The cytochrome b₆f complex: plastoquinol oxidation and regulation of electron transport in chloroplasts

In oxygenic photosynthetic systems, the cytochrome b₆f (Cytb₆f) complex (plastoquinol:plastocyanin oxidoreductase) is a heart of the hub that provides connectivity between photosystems (PS) II and I. In this review, the structure and function of the Cytb₆f complex are briefly outlined, being focused on the mechanisms of a bifurcated (two-electron) oxidation of plastoquinol (PQH₂). In plant chloroplasts, under a wide range of experimental conditions (pH and temperature), a diffusion of PQH₂ from PSII to the Cytb₆f does not limit the intersystem electron transport. The overall rate of PQH₂ turnover is determined mainly by the first step of the bifurcated oxidation of PQH₂ at the catalytic site Q_o, i.e., the reaction of electron transfer from PQH₂ to the Fe₂S₂ cluster of the high-potential Rieske iron-sulfur protein (ISP). This point has been supported by the quantum chemical analysis of PQH₂ oxidation within the framework of a model system including the Fe₂S₂ cluster of the ISP and surrounding amino acids, the low-potential heme b₆^L, Glu78 and 2,3,5-trimethylbenzoquinol (the tail-less analog of PQH₂). Other structure-function relationships and mechanisms of electron transport regulation of oxygenic photosynthesis associated with the Cytb₆f complex are briefly outlined: pH-dependent control of the intersystem electron transport and the regulatory balance between the operation of linear and cyclic electron transfer chains.