Am I right that [2Fe-2S] adrenodoxin (or ferredoxin) molecule is reduced (and can carry) one electron at a time? I've read several papers claiming that
Tobias is correct. The midpoint potential for mammalian adrenodoxins is around -270 mV. The physiologically relevant couple is FeIIFeII/FEIIIFeII (so ferric ferric to ferric ferrous; the reduced state has an unpaired electon and give a characteristic per spectrum.