title: Path of electron transfer in photosystem 1: direct evidence of forward electron transfer from A1 to Fe-Sx.
abstract: Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A1. At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P700+/A1-, it decays with t1/e = 23 microseconds, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t1/c = 200 ns in untreated photosystem 1, due to forward electron transfer from A1- to one of the iron-sulfur centers. This rate is unchanged in Fe-SA/B-depleted PS1 but is lost when the iron-sulfur center Fe-Sx is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t1/c = 1.3 microseconds, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P700-->A1-->Fe-SX-->Fe-SA/B.
authors.affiliation: Department of Biology, University College London, U.K.
authors.forename: P
authors.initials: P
authors.lastname: Moënne-Loccoz
authors.forename: P
authors.initials: P
authors.lastname: Heathcote
authors.forename: D J
authors.initials: DJ
authors.lastname: Maclachlan
authors.forename: M C
authors.initials: MC
authors.lastname: Berry
authors.forename: I H
authors.initials: IH
authors.lastname: Davis
authors.forename: M C
authors.initials: MC
authors.lastname: Evans
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D003080:Cold Temperature; D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D010944:Plants / Q000737:chemistry; D013696:Temperature; D010837:Vitamin K 1 / Q000378:metabolism*
chemical_list: D007506:Iron-Sulfur Proteins; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll; D010837:Vitamin K 1