Membrane-bound c-type cytochromes in Heliobacillus mobilis Characterisation by EPR and optical spectroscopy in membranes and detergent-solubilised material

The spectral and electrochemical parameters, as well as the orientations of the heme planes with respect to the membrane plane, of the c-type hemes present in membrane fragments from Heliobacillus mobilis were characterised by optical and EPR spectroscopy. Cytochrome c₅₅₃ was thereby shown to represent at least four and possibly five heme species with the following characteristics: E_m = -60 mV ± 10 mV, g_l = 2.92, 60°; E_m = +90 mV ± 10 mV, g_l = 2.92, 90°; E_m = +120 mV ± 20 mV, g_l = 3.03 ; and E_m = +170 mV ± 20 mV, g_l = 3.03. The latter component may correspond to two hemes with redox midpoint potentials of E_m = +160 mV ± 20 mV and E_m = +180 mV ± 20 mV (all E_m values at pH 7.0). For the heme species having g_l peaks at g≈3.03, determination of individual orientations was precluded due to the superposition of several differently oriented hemes. About one copy of each heme was found to be present per photosynthetic reaction centre, with the exception of the +120 mV component for which a stoichiometry of 2 hemes/reaction centre was obtained. The heme proteins were detergent-solubilised and partially purified. Three c-type cytochromes that migrated with apparent molecular masses of 18, 29 and 50 kDa were detected on SDS/PAGE. Optical redox titrations at pH 7.0 showed redox midpoint potentials of +160 mV ± 10 mV for the 18-kDa cytochrome, and -60 mV ± 10 mV, with possible contributions around +160 mV, for the 50-kDa cytochrome. A tentative attribution of heme species observed in membranes to the isolated heme proteins is presented. The results obtained on H. mobilis are compared with those reported for green sulphur bacteria.