title: Menaquinone as the Secondary Electron Acceptor in the Type I Homodimeric Photosynthetic Reaction Center of Heliobacterium modesticaldum.
abstract: The type I photosynthetic reaction center (RC) of heliobacteria (hRC) is a homodimer containing cofactors almost analogous to those in the plant photosystem I (PS I). However, its three-dimensional structure is not yet clear. PS I uses phylloquinone (PhyQ) as a secondary electron acceptor (A1), while the available evidence has suggested that menaquinone (MQ) in hRC has no function as A1. The present study identified a new transient electron spin-polarized electron paramagnetic resonance (ESP-EPR) signal, arising from the radical pair of the oxidized electron donor and the reduced electron acceptor (P800(+)MQ(-)), in the hRC core complex and membranes from Heliobacterium modesticaldum. The ESP signal could be detected at 5-20 K upon flash excitation only after prereduction of the iron-sulfur center, F(X), and was selectively lost by extraction of MQ with diethyl ether. MQ was suggested to be located closer to F(X) than PhyQ in PS I based on the simulation of the unique A/E (A, absorption; E, emission) ESP pattern, the reduction/oxidation rates of MQ, and the power saturation property of the static MQ(-) signal. The result revealed the quinone usage as the secondary electron acceptor in hRC, as in the case of PS I.
authors.forename: Toru
authors.initials: T
authors.lastname: Kondo
authors.forename: Shigeru
authors.initials: S
authors.lastname: Itoh
authors.affiliation: §Department of Biological Sciences, Graduate School of Science, Osaka University, Osaka 560-0043, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Matsuoka
authors.affiliation: §Department of Biological Sciences, Graduate School of Science, Osaka University, Osaka 560-0043, Japan.
authors.forename: Chihiro
authors.initials: C
authors.lastname: Azai
authors.affiliation: §Department of Biological Sciences, Graduate School of Science, Osaka University, Osaka 560-0043, Japan.
authors.forename: Hirozo
authors.initials: H
authors.lastname: Oh-oka
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002462:Cell Membrane / Q000737:chemistry; D000068016:Eubacteriales; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D004986:Ether / Q000737:chemistry; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000737:chemistry*; D013696:Temperature; D024482:Vitamin K 2 / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D045331:Photosystem I Protein Complex; D004986:Ether; D024482:Vitamin K 2