Probing the effect of ionic strength on the functional robustness of the triheme cytochrome PpcA from Geobacter sulfurreducens: a contribution for optimizing biofuel cell's power density

Thermodynamic properties of triheme cytochrome PpcF from Geobacter metallireducens reveal unprecedented functional mechanism

The bacterium Geobacter metallireducens is highly efficient in long-range extracellular electron transfer, a process that relies on an efficient bridging between the cytoplasmic electron donors and the extracellular acceptors. The periplasmic triheme cytochromes are crucial players in these processes and thus the understanding of their functional mechanism is crucial to elucidate the extracellular electron transfer processes in this microorganism. The triheme cytochrome PpcF from G. metallireducens has the lowest amino acid sequence identity with the remaining cytochromes from the PpcA-family of G. sulfurreducens and G. metallireducens, making it an interesting target for structural and functional studies. In this work, we performed a detailed functional and thermodynamic characterization of cytochrome PpcF by the complementary usage of NMR and visible spectroscopic techniques. The results obtained show that the heme reduction potentials are negative, different from each other and are also modulated by the redox and redox-Bohr interactions that assure unprecedented mechanistic features to the protein. The results showed that the order of oxidation of the hemes in cytochrome PpcF is maintained in the entire physiological pH range. The considerable separation of the hemes' redox potential values facilitates a sequential transfer within the chain of redox centers in PpcF, thus assuring electron transfer directionality to the electron acceptors.

Biomolecular Interaction Studies Between Cytochrome PpcA From Geobacter sulfurreducens and the Electron Acceptor Ferric Nitrilotriacetate (Fe-NTA)

Geobacter sulfurreducens bacterium exhibits an enormous respiratory versatility, including the utilization of several toxic and radioactive metals as electron acceptors. This versatility is also replicated in the capability of the most abundant cytochrome in G. sulfurreducens, the periplasmic triheme cytochrome PpcA, to reduce uranium, chromium and other metal ions. From all possible electron transfer pathways in G. sulfurreducens, those involved in the iron reduction are the best characterized to date. Previously, we provided structural evidence for the complex interface established between PpcA and the electron acceptor Fe(III)-citrate. However, genetic studies suggested that this acceptor is mainly reduced by outer membrane cytochomes. In the present work, we used UV-visible measurements to demonstrate that PpcA is able to directly reduce the electron acceptor ferric nitrilotriacetate (Fe-NTA), a more outer membrane permeable iron chelated form. In addition, the molecular interactions between PpcA and Fe-NTA were probed by Nuclear Magnetic Resonance (NMR) spectroscopy. The NMR spectra obtained for PpcA samples in the absence and presence of Fe-NTA showed that the interaction is reversible and encompasses a positively charged surface region located in the vicinity of the heme IV. Overall, the study elucidates the formation of an electron transfer complex between PpcA and a readily outer-membrane permeable iron chelated form. The structural and functional relationships obtained explain how a single cytochrome is designed to effectively interact with a wide range of G. sulfurreducens electron acceptors, a feature that can be explored for optimal bioelectrochemical applications.