Membrane-Bound c-Type Cytochromes in Heliobacillus mobilis. In Vivo Study of the Hemes Involved in Electron Donation to the Photosynthetic Reaction Center

Abstract

The amount of heme per photosynthetic reaction center (RC) was examined in whole cells of Heliobacillus mobilis, and a stoichiometry of 5-6 hemes c and 1-3 hemes b per RC was found. Virtually the full complement of heme was seen to be functionally connected to the pool of electron donors to the photosynthetic RC. The kinetic parameters of electron transfer between reduced c-type hemes and the photooxidized primary donor P798+ were studied in whole cells and membrane fragments. The in vivo half-times of electron donation (50% with t 1/2 = 110 microseconds, 50% with t 1/2 = 600 microseconds) were seen to slow down to half-times in the range of several and several tens of milliseconds following disruption of cells. A severe conformational alteration or a change in the identity of the donating heme is discussed. Redox titrations of the flash-induced absorption changes performed on whole cells in the presence of mediators yielded the following redox midpoint potentials: P798, Em = +240 mV; heme c553, Em = +190, +170, and +90 mV for the heme components oxidized after the first, second, and third flash, respectively. The results demonstrate that the pool of c553 hemes donating electrons to the RC is heterogeneous and that it consists of either several distinguishable cytochromes or multiheme cytochromes or both. The number of hemes reduced and the kinetics of heme rereduction after flash-induced oxidation were found to depend strongly on the degree of anaerobicity in the interior compartment of the cell. A model rationalizing the obtained results in terms of a set of differing redox components is proposed.

Tracking molecular evolution of photosynthesis by characterization of a major photosynthesis gene cluster from Heliobacillus mobilis

“…It also shows a high degree of similarity to cytochrome c 553 from Heliobacterium gestii (17). Cytochrome c 553 is involved in electron transfer between the cytochrome bc complex and the RC in H. mobilis (18). PetJ has a single heme c binding motif, CNSCH (starting at amino acid residue 79), with Cys-79 and Cys-82 as covalent heme binding ligands and His-83 and Met-125 as axial heme ligands.…”

The Thermodynamics and Kinetics of Electron Transfer between Cytochrome b6f and Photosystem I in the Chlorophyll d-dominated Cyanobacterium, Acaryochloris marina

“…The midpoint potential of the MKH 2 /MK couple is more negative than that of the PQH 2 /PQ couple, which acts as the electron carrier between PSII and the cyt b 6 f complex in oxygenic photosynthetic chain. Although the available reducing power for electron transfer from the quinol is thus larger, it is remarkable that not only the midpoint potential of the P ϩ /P couple but also those of the cyt c, b H , and b L from the cyt b 6 c complex are down-shifted in H. mobilis with respect to their homologous cofactors in oxygenic photosynthetic organisms (40,47). Therefore, the driving force for the oxidation of MKH 2 at the Q 0 site, on the one hand, and for the electron transfer between the cyt b 6 c cofactors and P ϩ , on the other hand, are preserved.…”

Heliobacterial Rieske/cytb complex

“…Heme c i is attached to the protein by a single cysteine residue, which is conserved in Heliobacteria and rationalizes the heme-staining activity of the b 6 subunit as observed on SDS PAGE (Nitschke et al 1995;Ducluzeau et al 2008). The EPR signature of this heme has recently been identified in an enriched preparation of Heliobacterium modesticaldum showing that heme c i is indeed a redox cofactor in the heliobacterial Rieske/cytb complex as it is in b 6 f complexes (Ducluzeau et al 2008).…”

“…Identification of the cytochrome donor to the RCI is complicated by the fact that all c-type cytochromes present in Heliobacteria exhibit an a-band absorption around 553 nm. Multiflash experiments on living cells showed that 5-6 cytochromes c/RC are photooxidisable with redox midpoint potentials of 190, 170, and 90 mV (Nitschke et al 1995). On SDS PAGE, five bands with heme-staining activity at 14, 16, 18, 30, and 50 kDa were attributed to cytochromes c (Trost and Blankenship 1990).…”

“…The menaquinone pool was titrated to -60 mV (Kramer et al 1997), and the cytochrome c 553 to about ?180 mV (Nitschke et al 1996). Cytochrome c 553 reduces the primary donor of the RCI which has a midpoint potential of ?240 mV (Nitschke et al 1995). For the cofactors of the Rieske/cytb complex, redox midpoint potentials of ?120 mV (Liebl et al 1990)/ ?150 mV (Liebl 1992) for the Rieske protein, -90 and -190 mV for the b-hemes were assigned (Kramer et al 1997) in redox titrations on membranes.…”