PARENT SESSION
Posters P8A Cytochrome b-c complexes. Abstracts (382-393)

The new heme of the cytochrome b₆f complex: an old acquaintance? Fabrice Rappaport^*,1, Yves Pierre², Daniel Picot², Jérôme Lavergne³, ¹ Institut de Biologie Physico-Chimique, CNRS UPR 1261, Paris, France² Institut de Biologie Physico-Chimique, CNRS UMR 7099, Paris, France³

ABSTRACT- We monitored the visible absorption spectra of the purified cytochrome b₆f complex from Chlamydomonas rheinardtii while varying the redox potential from −250 mV to + 500 mV. Four spectral and redox components were identified: in addition to heme f and to the two b-hemes, the fourth component must be the new heme c_i (Stroebel at al. 2003, Nature, 413-418) or x (Kurisu et al. 2003, Science, 302, 1009-1014) recently discovered in the crystallographic structures. Each component titrated as a one-electron Nernst curve, with midpoint potentials at pH 8.0 of −150 mV and −50 mV (hemes b), + 75 mV (new heme) and + 355 mV (heme f). For the b and f hemes, these E_m values are similar to those previously reported (Pierre et al., J. Biol. Chem. 1995, 29342-29349). The reduced minus oxidized spectrum of this new heme consists in a broad absorption increase centered around 425 nm and small changes in the green region. This is consistent with the absorption changes previously described for redox center "G" (Lavergne, 1983, BBA 725, 25-33). This species was shown to undergo a rapid redox equilibrium with a b heme, which was displaced (Gred box <-> Gox bred) under the influence of the membrane potential. The spectral and structural features found for the new heme render its identification with G very likely. Due to the proximity of heme c_i with the neighboring heme "b_i", an important electrostatic interaction is expected between them, raising the possibility that the latter corresponds to the species titrating around −150 mV rather than −50 mV. This disposition would also better account for the sensitivity of the G <-> b equilibrium toward the membrane potential.–

KEY WORDS: high spin heme, b6f complex, electron transfer