Abstract
Electrogenic microorganisms possess unique redox biological features, being capable of transferring electrons to the cell exterior and converting highly toxic compounds into nonhazardous forms. These microorganisms have led to the development of Microbial Electrochemical Technologies (METs), which include applications in the fields of bioremediation and bioenergy production. The optimization of these technologies involves efforts from several different disciplines, ranging from microbiology to materials science. Geobacter bacteria have served as a model for understanding the mechanisms underlying the phenomenon of extracellular electron transfer, which is highly dependent on a multitude of multiheme cytochromes (MCs). MCs are, therefore, logical targets for rational protein engineering to improve the extracellular electron transfer rates of these bacteria. However, the presence of several heme groups complicates the detailed redox characterization of MCs. In this Review, the main characteristics of electroactive Geobacter bacteria, their potential to develop microbial electrochemical technologies and the main features of MCs are initially highlighted. This is followed by a detailed description of the current methodologies that assist the characterization of the functional redox networks in MCs. Finally, it is discussed how this information can be explored to design optimal Geobacter-mutated strains with improved capabilities in METs.
Highlights
Electroactive microorganisms have been extensively studied since their discovery more than a century ago [1] and are defined by their ability to exchange electrons between intracellular donors and extracellular acceptors [2], a phenomenon designated ExtracellularElectron Transfer (EET)
These features make them interesting targets for Microbial Electrochemical Technologies (METs), ranging from bioenergy production [4,5] and bioremediation applications [3,6], to the fields of bioelectronics [7,8] and bionanotechnology [9]
The paramagnetic shifts of the heme methyls are proportional to the degree of oxidation of its heme group if the extrinsic paramagnetic shifts are negligible, as is the case of the selected heme methyls (121 CH3 I, 71 CH3 III and 121 CH3 IV ), which are illustrated on the heme core of PeriPlasmic Cytochrome A (PpcA) (Figure 7)
Summary
Electroactive microorganisms have been extensively studied since their discovery more than a century ago [1] and are defined by their ability to exchange electrons between intracellular donors and extracellular acceptors [2], a phenomenon designated Extracellular. It was discovered that it could withstand low levels of molecular oxygen, providing an explanation for its abundance in oxic subsurface environments [26] This bacterium is able to reduce a large variety of extracellular compounds, including Fe(III), U(VI) and Mn(IV) oxides, as well as many toxic organic substances that contaminate soils and wastewaters [27] and for this reason, Geobacter is being explored in bioremediation technologies (Figure 1). Rational-mutated Geobacter strain, containing several optimized EET components and increased respiratory rates, can putatively contribute to more efficient METs. Cytochromes c are proteins containing one or several c-type heme groups that often function as electron carriers in biological systems (Figure 2) [52,53]. The knowledge obtained from such studies is currently being explored to design Geobacter strains with higher EET efficiency, to improve Geobacter-based biotechnological applications
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