Cytochrome b 6 f Complex at the Heart of Energy Transduction and Redox Signaling

Abstract

The cytochrome (Cyt) b(6)f complex (plastoquinol plastocyanin or Cyt c(6) oxidoreductase) lies at the intersection of photosynthetic and respiratory electron transfer pathways in cyanobacteria and chloroplasts. It functions in both linear electron flow between the Photosystem II and I reaction centers and cyclic flow around Photosystem I, serving as an efficient coupling site that generates a majority of the proton gradient for ATP synthesis. The Cyt b(6)f complex is related to mitochondrial-bacterial Cyt bc(1) complexes and belongs to a larger family of energy transducing 'Rieske cytochrome b' complexes that are widespread among bacterial and archael lineages. In addition to energy conversion, Cyt b(6)f complexes serve as sensors of plastoquinone pool redox status and in signaling both short-term adjustments in photosynthesis and longer-term gene expression events. The past several years have witnessed remarkable advances in understanding of structures and reaction mechanisms. This chapter attempts to convey the excitement of these discoveries and the central role of the Cyt b(6)f complex in energy conversion and environmental responses of photosynthetic organisms. Important advances include: the high-resolution structures of Cyt b(6)f and Cyt bc(1) complexes; evidence for domain movement of the Rieske iron-sulfur protein subunit during catalysis; discovery of a previously undetected c-type heme required for transmembrane electron flow and activity of the plastoquinone-reductase site of Cyt b(6)f complexes; evidence for supercomplex formation in direct cyclic electron flow from Photosystem I to the Cyt b(6)f complex; roles for small subunits and peripherally associated proteins, and evidence for biologically significant intermonomer electron transfer in the dimeric Cyt bc(1)-b(6)f complexes. Many intriguing questions remain unanswered. I apologize to those whose contributions were not included or overlooked.