REDOX TITRATION OF 2 [4FE-4S] CLUSTERS IN THE PHOTOSYNTHETIC REACTION-CENTER FROM THE ANAEROBIC GREEN SULFUR BACTERIUM CHLOROBIUM-VIBRIOFORME

Anaerobic green sulfur bacteria contain photosynthetic reaction center s 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 accepters. In thi s work, the iron-sulfur clusters in Chlorobium vibrioforme were studie d using selective light-induced reduction protocols, spin quantificati ons, and chemical redox titrations coupled with EPR detection. Illumin ation of a dark-frozen sample at 12 K results in the appearance of a s pectrum termed signal I. Chemical reduction in darkness at solution po tentials between -414 mV and -492 mV results in the appearance of a di fferent spectrum termed signal II. Illumination of these chemically po ised samples at 12 K results in the appearance of signal I such that t he 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 lower ed to -545 mV, the spectrum shifts to yet a third set of resonances, t ermed signal III. Concomitant with this shift is a loss of low tempera ture light-induced reduction of signal I. Photoaccumulation of a sampl e poised at a solution potential of -50 mV results also in the appeara nce of signal III at nearly the same spin concentration as the chemica lly reduced sample. Spin quantifications imply that signals I and II a re both derived from the reduction of one iron-sulfur cluster, termed center I; signal III is derived from simultaneous reduction of two iro n-sulfur clusters, centers I and II. By measuring the EPR signal inten sities over a range of solution potentials, centers I and II were show n to have E(m) (pH 10.0) values of -446 mV and -501 mV, respectively T he observations are consistent with a structural and functional analog y of centers I and II with F-A and F-B of PS I.