The Cytochrome b 6 f Complex–Composition, Structure and Function

Abstract

The cytochrome b6f complex universally functions in oxygenic photosynthesis as plastoquinol-plastocyanin oxidoreductase or as plastoquinol-cytochrome c oxidoreductase between the two light reactions of Photosystem II and Photosystem I. It is structurally and functionally related to the cytochrome bc1 complex of respiratory organisms and comprises 4 redox centers–1 heme c, 2 hemes b and the Rieske 2Fe2S-center. These operate quinol oxidation in a concerted mechanism, called the Q-cycle, which translocates twice the protons that would be translocated by plastoquinone/quinol action alone. It is still under debate whether this efficient energy conservation is operative under all conditions. The redox centers are bound by the 3 proteins, cytochrome f (heme c), cytochrome b6 (2 hemes b) and the Rieske FeS-protein, whose genes have been sequenced for numerous plants and cyanobacteria. A fourth protein (subunit IV) is universally present and corresponds to the C-terminal part of cytochrome b of the bc1 complexes. While a high resolution 3D-structure is now available for cytochrome f (see Chapter 22), the 3D-strucrures of the Rieske FeS-protein and of cytochrome b6 plus subunit IV still depend on predictions from the sequences. The predicted folding for cytochrome b6 with its transmembrane arrangement of the 2 hemes is in reassuring consistency with the Q-cycle. In addition to its role as energy converter the cytochrome b6f complex functions as a redox sensor of the plastoquinone pool in regulating the light distribution between the two photosystems.