Ferredoxin from Methanosarcina barkeri: Evidence for the Presence of a Three‐Iron Center

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Abstract
Methanosarcina barkeri ferredoxin was purified and characterized by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. The purification procedure included chromatographic steps on DEAE‐cellulose and gel filtration. The isolated protein is unstable under aerobic conditions. The ferredoxin exhibits charge transfer bands at 283 nm and 405 nm with an absorption ratio A405/A283= 0.73. Its molecular weight has been estimated to be 20000–22000 by gel filtration chromatography. The native ferredoxin exhibits an intense EPR signal at g= 2.02 and only a very weak g= 1.94 signal develops upon reduction with dithionite. The Mössbauer spectra of the reduced protein are characteristic of a [3 Fe‐3S] center. The combined EPR and Mössbauer studies show that M. barkeri ferredoxin contains only [3 Fe‐3S] clusters, similar to Azotobacter vinelandii Fd [Emptage, M. H., Kent, T. A., Huynh, B. H., Rawlings, J., Orme‐Johnson, W. H. & Münck, M. (1980) J. Biol. Chem. 255, 1793–1796], Desulfovibrio gigas FdII [Huynh, B. H., Moura, J. J. G., Moura, I., Kent, T. A., LeGall, J., Xavier, A. V. & Münck, E. (1980) J. Biol. Chem. 255, 3242–3244] and mitochondrial beef heart aconitase [Kent, T. A., Dreyer, J.‐L., Kennedy, M. C., Huynh, B. H., Emptage, M. H., Beinert, H. & Münck, E. (1982) Proc. Natl Acad. Sci. USA, 79, 1096–1100].

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