The cytochrome complex functions in oxygenic photosynthetic membranes as the redox link between the photosynthetic reaction center complexes II and I and also functions in proton translocation. It is an ideal integral membrane protein complex in which to study structure and function because of the existence of a large amount of primary sequence data, purified complex, the emergence of structures, and the ability of flash kinetic spectroscopy to assay function in a readily accessible ms–100 μs time domain. The redox active polypeptides are cytochromes and (organelle encoded) and the Rieske iron-sulfur protein (nuclear encoded) in a mol wt = 210,000 dimeric complex that is believed to contain 22–24 transmembrane helices. The high resolution structure of the lumen-side domain of cytochrome shows it to be an elongate (75 Å long) mostly β-strand, two-domain protein, with the N-terminal α-amino group as orthogonal heme ligand and an internal linear 11-Å bound water chain. An unusual electron transfer event, the oxidant-induced reduction of a significant fraction of the (lumen)-side cytochrome heme by plastosemiquinone indicates that the electron transfer pathway in the complex can be described by a version of the Q-cycle mechanism, originally proposed to describe similar processes in the mitochondrial and bacterial complexes.


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  • Article Type: Review Article
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