Cytochrome b6f: structure for signalling and vectorial metabolism

Following decades of detailed kinetic and spectroscopic evidence, two new, independent X-ray structures for the cytochrome b₆f complex of photosynthesis now reveal the arrangement of its key electron carriers relative to each other, and to their protein ligands. But these are not predictable additions to the structural collection. The complex is dimeric, and encloses a central chamber in which plastoquinone and its redox intermediates couple proton translocation with cytochrome oxidation and reduction. The structures also announce a fourth, wholly unexpected haem, that could be the long-sought, missing link of photosystem I cyclic photophosphorylation. One chlorophyll molecule and one carotenoid molecule add to the enigma of this dark, downhill electron transfer complex, linking the real photosystems I and II. Conserved structural features offer clues to the evolution of photosynthesis, and to the initiation of redox signals required for genome function.