The rising interest in the use of Geobacter bacteria for biotechnological applications demands a deep understanding of how these bacteria are able to thrive in a variety of environments and perform extracellular electron transfer. The Geobacter metallireducens bacterium can couple the oxidation of a wide range of compounds to the reduction of several extracellular acceptors, including heavy metals, toxic organic compounds or electrode surfaces. The periplasmic c-type cytochrome PpcA from this bacterium is a member of a family composed of five periplasmic triheme cytochromes, which are important to bridge the electron transfer between the cytoplasm and the extracellular environment. To better understand the functional mechanism of PpcA it is essential to obtain structural data for this cytochrome. In this work, the geometry of the heme axial ligands, as well as the magnetic properties of the hemes were determined for the oxidized form of the cytochrome, using the 13C NMR chemical shifts of the heme α-substituents. The results were further compared with those previously obtained for the homologous cytochrome from Geobacter sulfurreducens. The orientations of the axial histidine planes and the magnetic properties of the hemes are conserved in both proteins. Overall, the results obtained allowed the definition of the orientation of the magnetic axes of PpcA from G. metallireducens, which will be used as constraints to assist the solution structure determination of the cytochrome in the oxidized form.

The unambiguous assignment of the nuclear magnetic resonance (NMR) signals of the α-substituents of the haems in the tetrahaem cytochrome isolated from Shewanella frigidimarina NCIMB400, was made using a combination of homonuclear and heteronuclear experiments. The paramagnetic 13C shifts of the nuclei directly bound to the porphyrin of each haem group were analysed in the framework of a model for the haem electronic structure. The analysis yields g-tensors for each haem, which allowed the assignment of some electron paramagnetic resonance (EPR) signals to specific haems, and the orientation of the magnetic axes relative to each haem to be established. The orientation of the axial ligands of the haems was determined semi-empirically from the NMR data, and the structural results were compared with those of the homologous tetrahaem cytochrome from Shewanella oneidensis MR-1 showing significant similarities between the two proteins.

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.

Geobacter bacteria have a remarkable respiratory versatility that includes the dissimilatory reduction of insoluble metal oxides in natural habitats and electron transfer to electrode surfaces from which electricity can be harvested. In both cases, electrons need to be exported from the cell interior to the exterior via a mechanism designated as extracellular electron transfer (EET). Several c-type cytochromes from G. sulfurreducens (Gs) were identified as key players in this process. Biochemical and biophysical data have been obtained for ten Gs cytochromes, including inner-membrane associated (MacA), periplasmic (PpcA, PpcB, PpcC, PpcD, PpcE and GSU1996) and outer membrane-associated (OmcF, OmcS and OmcZ). The redox properties of these cytochromes have been determined, except for PpcC and GSU1996. In this perspective, the reduction potentials of these two cytochromes were determined by potentiometric redox titrations followed by visible spectroscopy. The data obtained are taken together with those available for other key cytochromes to present a thorough overview of the current knowledge of Gs EET mechanisms and provide a possible rationalization for the existence of several multiheme cytochromes involved in the same respiratory pathways.