Thermodynamic resolution of the iron-sulfur centers of the succinic dehydrogenase of Rhodopseudomonas sphaeroides.

A single alkaline wash removes most of the succinic dehydrogenase activity from chromatophores of Rhodopseudomonas sphaeroides. Three iron-sulfur centers are also removed by this washing. Two of these are ferredoxin-like centers with electron paramagnetic resonance signals at g_v = 1.94 and midpoint potentials of +50 and ?250 mV at pH 7. The third is a high-potential iron-sulfur protein type signal centered at g 2.01 and a midpoint potential of +80 mV at pH 7. These centers have very similar properties to those of the well-characterized mammalian succinic dehydrogenase and account for the majority of iron-sulfur centers observed in chromatophores. Because it is so easily removed, it is concluded that succinic dehydrogenase is located on the outer surface of the chromatophore membrane, a conclusion supported by the fact that removal of the enzyme does not interfere with the kinetics of light-induced electron flow, nor does it allow cytochrome c₂ to escape from inside the chromatophore vesicles.