Bacteria of the genus Shewanella contain an abundant small tetraheme cytochrome in their periplasm when growing anaerobically. Data collected for the protein isolated from S. oneidensis MR-1 and S. frigidimarina indicate differences in the order of oxidation of the hemes. A detailed thermodynamic characterization of the cytochrome from S. oneidensis MR-1 in the physiological pH range was performed, with data collected in the pH range 5.5-9.0 from NMR experiments using partially oxidized samples and from redox titrations followed by visible spectroscopy. These data allow the parsing of the redox and redox-protonation interactions that occur during the titration of hemes. The results show that electrostatic effects dominate the heme-heme interactions, in agreement with modest redox-linked structural modifications, and protonation has a considerable influence on the redox properties of the hemes in the physiological pH range. Theoretical calculations using the oxidized and reduced structures of this protein reveal that the bulk redox-Bohr effect arises from the aggregate fractional titration of several of the heme propionates. This detailed characterization of the thermodynamic properties of the cytochrome shows that only a few of the multiple microscopic redox states that the protein can access are significantly populated at physiological pH. On this basis a functional pathway for the redox activity of the small tetraheme cytochrome from S. oneidensis MR-1 is proposed, where reduction and protonation are thermodynamically coupled in the physiological range. The differences between the small tetraheme cytochromes from the two organisms are discussed in the context of their biological role.

eThe purple bacterium Rhodopseudomonas palustris TIE-1 expresses multiple small high-potential redox proteins during photoautotrophic growth, including two high-potential iron-sulfur proteins (HiPIPs) (PioC and Rpal_4085) and a cytochrome c 2 . We evaluated the role of these proteins in TIE-1 through genetic, physiological, and biochemical analyses. Deleting the gene encoding cytochrome c 2 resulted in a loss of photosynthetic ability by TIE-1, indicating that this protein cannot be replaced by either HiPIP in cyclic electron flow. PioC was previously implicated in photoferrotrophy, an unusual form of photosynthesis in which reducing power is provided through ferrous iron oxidation. Using cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and flash-induced spectrometry, we show that PioC has a midpoint potential of 450 mV, contains all the typical features of a HiPIP, and can reduce the reaction centers of membrane suspensions in a light-dependent manner at a much lower rate than cytochrome c 2 . These data support the hypothesis that PioC linearly transfers electrons from iron, while cytochrome c 2 is required for cyclic electron flow. Rpal_4085, despite having spectroscopic characteristics and a reduction potential similar to those of PioC, is unable to reduce the reaction center. Rpal_4085 is upregulated by the divalent metals Fe(II), Ni(II), and Co(II), suggesting that it might play a role in sensing or oxidizing metals in the periplasm. Taken together, our results suggest that these three small electron transfer proteins perform different functions in the cell.

Biological macromolecules involved in electron transfer reactions display chains of closely packed redox cofactors when long distances must be bridged. This is a consequence of the need to maintain a rate of transfer compatible with metabolic activity in the framework of the exponential decay of electron tunneling with distance. In this work intermolecular electron transfer was studied in kinetic experiments performed with the small tetraheme cytochrome from Shewanella oneidensis MR-1 and from Shewanella frigidimarina NCIMB400 using non-physiological redox partners. This choice allowed the effect of specific recognition and docking to be eliminated from the measured rates. The results were analyzed with a kinetic model that uses the extensive thermodynamic characterization of these proteins reported in the literature to discriminate the kinetic contribution of each heme to the overall rate of electron transfer. This analysis shows that, in this redox chain that spans 23 Å , the kinetic properties of the individual hemes establish a functional specificity for each redox center. This functional specificity combined with the thermodynamic properties of these soluble proteins ensures directional electron flow within the cytochrome even outside of the context of a functioning respiratory chain.Shewanella are facultative anaerobic ␥-proteobacteria capable of reducing a multitude of organic and inorganic substrates, including soluble and insoluble metallic compounds containing iron, manganese, uranium, or chromium (1). Shewanella arouse widespread interest in the science and in the engineering communities due to their role in geological phenomena such as global weathering and formation of minerals, their possible application in bioremediation of contaminated environments polluted with heavy metals, and their biotechnological applications for energy production in microbial fuel cells (1-3).The anaerobic respiratory flexibility found in these bacteria is associated with the presence of numerous multiheme cytochromes (4). One of the most abundant is the small tetraheme cytochrome (STC) 2 that has a 12-kDa molecular mass and contains four c-type hemes (5). Although the physiological role of STC in Shewanella oneidensis MR-1 (SoSTC) is still unknown, in S. frigidimarina NCIMB400 (SfSTC) it was shown to participate in iron respiration (6). The three-dimensional structure of SoSTC was determined for the reduced and oxidized states by x-ray crystallography (7), and recently the solution structure was solved for the reduced state of SfSTC (8). These studies showed that, despite the diversity found in the amino acid sequence of these cytochromes (64% identical), the architecture of their heme core is highly conserved with the hemes organized in a chain spanning 23 Å for the most distant heme irons (7,8). The determination of the structure of SoSTC led to the proposal that this protein may work as a nonspecific electron harvester (7). This proposal was refined by considering that hemes I-III would feed electrons to heme IV on t...

Background:The di-heme cytochrome FoxE is predicted to be the iron oxidoreductase of the photoferrotroph Rhodobacter ferrooxidans SW2. Results: The thermodynamic and kinetic aspects of iron oxidation by FoxE are described. Conclusion: FoxE is thermodynamically and kinetically able to oxidize iron in a pH-dependent manner. Significance: This is the first functional characterization of a putative iron oxidoreductase involved in anoxygenic photosynthesis.

a b s t r a c tThe geometry of the axial ligands of the hemes in the triheme cytochrome PpcA from Geobacter sulfurreducens was determined in solution for the ferric form using the unambiguous assignment of the NMR signals of the a-substituents of the hemes. The paramagnetic 13 C shifts of the hemes can be used to define the heme electronic structure, the geometry of the axial ligands, and the magnetic susceptibility tensor. The latter establishes the magnitude and geometrical dependence of the pseudocontact shifts, which are crucial to warrant reliable structural constraints for a detailed structural characterization of this paramagnetic protein in solution.