title: ESR signal of the iron-sulfur center F(X) and its function in the homodimeric reaction center of Heliobacterium modesticaldum.
abstract: Electron transfer in the membranes and the type I reaction center (RC) core protein complex isolated from Heliobacterium modesticaldum was studied by optical and ESR spectroscopy. The RC is a homodimer of PshA proteins. In the isolated membranes, illumination at 14 K led to accumulation of a stable ESR signal of the reduced iron-sulfur center F(B)(-) in the presence of dithiothreitol, and an additional 20 min illumination at 230 K induced the spin-interacting F(A)(-)/F(B)(-) signal at 14 K. During illumination at 5 K in the presence of dithionite, we detected a new transient signal with the following values: g(z)= 2.040, g(y)= 1.911, and g(x)= 1.896. The signal decayed rapidly with a 10 ms time constant after the flash excitation at 5 K and was attributed to the F(X)(-)-type center, although the signal shape was more symmetrical than that of F(X)(-) in photosystem I. In the purified RC core protein, laser excitation induced the absorption change of a special pair, P800. The flash-induced P800(+) signal recovered with a fast 2-5 ms time constant below 150 K, suggesting charge recombination with F(X)(-). Partial destruction of the RC core protein complex by a brief exposure to air increased the level of the P800(+)A(0)(-) state that gave a lifetime (t(1/2)) of 100 ns at 77 K. The reactions of F(X) and quinone were discussed on the basis of the three-dimensional structural model of RC that predicts the conserved F(X)-binding site and the quinone-binding site, which is more hydrophilic than that in the photosystem I RC.
authors.affiliation: Division of Material Science, Graduate School of Science, Nagoya University, Nagoya 456-8602, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Miyamoto
authors.forename: Masayo
authors.initials: M
authors.lastname: Iwaki
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Mino
authors.forename: Jiro
authors.initials: J
authors.lastname: Harada
authors.forename: Shigeru
authors.initials: S
authors.lastname: Itoh
authors.forename: Hirozo
authors.initials: H
authors.lastname: Oh-Oka
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D016998:Helicobacter / Q000737:chemistry / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013696:Temperature
chemical_list: D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins