EPR studies of the cytochrome-d complex of Escherichia coli

We have examined the thermodynamic and EPR properties of one of the ubiquinol oxidase systems (the cytochrome d complex) of Escherichia coli, and have assigned the EPR-detectable signals to the optically identified cytochromes. The axial high spin g = 6.0 signal has been assigned to cytochrome d basedon the physicochemical properties of this signal and those of the optically defined cytochrome d. A rhombic low spin species at g_x,y,z = 1.85, 2.3, 2.5 exhibited similar properties but was present at only one-fifth the concentration of the axial high spin species. Both species have an E_m7 of 260 mV and follow a -60 mV/pH unit dependence from pH 6 to 10. The rhombic high spin signal with g_y,z = 5.5 and 6.3 has been assigned to cytochrome b-595. This component has an E_m7 of 136 mV and follows a -30 mV/pH unit dependence from pH 6 to 10. Lastly, the low spin g_z = 3.3 signal whichtitrates with an E_m7 of 195 mV and follows a -40 mV/pHunit dependence from pH 6 to 10 has been assigned to cytochrome b-558. Spin quantitation of the high-spin signals indicates that cytochrome d and b-595 are present in approximately equal amounts. These observations are discussed in termsof the stoichiometry of the prosthetic groups and its implications on the mechanism of electron transport.