SOME NEW STRUCTURAL ASPECTS AND OLD CONTROVERSIES CONCERNING THE CYTOCHROME<i>b</i>₆<i>f</i>COMPLEX OF OXYGENIC PHOTOSYNTHESIS

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 mus 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 A long) mostly beta-strand, two-domain protein, with the N-terminal alpha-amino group as orthogonal heme ligand and an internal linear 11-A 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.