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

PD/00193/2012 UID/FIS/00068/2019 PTDC/BBBBQB/3554/2014 PTDC/BIA-BQM/31981/2017 PD/BD/114445/2016 UID/Multi/04378/2019 ROTEIRO/0031/2013 -PINFRA/22161/2016The monoheme outer membrane cytochrome F (OmcF) from Geobacter sulfurreducens plays an important role in Fe(III) reduction and electric current production. The electrochemical characterization of this cytochrome has shown that its redox potential is modulated by the solution pH (redox-Bohr effect) endowing the protein with the necessary properties to couple electron and proton transfer in the physiological range. The analysis of the OmcF structures in the reduced and oxidized states showed that with the exception of the side chain of histidine 47 (His47), all other residues with protonatable side chains are distant from the heme iron and, therefore, are unlikely to affect the redox potential of the protein. The protonatable site at the imidazole ring of His47 is in the close proximity to the heme and, therefore, this residue was suggested as the redox-Bohr center. In the present work, we tested this hypothesis by replacing the His47 with non-protonatable residues (isoleucine – OmcFH47I and phenylalanine – OmcFH47F). The structure of the mutant OmcFH47I was determined by X-ray crystallography to 1.13 Å resolution and showed only minimal changes at the site of the mutation. Both mutants were 15N-labeled and their overall folding was confirmed to be the same as the wild-type by NMR spectroscopy. The pH dependence of the redox potential of the mutants was measured by cyclic voltammetry. Compared to the wild-type protein, the magnitude of the redox-Bohr effect in the mutants was smaller, but not fully abolished, confirming the role of His47 on the pH modulation of OmcF’s redox potential. However, the pH effect on the heme substituents’ NMR chemical shifts suggested that the heme propionate P13 also contributes to the overall redox-Bohr effect in OmcF. In physiological terms, the contribution of two independent acid–base centers to the observed redox-Bohr effect confers OmcF a higher versatility to environmental changes by coupling electron/proton transfer within a wider pH range.publishersversionpublishe

A Zinc Catalyzed Two-Electron Nickel(IV/II) Redox Couple: New Catholyte Design for Redox Flow Batteries

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Herein, we investigated the redox cycle of nickel (II) bis(diethyldithiocarbamate), NiII(dtc)2, for potential use as a multi-electron storage catholyte in non-aqueous redox flow batteries (RFBs). Previous studies have shown the unique redox cycle of NiII(dtc)2 offers 2e- chemistry upon oxidation from NiII → NiIV but 1e- chemistry upon reduction from NiIV → NiIII → NiII. Electrochemical experiments presented here show that the addition of as little as 10 mol% ZnII(ClO4)2 to the electrolyte consolidates the two 1e- reduction peaks into a single 2e- reduction where [NiIV(dtc)3]+ is reduced directly to NiII(dtc)2. This catalytic enhancement is believed to be due to ZnII removal of a dtc- ligand from a NiIII(dtc)3 intermediate, resulting in more facile reduction to NiII(dtc)2. The addition of ZnII also improves the 2e- oxidation, shifting the anodic peak negative and decreasing the 2e- peak splitting. H-cell cycling experiments showed that 97% coulombic efficiency and 98% charge storage efficiency was maintained for 50 cycles over 25 h using 0.1 M ZnII(ClO4)2 as supporting electrolyte. If ZnII(ClO4)2 was replaced with TBAPF6 in the electrolyte, the coulombic efficiency fell to 78%. The use of ZnII to increase the reversibility of 2e- transfer is a promising result that points to the ability to use nickel dithiocarbonates for multi-electron storage in RFBs

Rational engineering of Geobacter sulfurreducens electron transfer components: a foundation for building improved Geobacter-based bioelectrochemical technologies

Multiheme cytochromes have been implicated in Geobacter sulfurreducens extracellular electron transfer (EET). These proteins are potential targets to improve EET and enhance bioremediation and electrical current production by G. sulfurreducens. However, the functional characterization of multiheme cytochromes is particularly complex due to the co-existence of several microstates in solution, connecting the fully reduced and fully oxidized states. Over the last decade, new strategies have been developed to characterize multiheme redox proteins functionally and structurally. These strategies were used to reveal the functional mechanism of G. sulfurreducens multiheme cytochromes and also to identify key residues in these proteins for EET. In previous studies, we set the foundations for enhancement of the EET abilities of G. sulfurreducens by characterizing a family of five triheme cytochromes (PpcA-E). These periplasmic cytochromes are implicated in electron transfer between the oxidative reactions of metabolism in the cytoplasm and the reduction of extracellular terminal electron acceptors at the cell's outer surface. The results obtained suggested that PpcA can couple e−/H+ transfer, a property that might contribute to the proton electrochemical gradient across the cytoplasmic membrane for metabolic energy production. The structural and functional properties of PpcA were characterized in detail and used for rational design of a family of 23 single site PpcA mutants. In this review, we summarize the functional characterization of the native and mutant proteins. Mutants that retain the mechanistic features of PpcA and adopt preferential e−/H+ transfer pathways at lower reduction potential values compared to the wild-type protein were selected for in vivo studies as the best candidates to increase the electron transfer rate of G. sulfurreducens. For the first time G. sulfurreducens strains have been manipulated by the introduction of mutant forms of essential proteins with the aim to develop and improve bioelectrochemical technologies.This work was supported by project grants: PTDC/BBB-BEP/0753/2012 (to CS), L'Oréal Portugal Medals of Honor for Women in Science 2012 (to LM), SFRH/BD/89701/2012 (to JD), UID/Multi/04378/2013 from Fundação para a Ciência e a Tecnologia (FCT), Portugal and CTQ2011-22514 from the Ministerio De Economia y Competitividad. Cytochrome work at Argonne National Laboratory (MS, YL, and PP) was previously supported by the DOE Office of Biological and Environmental Research program. Currently, PP is partially supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy under contract no. DE-AC02-06CH11357. Geobacter sulfurreducens cells and cloning protocols referred in this work were kindly provided by Prof. Derek Lovley from the University of Massachusetts Amherst (USA).Peer reviewedPeer Reviewe

Dissecting the functional role of key residues in triheme cytochrome PpcA: a path to rational design of G. sulfurreducens strains with enhanced electron transfer capabilities.

