Two types of cytochrome cd₁ in the aerobic photosynthetic bacterium, Erythrobacter sp. OCh 114

Components I and II of cytochrome cd₁ which had different spectral features were purified from the aerobic photosynthetic bacterium, Erythrobacter sp. strain OCh 114. Component I showed an absorption maxima at 700 and 406 nm in the oxidized form, and at 621, 552.5, 548 and 416 nm in the reduced form. Component II showed an absorption maxima at 635 and 410 nm in the oxidized form and at 628, 552,5, 548 and 417 nm in the reduced form. The relative molecular mass, M_r, of both cytochromes was determined to be 135000 with two identical subunits. Components I and II showed pI values of 7.6 and 6.8, respectively. The redox potential of hemes ranged form +234 mV to +242 mV, except for the heme d₁ of component I (E_m7=+ 134 mV). Components I and II showed both cytochrome c oxidase and nitrite reductase activities. Cytochrome c oxidase activity was strongly inhibited by a low concentration of nitrite and cyanide. Erythrobacter cytochromes c‐551 and c‐552 were utilized as electron donors for the cytochrome c oxidase reaction. The high affinity of cytochrome c‐552 to component II (K_m= 1.27 μM) suggested a physiological significance for this cytochrome. Erythrobacter cytochromes cd₁ are unique in their presence in cells grown under aerobic conditions as compared to other bacterial cytochromes cd₁ which are formed only under denitrifying conditions.