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

In oxygenic photosynthetic systems, the cytochrome b(6)f (Cytb(6)f) 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 Cytb(6)f complex are briefly outlined, being focused on the mechanisms of a bifurcated (two-electron) oxidation of plastoquinol (PQH(2)). In plant chloroplasts, under a wide range of experimental conditions (pH and temperature), a diffusion of PQH(2) from PSII to the Cytb(6)f does not limit the intersystem electron transport. The overall rate of PQH(2) turnover is determined mainly by the first step of the bifurcated oxidation of PQH(2) at the catalytic site Q(o), i.e., the reaction of electron transfer from PQH(2) to the Fe2S2 cluster of the high-potential Rieske iron-sulfur protein (ISP). This point has been supported by the quantum chemical analysis of PQH(2) oxidation within the framework of a model system including the Fe2S2 cluster of the ISP and surrounding amino acids, the low-potential heme b(6)(L), Glu78 and 2,3,5-trimethylbenzoquinol (the tail-less analog of PQH(2)). Other structure-function relationships and mechanisms of electron transport regulation of oxygenic photosynthesis associated with the Cytb(6)f 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.

Automated summary

In oxygenic photosynthetic systems, the Cytb(6)f complex plays a crucial role in connecting photosystem II and I. This review focuses on the mechanism of the bifurcated oxidation of plastoquinol within the Cytb(6)f complex. The diffusion of plastoquinol from PSII to Cytb(6)f does not limit intersystem electron transport, and the rate of plastoquinol turnover is mainly determined by the oxidation at the catalytic site Q(o).