Cyclic electron transfer in Heliobacillus mobilis involving a menaquinol-oxidizing cytochrome bc complex and an RCI-type reaction center

Flash-induced absorption changes arising from b-type hemes were studied on whole cells of Heliobacillus mobilis under physiological and redox- controlled conditions. The sensitivity of the monitored redox changes to inhibitors of cytochrome bc complexes and the redox potential dependence of reduction and oxidation reactions of cytochrome b-hemes demonstrate that the respective b-hemes are part of a cytochrome bc complex. Both the half-time and the extent of flash-induced reduction of cytochrome b titrated with apparent potentials of about -60 and -50 mV (both n = 2), respectively, i.e., close to the E(m,7) value of the menaquinone (MK) pool, indicating a collisional interaction between menaquinol and the Q(o) site of the cytochrome bc complex. At strongly reducing ambient potentials (<-150 mV), a net flash-induced oxidation of b-hemes was observed in agreement with the E(m,7) values of the individual hemes of -90 mV (b(b)) and -190 mV (b(l)) determined in equilibrium redox titrations on membrane fragments. From the extent of photooxidized b- and c-type hemes as well as P₇₉₈⁺, a stoichiometry of 0.6-0.75 cytochrome bc complexes per photosynthetic reaction center was estimated. The kinetic behavior and also the energy profiles for Q-cycle turnover of the heliobacterial complex are compared to those of cytochrome bc(l) complexes from purple bacteria and of cytochrome b₆f complexes from chloroplasts.