Spectral and redox characterization of the heme c_i of the cytochrome b_6f complex

摘要

Absorption spectra of the purified cytochrome b_6f complex from Chlamydomonas reinhardtii were monitored as a function of the redox potential. Four spectral and redox components were identified: in addition to heme f and the two b hemes, the fourth component must be the new heme c_i (also denoted x) recently discovered in the crystallographic structures. This heme is co-valently attached to the protein, but has no amino acid axial ligand. It is located in the plastoquinone-reducing site Q_i in the immediate vicinity of a b heme. Each heme titrated as a one-electron Nernst curve, with midpoint potentials at pH 7.0 of -130 mV and -35 mV (hemes b), +100 mV (heme c_i), and +355 mV (heme f). The reduced minus oxidized spectrum of heme c_i consists of a broad absorption increase centered ≈425 nm. Its potential has a dependence of -60 mV/pH unit, implying that the reduced form binds one proton in the pH 6-9 range. The Q_i site inhibitor 2-n-nonyl-4-hydroxyquin-oline N-oxide, a semiquinone analogue, induces a shift of this potential by about -225 mV. The spectrum of c_i matches the absorption changes previously observed in vivo for an unknown redox center denoted "G." The data are discussed with respect to the effect of the membrane potential on the electron transfer equilibrium between G and heme b_H found in earlier experiments.