title: Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways.
abstract: At least two features of the crystal structures of the cytochrome b6f complex from the thermophilic cyanobacterium, Mastigocladus laminosus and a green alga, Chlamydomonas reinhardtii, have implications for the pathways and mechanism of charge (electron/proton) transfer in the complex: (i) The narrow 11 x 12 A portal between the p-side of the quinone exchange cavity and p-side plastoquinone/quinol binding niche, through which all Q/QH2 must pass, is smaller in the b6f than in the bc1 complex because of its partial occlusion by the phytyl chain of the one bound chlorophyll a molecule in the b6f complex. Thus, the pathway for trans-membrane passage of the lipophilic quinone is even more labyrinthine in the b6f than in the bc1 complex. (ii) A unique covalently bound heme, heme cn, in close proximity to the n-side b heme, is present in the b6f complex. The b6f structure implies that a Q cycle mechanism must be modified to include heme cn as an intermediate between heme bn and plastoquinone bound at a different site than in the bc1 complex. In addition, it is likely that the heme bn-cn couple participates in photosytem I-linked cyclic electron transport that requires ferredoxin and the ferredoxin: NADP+ reductase. This pathway through the n-side of the b6f complex could overlap with the n-side of the Q cycle pathway. Thus, either regulation is required at the level of the redox state of the hemes that would allow them to be shared by the two pathways, and/or the two different pathways are segregated in the membrane.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA. waclab@purdue.edu
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
authors.forename: Huamin
authors.initials: H
authors.lastname: Zhang
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016227:Benzoquinones / Q000378:metabolism; D001665:Binding Sites; D016825:Chlamydomonas reinhardtii / Q000201:enzymology; D002736:Chloroplasts / Q000201:enzymology; D000458:Cyanobacteria / Q000201:enzymology; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000502:physiology; D004579:Electron Transport; D005287:Ferredoxin-NADP Reductase / Q000378:metabolism; D005288:Ferredoxins / Q000378:metabolism; D006418:Heme / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D021122:Protein Subunits / Q000737:chemistry / Q000502:physiology; D017386:Sequence Homology, Amino Acid
chemical_list: D016227:Benzoquinones; D005288:Ferredoxins; D021122:Protein Subunits; C004532:quinone; D006418:Heme; D045346:Cytochrome b6f Complex; D005287:Ferredoxin-NADP Reductase