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The low potential electron transfer chain in the cytochrome b f complex

, : The low potential electron transfer chain in the cytochrome b f complex. Biochimica et Biophysica Acta 933(2): 319-333

The oxidized-minus-reduced spectra of the low- and high-potential cytochromes b6 were measured from 380 to 590 nm under anaerobic conditions in a mutant of Chlorella sorokiniana lacking of Photosystem II (PS II). The two spectra are similar in the Soret band; in the .alpha.-band, cytochrome b1 and cytochrome bh peak at 563.4 and 564 nm, respectively. The spectrum of a third component, G, located on the outer face of the membrane and able to exchange electrons with cytochrome bh (Lavergne, J. (1983) Biochim. Biophys. Acta 725, 25-33) was characterized. The oxidized minus-reduced spectrum of G displays a negative band peaking around 424 nm (probably a double peak) and a broad and small negative band in the green region. This spectrum resembles that of a soluble high-spin cytochrome c', as characterized in several classes of photosynthetic bacteria. As other cytochromes c', G binds CO with a low affinity, which inhibits electron exchange with cytochrome bh. The kinetics of reduction of cytochrome bh and cytochrome b1 were measured in algae under conditions, following illumination by weak continuous light or saturating flashes. From the kinetic behaviour, the difference between the midpoint potentials of cytochrome bh and cytochrome b1 was estimated at approx. 140 mV. Two populations can be distinguished among cytochrome b/f complexes. In a first fraction, the complexes are associated with the carrier G, which very likely takes part in an electron-transfer chain mediating cytochrome b reduction by a stromal reductant. This chain could be involved in cyclic electron flow around Photosystem I. In the second fraction, where G is not attached to the complex, the reduction of the b-cytochromes is a much slower process. Cytochrome bh and G are in equilibrium, the midpoint potential G being approx. 20 mV higher than that of cytochrome bh. From the study of the PS II-driven photoreduction of cytochrome bh and G (under aerobic conditions) in a mutant which lacks of PS I, we conclude that the affinity of plastosemiquinone for site C is much higher than that of plastiquinol, which makes the properties of this site similar to those of PS II secondary acceptor QB. These results are discussed in terms of a model in which the semiquinone can rapidly move from site Z to site C.

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