The cytochrome b(6)f complex: plastoquinol oxidation and regulation of electron transport in chloroplasts.

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