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

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 = 110µs, 50% with t_1/2 = 600µs) 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, E_m = +240 mV; heme C₅₅₃, E_m = +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 components is proposed.