Replacement of Lysine 45 by Uncharged Residues Modulates the Redox-Bohr Effect in Tetraheme Cytochromec3ofDesulfovibrio vulgaris(Hildenborough)†

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.

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

Biochemical Journal ◽

10.1042/bcj20210365 ◽

2021 ◽

Author(s):

Marta A. Silva ◽

Pilar C. Portela ◽

Carlos A Salgueiro

Keyword(s):

Proton Transfer ◽

Rational Design ◽

Atp Synthesis ◽

Geobacter Sulfurreducens ◽

Bohr Effect ◽

Extracellular Electron Transfer ◽

Electrochemical Gradient ◽

Ph Range ◽

Electron Proton Transfer ◽

Redox Bohr Effect

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.

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