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 studie d on whole cells of Heliobacillus mobilis under physiological and redo x-controlled conditions. The sensitivity of the monitored redox change s to inhibitors of cytochrome be complexes and the redox potential dep endence of reduction and oxidation reactions of cytochrome b-hemes dem onstrate that the respective b-hemes are part of a cytochrome be compl ex. Both the half-time and the extent of flash-induced reduction of cy tochrome 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 mena quinone (MK) pool, indicating a collisional interaction between menaqu inol and the Q(o) site of the cytochrome be complex. At strongly reduc ing ambient potentials (<-150 mV), a net flash-induced oxidation of b- hemes was observed in agreement with the Em,7 values of the individual hemes of -90 mV (b(h)) and -190 mV (b(l)) determined in equilibrium r edox titrations on membrane fragments. From the extent of photooxidize d b- and c-type hemes as well as P-798(+), a stoichiometry of 0.6-0.75 cytochrome be complexes per photosynthetic reaction center was estima ted. The kinetic behavior and also the energy profiles for Q-cycle tur nover of the heliobacterial complex are compared to those of cytochrom e bc(1) complexes from purple bacteria and of cytochrome b(6)f complex es from chloroplasts.