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any of a group of water-soluble proteins whose molecules contain iron atoms in nonheme form bonded to labile sulfur atoms. All ferredoxins are both enzymes and electron carriers with a highly negative redox potential (–0.39 to –0.49 volt). They are known to occur in bacteria and in the chloroplasts of plants.

All known ferredoxins are composed of a single polypeptide chain. Ferredoxins in bacteria (molecular weight, 5,500–10,000; 55–81 amino acid residues) contain from three (Chromatium) to eight (Clostridium) Fe atoms per molecule and an equal number of labile sulfur atoms. They participate in the fixation of atmospheric nitrogen and the reduction of the latter to ammonia, in the oxidation of gaseous hydrogen, and in the reduction of nicotinamide adenine dinucleotide phosphate (NADP) through the light reactions of photosynthesis in photosynthesizing bacteria.

Ferredoxins in chloroplasts (molecular weight, 11,500–13,000; 96–97 amino acid residues) contain two iron atoms bound to the protein by two sulfur atoms of cysteine molecules; the iron atoms are also bound to each other by two sulfur atoms. These ferredoxins figure in the photoreduction of NADP during photosynthesis. The sequence of amino acid residues has been determined for a number of bacterial and plant ferredoxins. A chemical synthesis of the peptide chain of the ferredoxins in the bacterium Clostridium pasteurinianum has been carried out.


Mukhin, E. N. “Rol’ ferredoksina v fotosinteze.” Uspekhi sovremennoi biologii, 1969, vol. 67, no. 2.
Buchanan, B. B., and D. J. Arnon. “Ferredoxins.” Advances in Enzymology, 1970, vol. 33.


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