Rational design of electron/proton transfer mechanisms in the exoelectrogenic bacteria Geobacter sulfurreducens

The redox potential values of cytochromes can be modulated by the protonation/deprotonation of neighbor groups (redox-Bohr effect), a mechanism that permits the proteins to couple electron/proton transfer. In the respiratory chains, this effect is particularly relevant if observed in the physiological pH range, as it may contribute to the electrochemical gradient for ATP synthesis. A constitutively produced family of five triheme cytochromes (PpcA−E) from the bacterium Geobacter sulfurreducens plays a crucial role in extracellular electron transfer, a hallmark that permits this bacterium to be explored for several biotechnological applications. Two members of this family (PpcA and PpcD) couple electron/proton transfer in the physiological pH range, a feature not shared with PpcB and PpcE. That ability is crucial for G. sulfurreducens’ growth in Fe(III)-reducing habitats since extra contributors to the electrochemical gradient are needed. It was postulated that the redox-Bohr effect is determined by the nature of residue 6, a leucine in PpcA/PpcD and a phenylalanine in PpcB/PpcE. To confirm this hypothesis, Phe6 was replaced by leucine in PpcB and PpcE. The functional properties of these mutants were investigated by NMR and UV-visible spectroscopy to assess their capability to couple electron/proton transfer in the physiological pH range. The results obtained showed that the mutants have an increased redox-Bohr effect and are now capable of coupling electron/proton transfer. This confirms the determinant role of the nature of residue 6 in the modulation of the redox-Bohr effect in this family of cytochromes, opening routes to engineer Geobacter cells with improved biomass production.

Download Full-text

Fine Tuning of Redox Networks on Multiheme Cytochromes fromGeobacter sulfurreducensDrives Physiological Electron/Proton Energy Transduction

Bioinorganic Chemistry and Applications ◽

10.1155/2012/298739 ◽

2012 ◽

Vol 2012 ◽

pp. 1-9 ◽

Cited By ~ 17

Author(s):

Leonor Morgado ◽

Joana M. Dantas ◽

Marta Bruix ◽

Yuri Y. Londer ◽

Carlos A. Salgueiro

Keyword(s):

Microbial Fuel Cells ◽

Geobacter Sulfurreducens ◽

Cellular Growth ◽

Bohr Effect ◽

Fine Tuning ◽

Physiological Relevance ◽

Ph Range ◽

Coupling Mechanisms ◽

Redox Bohr Effect

The bacteriumGeobacter sulfurreducens (Gs)can grow in the presence of extracellular terminal acceptors, a property that is currently explored to harvest electricity from aquatic sediments and waste organic matter into microbial fuel cells. A family composed of five triheme cytochromes (PpcA-E) was identified inGs. These cytochromes play a crucial role by bridging the electron transfer from oxidation of cytoplasmic donors to the cell exterior and assisting the reduction of extracellular terminal acceptors. The detailed thermodynamic characterization of such proteins showed that PpcA and PpcD have an important redox-Bohr effect that might implicate these proteins in the e−/H+coupling mechanisms to sustain cellular growth. The physiological relevance of the redox-Bohr effect in these proteins was studied by determining the fractional contribution of each individual redox-microstate at different pH values. For both proteins, oxidation progresses from a particular protonated microstate to a particular deprotonated one, over specific pH ranges. The preferred e−/H+transfer pathway established by the selected microstates indicates that both proteins are functionally designed to couple e−/H+transfer at the physiological pH range for cellular growth.

Download Full-text

Making protons tag along with electrons

Biochemical Journal ◽

10.1042/bcj20210592 ◽

2021 ◽

Vol 478 (23) ◽

pp. 4093-4097

Author(s):

Matthew J. Guberman-Pfeffer ◽

Nikhil S. Malvankar

Keyword(s):

Proton Transfer ◽

Soil Microbes ◽

Electron Acceptors ◽

Geobacter Sulfurreducens ◽

Harsh Environments ◽

Extracellular Electron Transfer ◽

Necessary Condition ◽

Engineering Strategy ◽

Electron Proton Transfer

Every living cell needs to get rid of leftover electrons when metabolism extracts energy through the oxidation of nutrients. Common soil microbes such as Geobacter sulfurreducens live in harsh environments that do not afford the luxury of soluble, ingestible electron acceptors like oxygen. Instead of resorting to fermentation, which requires the export of reduced compounds (e.g. ethanol or lactate derived from pyruvate) from the cell, these organisms have evolved a means to anaerobically respire by using nanowires to export electrons to extracellular acceptors in a process called extracellular electron transfer (EET) [ 1]. Since 2005, these nanowires were thought to be pili filaments [ 2]. But recent studies have revealed that nanowires are composed of multiheme cytochromes OmcS [ 3, 4] and OmcZ [ 5] whereas pili remain inside the cell during EET and are required for the secretion of nanowires [ 6]. However, how electrons are passed to these nanowires remains a mystery ( Figure 1A). Periplasmic cytochromes (Ppc) called PpcA-E could be doing the job, but only two of them (PpcA and PpcD) can couple electron/proton transfer — a necessary condition for energy generation. In a recent study, Salgueiro and co-workers selectively replaced an aromatic with an aliphatic residue to couple electron/proton transfer in PpcB and PpcE (Biochem. J. 2021, 478 (14): 2871–2887). This significant in vitro success of their protein engineering strategy may enable the optimization of bioenergetic machinery for bioenergy, biofuels, and bioelectronics applications.

