Purification and properties of high potential iron-sulphur protein from Thiocapsa roseopersicina.

High potential iron-sulphur protein (HiPIP) has been purified to electrophoretic homogeneity from the photosynthetic bacterium Thiocapsa roseopersicina. The protein has a single polypeptide chain (molecular mass 10 kDa) containing one 4Fe-4S cluster. The midpoint redox potential (E = 0.35 V), isoelectric points and pH profile, as well as the absorption, circular dichroism and Mössbauer spectroscopic properties in the reduced and oxidized states have been determined. The protein is in the reduced state as isolated; upon oxidation by ferricyanide there are characteristic changes in its visible absorption and circular dichroism spectra. HiPIP contains no alpha helix, about half of the polypeptide chain assumes beta sheet conformation. Pronounced structural differences between the oxidized and reduced states have been observed in the aromatic amino acid and Fe-S cluster spectral regions. Mössbauer spectra of the HiPIP in the two redox states reveal further differences. The possible contribution of aromatic amino acid residues, to the redox transition is discussed.