaic virus, can be decorated with ferrocene derivatives, of various linker lengths with amine and carboxylategroups, on the external surface using a range of conjugation strategies. The multiple, organometallic, redox-active ferrocene moieties on the outer surface of the virus are electrochemically independent with reduction potentials that span a potential window of 0.16 V that are dependent on the site of modification and the nature of the ferrocene derivative. The number of ferrocenes coupled to each virus ranges from about 100 to 240 depending upon the conjugation site and the linker length and these redox active units can provide multielectron reservoirs

Characterization of an electron conduit between bacteria and the extracellular environment

A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning ß-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment

Exploring the biochemistry at the extracellular redox frontier of bacterial mineral Fe(III) respiration

Many species of the bacterial Shewanella genus are notable for their ability to respire in anoxic environments utilizing insoluble minerals of Fe(III) and Mn(IV) as extracellular electron acceptors. In Shewanella oneidensis, the process is dependent on the decahaem electron-transport proteins that lie at the extracellular face of the outer membrane where they can contact the insoluble mineral substrates. These extracellular proteins are charged with electrons provided by an inter-membrane electron-transfer pathway that links the extracellular face of the outer membrane with the inner cytoplasmic membrane and thereby intracellular electron sources. In the present paper, we consider the common structural features of two of these outer-membrane decahaem cytochromes, MtrC and MtrF, and bring this together with biochemical, spectroscopic and voltammetric data to identify common and distinct properties of these prototypical members of different clades of the outer-membrane decahaem cytochrome superfamily

Bespoke Biomolecular Wires for Transmembrane Electron Transfer: Spontaneous Assembly of a Functionalized Multiheme Electron Conduit.

Shewanella oneidensis exchanges electrons between cellular metabolism and external redox partners in a process that attracts much attention for production of green electricity (microbial fuel cells) and chemicals (microbial electrosynthesis). A critical component of this pathway is the outer membrane spanning MTR complex, a biomolecular wire formed of the MtrA, MtrB, and MtrC proteins. MtrA and MtrC are decaheme cytochromes that form a chain of close-packed hemes to define an electron transfer pathway of 185 Å. MtrA is wrapped inside MtrB for solubility across the outer membrane lipid bilayer; MtrC sits outside the cell for electron exchange with external redox partners. Here, we demonstrate tight and spontaneous in vitro association of MtrAB with separately purified MtrC. The resulting complex is comparable with the MTR complex naturally assembled by Shewanella in terms of both its structure and rates of electron transfer across a lipid bilayer. Our findings reveal the potential for building bespoke electron conduits where MtrAB combines with chemically modified MtrC, in this case, labeled with a Ru-dye that enables light-triggered electron injection into the MtrC heme chain