Membrane Protein Complexes: Structure and Function

Alexander N. Tikhonov, Tikhonov, Alexander N.

This chapter presents an overview of structural properties of the cytochrome (Cyt) b6f complex and its functioning in chloroplasts. The Cyt b6f complex stands at the crossroad of photosynthetic electron transport pathways, providing connectivity between Photosystem (PSI) and Photosysten II (PSII) and pumping protons across the membrane into the thylakoid lumen. After a brief review of the chloroplast electron transport chain, the consideration is focused on the structural organization of the Cyt b6f complex and its interaction with plastoquinol (PQH2, reduced form of plastoquinone), a mediator of electron transfer from PSII to the Cyt b6f complex. The processes of PQH2oxidation by the Cyt b6f complex have been considered within the framework of the Mitchell's Q-cycle. The overall rate of the intersystem electron transport is determined by PQH2turnover at the quinone-binding site Qoof the Cyt b6f complex. The rate of PQH2oxidation is controlled by the intrathylakoid pHin, which value determines the protonation/deprotonation events in the Qo-center. Two other regulatory mechanisms associated with the Cyt b6f complex are briefly overviewed: (i) redistribution of electron fluxes between alternative (linear and cyclic) pathways, and (ii) "state transitions" related to redistribution of solar energy between PSI and PSII.