PpcA is the most abundant member of a family of five triheme cytochromes c7 in the bacterium Geobacter sulfurreducens (Gs) and is the most likely carrier of electrons destined for outer surface during respiration on solid metal oxides, a process that requires extracellular electron transfer. This cytochrome has the highest content of lysine residues (24%) among the family, and it was suggested to be involved in e-/H(+) energy transduction processes. In the present work, we investigated the functional role of lysine residues strategically located in the vicinity of each heme group. Each lysine was replaced by glutamine or glutamic acid to evaluate the effects of a neutral or negatively charged residue in each position. The results showed that replacing Lys9 (located near heme IV), Lys18 (near heme I) or Lys22 (between hemes I and III) has essentially no effect on the redox properties of the heme groups and are probably involved in redox partner recognition. On the other hand, Lys43 (near heme IV), Lys52 (between hemes III and IV) and Lys60 (near heme III) are crucial in the regulation of the functional mechanism of PpcA, namely in the selection of microstates that allow the protein to establish preferential e-/H(+) transfer pathways. The results showed that the preferred e-/H(+) transfer pathways are only established when heme III is the last heme to oxidize, a feature reinforced by a higher difference between its reduction potential and that of its predecessor in the order of oxidation. We also showed that K43 and K52 mutants keep the mechanistic features of PpcA by establishing preferential e-/H+ transfer pathways at lower reduction potential values than the wild-type protein, a property that can enable rational design of Gs strains with optimized extracellular electron transfer capabilities

Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Herein, we investigated the redox cycle of nickel(II) bis(diethyldithiocarbamate), NiII(dtc)2, for potential use as a multielectron storage catholyte in nonaqueous redox flow batteries (RFBs). Previous studies have shown that the unique redox cycle of NiII(dtc)2 offers 2e– chemistry upon oxidation from NiII → NiIV but 1e– chemistry upon reduction from NiIV → NiIII → NiII. Electrochemical experiments presented here show that the addition of as little as 10 mol % ZnII(ClO4)2 to the electrolyte consolidates the two 1e– reduction peaks into a single 2e– reduction where [NiIV(dtc)3]+ is reduced directly to NiII(dtc)2. This catalytic enhancement is believed to be due to ZnII removal of a dtc– ligand from a NiIII(dtc)3 intermediate, resulting in more facile reduction to NiII(dtc)2. The addition of ZnII also improves the 2e– oxidation, shifting the anodic peak negative and decreasing the 2e– peak separation. H-cell cycling experiments showed that 97% Coulombic efficiency and 98% charge storage efficiency was maintained for 50 cycles over 25 h using 0.1 M ZnII(ClO4)2 as the supporting electrolyte. If ZnII(ClO4)2 was replaced with TBAPF6 in the electrolyte, the Coulombic efficiency fell to 78%. The use of ZnII to increase the reversibility of 2e– transfer is a promising result that points to the ability to use nickel dithiocarbonates for multielectron storage in RFBs

Multimerization of Solution-State Proteins by Tetrakis(4-sulfonatophenyl)porphyrin

Surface binding and interactions of anionic porphyins bound to cationic proteins have been studied for nearly three decades and are relevant as models for protein surface molecular recognition and photoinitiated electron transfer. However, interpretation of data in nearly all reports explicitly or implicitly assumed interaction of porphyrin with monodisperse proteins in solutions. In this report, using small-angle X-ray scattering with solution phase samples, we demonstrate that horse heart cytochrome (cyt) <i>c</i>, triheme cytochrome <i>c</i>₇ PpcA from <i>Geobacter sulfurreducens</i>, and hen egg lysozyme multimerize in the presence of zinc tetrakis­(4-sulfonatophenyl)­porphyrin (ZnTPPS). Multimerization of cyt <i>c</i> showed a pH dependence with a stronger apparent binding affinity under alkaline conditions and was weakened in the presence of a high salt concentration. Ferric-cyt <i>c</i> formed complexes larger than those formed by ferro-cyt c. Free base TPPS and FeTPPS facilitated formation of complexes larger than those of ZnTPPS. No increase in protein aggregation state for cationic proteins was observed in the presence of cationic porphyrins. All-atom molecular dynamics simulations of cyt <i>c</i> and PpcA with free base TPPS corroborated X-ray scattering results and revealed a mechanism by which the tetrasubstituted charged porphyrins serve as bridging ligands nucleating multimerization of the complementarily charged protein. The final aggregation products suggest that multimerization involves a combination of electrostatic and hydrophobic interactions. The results demonstrate an overlooked complexity in the design of multifunctional ligands for protein surface recognition

Extracytoplasmic PAS-Like Domains Are Common in Signal Transduction Proteins▿ †

We present the crystal structure of the extracytoplasmic domain of the Bacillus subtilis PhoR sensor histidine kinase, part of a two-component system involved in adaptation to low environmental phosphate concentrations. In addition to the PhoR structure, we predict that the majority of the extracytoplasmic domains of B. subtilis sensor kinases will adopt a fold similar to the ubiquitous PAS domain