Iron-sulfur proteins of the green photosynthetic bacterium Chlorobium

The iron-sulfur proteins of the green photosynthetic bacterium Chlorobium have been characterized by oxidation-reduction potentiometry in conjunction with low-temperature electron paramagnetic resonance spectroscopy. Chlorobium ferredoxin was the only iron-sulfur protein detected in the soluble fraction; no high-potential iron-sulfur protein was observed. In addition, high-potential iron-sulfur protein was not detected in the chromatophores. Four chromatophore-bound iron-sulfur proteins were detected. One is the “Rieske” type iron-sulfur protein with a g-value of 1.90 in the reduced state; the protein has a midpoint potential of +160 mV (pH 7.0), and this potential is pH dependent. Three g = 1.94 chromatophore-bound iron-sulfur proteins were observed, with midpoint potentials of ?25, ?175, and about ?550 mV. A possible role for the latter iron-sulfur protein in the primary photochemical reaction in Chlorobium is considered.