Spectrally distinct cytochrome b-563 components in a chloroplast cytochrome b-f complex: Interaction with a...

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 degrees 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 alpha 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 alpha-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 approximately 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.