title: An atypical haem in the cytochrome b(6)f complex.
abstract: Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b(6)f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc(1), cytochrome b(6)f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 A of cytochrome b(6)f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc(1) but also exhibits some unique features, such as binding chlorophyll, beta-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.
authors.affiliation: Laboratoire de Physico-Chimie Moléculaire des Membranes Biologiques, CNRS/Université Paris 7, UMR 7099, France.
authors.forename: David
authors.initials: D
authors.lastname: Stroebel
authors.forename: Yves
authors.initials: Y
authors.lastname: Choquet
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Popot
authors.forename: Daniel
authors.initials: D
authors.lastname: Picot
mesh_terms: D000818:Animals; D016227:Benzoquinones / Q000378:metabolism; D001665:Binding Sites; D016825:Chlamydomonas reinhardtii / Q000201:enzymology*; D002734:Chlorophyll / Q000378:metabolism; D018360:Crystallography, X-Ray; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000378:metabolism*; D019281:Dimerization; D006418:Heme / Q000378:metabolism*; D008024:Ligands; D050356:Lipid Metabolism; D008958:Models, Molecular; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D019207:beta Carotene / Q000378:metabolism
chemical_list: D016227:Benzoquinones; D008024:Ligands; D019207:beta Carotene; D002734:Chlorophyll; C004532:quinone; D006418:Heme; D045346:Cytochrome b6f Complex