title: Redox titration of two [4Fe-4S] clusters in the photosynthetic reaction center from the anaerobic green sulfur bacterium Chlorobium vibrioforme.
abstract: Anaerobic green sulfur bacteria contain photosynthetic reaction centers analogous to photosystem I (PS I) of plants and cyanobacteria. These reaction centers, termed type I, are characterized by the presence of bound iron-sulfur clusters as the terminal electron acceptors. In this work, the iron-sulfur clusters in Chlorobium vibrioforme were studied using selective light-induced reduction protocols, spin quantifications, and chemical redox titrations coupled with EPR detection. Illumination of a dark-frozen sample at 12 K results in the appearance of a spectrum termed signal I. Chemical reduction in darkness at solution potentials between -414 mV and -492 mV results in the appearance of a different spectrum termed signal II. Illumination of these chemically poised samples at 12 K results in the appearance of signal I such that the sum of the intensity of signal I + signal II is nearly constant for every ratio of signal I/signal II. As the solution potential is lowered to -545 mV, the spectrum shifts to yet a third set of resonances, termed signal III. Concomitant with this shift is a loss of low temperature light-induced reduction of signal I. Photoaccumulation of a sample poised at a solution potential of -50 mV results also in the appearance of signal III at nearly the same spin concentration as the chemically reduced sample. Spin quantifications imply that signals I and II are both derived from the reduction of one iron-sulfur cluster, termed center I; signal III is derived from simultaneous reduction of two iron-sulfur clusters, centers I and II. By measuring the EPR signal intensities over a range of solution potentials, centers I and II were shown to have Em (pH 10.0) values of -446 mV and -501 mV, respectively. The observations are consistent with a structural and functional analogy of centers I and II with F(A) and F(B) of PS I.
authors.affiliation: The University of Nebraska, Department of Biochemistry, Lincoln, USA.
authors.forename: M P
authors.initials: MP
authors.lastname: Scott
authors.forename: B
authors.initials: B
authors.lastname: Kjoer
authors.forename: H V
authors.initials: HV
authors.lastname: Scheller
authors.forename: J H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D000693:Anaerobiosis; D019414:Chlorobi / Q000737:chemistry*; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000528:radiation effects; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000528:radiation effects; D013455:Sulfur / Q000737:chemistry
chemical_list: D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D013455:Sulfur; D007501:Iron