title: Photosynthetic electron transfer through the cytochrome b6f complex can bypass cytochrome f.
abstract: The cytochrome b(6)f complex is an obligatory electron transfer and proton-translocating enzyme in all oxygenic photosynthesis. Its operation has been described by the "Q-cycle." This model proposes that electrons are transferred from plastoquinol to plastocyanin (the reductant of P700 in Photosystem I) through, obligatorily in series, the iron-sulfur and the cytochrome f redox centers in the cytochrome b(6)f complex. However, here we demonstrate that (a) the iron-sulfur center-dependent reductions of plastocyanin and P700 are much faster than cytochrome f reduction, both in Chlamydomonas reinhardtii cytochrome f mutants and in the wild type, and (b) the steady-state photosynthetic electron transport does not correlate with strongly inhibited cytochrome f reduction kinetics in the mutants. Thus, cytochrome f is not an obligatory intermediate for electrons flowing through the cytochrome b(6)f complex. The oxidation equivalents from Photosystem I are delivered to the high potential chain of the cytochrome b(6)f complex both at the cytochrome f level and, independently, at another site connected to the quinol-oxidizing site, possibly the iron-sulfur center.
authors.affiliation: Department of Plant and Microbial Biology, University of California, Berkeley, California 94720, USA. jfernand@nature.berkeley.edu
authors.forename: J G
authors.initials: JG
authors.lastname: Fernández-Velasco
authors.forename: A
authors.initials: A
authors.lastname: Jamshidi
authors.forename: X S
authors.initials: XS
authors.lastname: Gong
authors.forename: J
authors.initials: J
authors.lastname: Zhou
authors.forename: R Y
authors.initials: RY
authors.lastname: Ueng
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D003573:Cytochrome b Group / Q000378:metabolism*; D045346:Cytochrome b6f Complex; D003580:Cytochromes / Q000378:metabolism*; D045348:Cytochromes f; D004579:Electron Transport; D007700:Kinetics; D010084:Oxidation-Reduction; D010788:Photosynthesis; D010970:Plastocyanin / Q000378:metabolism
chemical_list: D003573:Cytochrome b Group; D003580:Cytochromes; D002734:Chlorophyll; C001785:chlorophyll P 700; D010970:Plastocyanin; D045346:Cytochrome b6f Complex; D045348:Cytochromes f