Thermodynamic and EPR characteristics of a HiPIP-type iron-sulfur center in the succinate dehydrogenase of the respiratory chain.

Abstract

In addition to the two species of ferredoxin-type iron-sulfur centers (Centers S-l and S-2), a third iron-sulfur center (Center S-3), which is paramagnetic in the oxidized state analogous to the bacterial high potential iron-sulfur protein, has been detected in the reconstitutively active soluble succinate dehydrogenase preparation.Midpoint potential (at pH 7.4) of Center S-3 determined in a particulate succinate-cytochrome c reductase is +60 =t 15 mV.In soluble form, Center S-3 becomes extremely labile towards oxygen or ferricyanide plus phenazine methosulfate similar to reconstitutive activity of the dehydrogenase.Thus, even freshly prepared reconstitutively active enzyme preparations show EPR spectra of Center S-3 which correspond approximately to 0.5 eq per flavin; in particulate preparations this component was found in a 1: 1 ratio to flavin.All reconstitutively inactive dehydrogenase preparations so far examined give rise to no Center S-3 spectra or, if any, highly modified.These observations indicate that Center S-3 is an innate constituent of succinate dehydrogenase and plays an important role in mediating electrons from the flavoprotein subunit to most probably ubiquinone and then to the cytochrome chain.