Download Full-text

Modulation of the Redox Potential and Electron/Proton Transfer Mechanisms in the Outer Membrane Cytochrome OmcF From Geobacter sulfurreducens

Frontiers in Microbiology ◽

10.3389/fmicb.2019.02941 ◽

2020 ◽

Vol 10 ◽

Cited By ~ 1

Author(s):

Liliana R. Teixeira ◽

Cristina M. Cordas ◽

Marta P. Fonseca ◽

Norma E. C. Duke ◽

Phani Raj Pokkuluri ◽

...

Keyword(s):

Proton Transfer ◽

Redox Potential ◽

Outer Membrane ◽

Geobacter Sulfurreducens ◽

Electron Proton Transfer ◽

Transfer Mechanisms

Download Full-text

Electrostatics and proton transfer in photosynthetic water oxidation

Philosophical Transactions of the Royal Society B Biological Sciences ◽

10.1098/rstb.2002.1137 ◽

2002 ◽

Vol 357 (1426) ◽

pp. 1407-1418 ◽

Cited By ~ 93

Author(s):

Wolfgang Junge ◽

Michael Haumann ◽

Ralf Ahlbrink ◽

Armen Mulkidjanian ◽

Jürgen Clausen

Keyword(s):

Electron Transfer ◽

Proton Transfer ◽

Water Oxidation ◽

Bound Water ◽

Bohr Effect ◽

Primary Donor ◽

Mn Cluster ◽

Electron Proton Transfer ◽

Photosynthetic Water Oxidation ◽

Hydrogen Bonded

Photosystem II (PSII) oxidizes two water molecules to yield dioxygen plus four protons. Dioxygen is released during the last out of four sequential oxidation steps of the catalytic centre (S 0 ⇒ S 1 , S 1 ⇒ S 2 , S 2 ⇒ S 3 , S 3 ⇒ S 4 → S 0 ). The release of the chemically produced protons is blurred by transient, highly variable and electrostatically triggered proton transfer at the periphery (Bohr effect). The extent of the latter transiently amounts to more than one H + /e – under certain conditions and this is understood in terms of electrostatics. By kinetic analyses of electron–proton transfer and electrochromism, we discriminated between Bohr–effect and chemically produced protons and arrived at a distribution of the latter over the oxidation steps of 1 : 0 : 1 : 2. During the oxidation of tyr–161 on subunit D1 (Y Z ), its phenolic proton is not normally released into the bulk. Instead, it is shared with and confined in a hydrogen–bonded cluster. This notion is difficult to reconcile with proposed mechanisms where Y Z acts as a hydrogen acceptor for bound water. Only in manganese (Mn) depleted PSII is the proton released into the bulk and this changes the rate of electron transfer between Y Z and the primary donor of PSII P + 680 from electron to proton controlled. D1–His190, the proposed centre of the hydrogen–bonded cluster around Y Z , is probably further remote from Y Z than previously thought, because substitution of D1–Glu189, its direct neighbour, by Gln, Arg or Lys is without effect on the electron transfer from Y Z to P + 680 (in nanoseconds) and from the Mn cluster to Y ox Z .

Download Full-text

Role of Met58 in the regulation of electron/proton transfer in trihaem cytochrome PpcA from Geobacter sulfurreducens

Bioscience Reports ◽

10.1042/bsr20120086 ◽

2012 ◽

Vol 33 (1) ◽

Cited By ~ 8

Author(s):

Leonor Morgado ◽

Joana M. Dantas ◽

Telma Simões ◽

Yuri Y. Londer ◽

P. Raj Pokkuluri ◽

...

Keyword(s):

Electron Transfer ◽

Solvent Accessibility ◽

Potential Gradient ◽

Geobacter Sulfurreducens ◽

Extracellular Electron Transfer ◽

Redox States ◽

Electron Transfer Pathways ◽

Electron Proton Transfer ◽

Fine Tune

The bacterium Gs (Geobacter sulfurreducens) is capable of oxidizing a large variety of compounds relaying electrons out of the cytoplasm and across the membranes in a process designated as extracellular electron transfer. The trihaem cytochrome PpcA is highly abundant in Gs and is most probably the reservoir of electrons destined for the outer surface. In addition to its role in electron transfer pathways, we have previously shown that this protein could perform e−/H+ energy transduction. This mechanism is achieved by selecting the specific redox states that the protein can access during the redox cycle and might be related to the formation of proton electrochemical potential gradient across the periplasmic membrane. The regulatory role of haem III in the functional mechanism of PpcA was probed by replacing Met58, a residue that controls the solvent accessibility of haem III, with serine, aspartic acid, asparagine or lysine. The data obtained from the mutants showed that the preferred e−/H+ transfer pathway observed for PpcA is strongly dependent on the reduction potential of haem III. It is striking to note that one residue can fine tune the redox states that can be accessed by the trihaem cytochrome enough to alter the functional pathways.

Download Full-text

Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1500245112 ◽

2015 ◽

Vol 112 (16) ◽

pp. 4935-4940 ◽

Cited By ~ 91

Author(s):

Na Song ◽

Javier J. Concepcion ◽

Robert A. Binstead ◽

Jennifer A. Rudd ◽

Aaron K. Vannucci ◽

...

Keyword(s):

Aqueous Solution ◽

Proton Transfer ◽

Water Oxidation ◽

Proton Acceptor ◽

Half Time ◽

X Ray ◽

X Ray Crystallography ◽

Buffer Base ◽

Electron Proton Transfer ◽

C Nmr

In aqueous solution above pH 2.4 with 4% (vol/vol) CH3CN, the complex [RuII(bda)(isoq)2] (bda is 2,2′-bipyridine-6,6′-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [RuII(CO2-bpy-CO2−)(isoq)2(NCCH3)], as shown by 1H and 13C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO43–, the calculated half-time for water oxidation is ∼7 μs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom–proton transfer (APT) or concerted electron–proton transfer (EPT) pathways.

Download Full-text