SOME NEW STRUCTURAL ASPECTS AND OLD CONTROVERSIES CONCERNING THE CYTOCHROME b6f COMPLEX OF OXYGENIC PHOTOSYNTHESIS

Abstract

▪ Abstract  The cytochrome b6f 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 f and b6 (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 f 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 p (lumen)-side cytochrome b heme by plastosemiquinone indicates that the electron transfer pathway in the b6f complex can be described by a version of the Q-cycle mechanism, originally proposed to describe similar processes in the mitochondrial and bacterial bc1 complexes.