Structure of the cytochrome b6f complex:: new prosthetic groups, Q-space, and the hors d'oeuvres hypothesis for assembly of the complex

3-angstrom crystal structures of the cytochrome b(6) f complex have provided a structural framework for the photosynthetic electron transport chain. The structures of the 220,000 molecular weight dimeric cytochrome b(6) f complex from the thermophilic cyanobacterium, Mastigocladis laminosus (Kurisu et al. 2003, Science 302: 1009 - 1014), and the green alga, Chlamydomonas reinhardtii (Stroebel et al. 2003, Nature 426: 413 418), are very similar. The latter is the first structure of a integral membrane photosynthetic electron transport complex from a eukaryotic source. The M. laminosus and C. reinhardtii structures have provided structural information and experimental insights to the properties and functions of three native and novel prosthetic groups, a chlorophyll a, a beta-carotene, and a unique heme x, one copy of which is found in each monomer of the cytochrome b(6) f complex, but not the cytochrome bc(1) complex from the mitochondrial respiratory chain of animals and yeast. Several functional insights have emerged from the structures including the function of the dimer; the properties of heme x; the function of the inter-monomer quinone-exchange cavity; a quinone diffusion pathway through relatively narrow crevices or portals; a modified reaction scheme for n-side quinone redox reactions; a necessarily novel mechanism for quenching of the bound chlorophyll triplet state; a possible role for the bound chlorophyll a in activation of the LHC kinase; and a structural and assembly role for the four small PetG, L, M, and N subunits. An 'hors d'oeuvres hypothesis' for assembly of the complex is proposed for the small 'hydrophobic stick' or 'picket fence' polypeptides at the periphery of the complex, based on the cis-positive orientation of the small hydrophobic subunits and the 'toothpick' binding mode of the beta-carotene.