ELECTRON-TRANSFER BETWEEN PHOTOSYSTEM-II AND THE CYTOCHROME-B/F COMPLEX - MECHANISTIC AND STRUCTURAL IMPLICATIONS

Electron transfer between Photosystem II (PSII) centers, cytochrome b/f complexes and Photosystem I (PSI) centers have been studied in isolated spinach chloroplasts in the presence of ferricyanide. The analysis of the reduction of cytochrome b(h) under saturating illumination shows that about 2/3 of cytochrome b/f complexes are rapidly accessible to PSII-formed plastoquinol (< 10 ms), while for the remaining fraction, cytochrome b(h) is reduced in the 100 ms range. These results are interpreted according to a model proposed by Joliot et al. (Joliot, P., Lavergne, J. and Beal, D. (1992) Biochim. Biophys. Acta 1101, 1-12), in which the diffusion of plastoquinone is restricted to domains, including an average of 4 PSII centers. Cytochrome b/f complexes included in the grana regions are involved in the linear electron flow from PSII to PSI; long-range electron transfer reactions are exclusively mediated by plastocyanin. Under flash excitation, the rate of electron transfer from PSII to PSI is limited neither by the diffusion nor by the binding of plastoquinol to cytochrome b/f complexes, but by electron transfer processes occurring within the cytochrome b/f complexes. The rate of these limiting processes is very likely controlled by the redox state of the high potential chain (cytochrome f and Rieske protein). Electron transfer through the cytochrome b/f complexes is discussed in terms of a Q-cycle or a semiquinone cycle mechanism.