Spectrally distinct cytochrome b -563 components in a chloroplast cytochrome b — f complex: Interaction with a hydroxyquinoline N -oxide

The two heme equivalents of cytochrome b -563 in the photosynthetic cytochrome b — f complex can be distinguished by their rate of reduction with dithionite at 25°C and by their optical absorption spectra at 77 K. The cytochrome b component that is rapidly reduced after addition of dithionite or reduced ferredoxin possesses an α band that splits at 77 K into two peaks, at 557 and 561 nm. Prolonged incubation with reductant reveals a second, approximately equimolar cytochrome b component that has at 77 K an unsplit α-band maximum at 561 nm. The designations cytochrome b -563 H and cytochrome b -563 L , respectively, are proposed for the rapidly and more slowly reduced cytochrome b -563 components. Potentiometric titration establishes a midpoint potential, E m , of -30 mV (electron change n ≈ 2) for cytochrome b -563 H and -150 mV ( n = 1) for cytochrome b -563 L at pH 7.5. The reduction potential of these components is raised by 2-heptyl-4-hydroxyquinoline N -oxide, giving E m values of +57 and -34 mV, respectively, with each titration slope approximating n = 2.