Title: Ferredoxins
Literature References: A group of electron transfer factors found in plants and bacteria, which are non-heme iron-sulfur proteins and which play an important role in photosynthesis, nitrogen and carbon dioxide fixation, and respiration. They are generally classified by the presence of either 2 or 4 iron atom clusters and an equivalent amount of inorganic or "acid-labile" sulfide bonded to the peptide chain through 4 cysteine sulfhydryl groups. The two-iron ferredoxins are found primarly in plants and blue-green algae and are sometimes referred to as chloroplast or "plant type" ferredoxins; the four-iron ferredoxins are predominant in bacteria. Mol wts of chloroplast ferredoxins are about 12,000; those of bacterial ferredoxins range from about 6,000 to 24,000. Isoln from Clostridium pasteurianum: L. E. Mortenson et al., Biochem. Biophys. Res. Commun. 7, 448 (1962). Prepn of bacterial ferredoxins: L. E. Mortenson, US 3344130 (1960 to duPont). The amino acid sequence of several bacterial and chloroplast ferredoxins has been elucidated. Amino acid sequence of ferredoxin from Clostridium pasteurianum: Tanaka et al., Biochem. Biophys. Res. Commun. 16, 422 (1964); eidem, Biochemistry 5, 1666 (1966); from spinach: Matsubara, Sasaki, J. Biol. Chem. 243, 1732 (1968). Synthesis of peptide chain of C. pasteurianum ferredoxins: Bayer et al., Tetrahedron 24, 4853 (1968); Trakatellis, Schwartz, Proc. Natl. Acad. Sci. USA 63, 436 (1969). Reviews: Buchanan, Struct. Bonding 1, 109 (1966); Malkin, Rabinowitz, Annu. Rev. Biochem. 36, 113 (1967); Arnon, Naturwissenschaften 56, 295 (1969); Buchanan, Arnon, Adv. Enzymol. 33, 119 (1970); W. Lovenberg, "Ferredoxin and Rubredoxin" in Microbial Iron Metabolism, J. B. Neilands, Ed. (Academic Press, New York, 1974) pp 161-182; D. C. Yoch, R. P. Carithers, Microbiol. Rev. 43, 384-421 (1979).
Properties: Absorption max of bacterial ferredoxins: 280, 385-400 nm; of plant ferredoxins: 280, 325, 420, 463 nm. All show negative oxidation-reduction potentials near that of the hydrogen electrode: -0.390 V (C. pasteurianum); -0.420 V (spinach): Tagawa, Arnon, Biochim. Biophys. Acta 153, 602 (1968). Autoxidizable. Acidification, as well as treatment with iron-chelating agents or mercurials, results in evolution of H2S and loss of the visible absorption.
Absorption maximum: Absorption max of bacterial ferredoxins: 280, 385-400 nm; of plant ferredoxins: 280, 325, 420, 463 nm
Ferric Acetate, Basic Ferric Albuminate Ferric Ammonium Citrate Ferric and Ammonium Acetate Solution Ferric Bromide

Ferredoxins (from Latin ferrum: iron + redox, often abbreviated "fd") are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied to the "iron protein" first purified in 1962 by Mortenson, Valentine, and Carnahan from the anaerobic bacterium Clostridium pasteurianum.[1][2]

Another redox protein, isolated from spinach chloroplasts by Tagawa and Arnon in 1962, was termed "chloroplast ferredoxin".[3] The chloroplast ferredoxin is involved in both cyclic and non-cyclic photophosphorylation reactions of photosynthesis. In non-cyclic photophosphorylation, ferredoxin is the last electron acceptor and reduces the enzyme NADP+ reductase. It accepts electrons produced from sunlight-excited chlorophyll and transfers them to the enzyme ferredoxin:NADP+ oxidoreductase EC

Ferredoxins are small proteins containing iron and sulfur atoms organized as iron-sulfur clusters. These biological "capacitors" can accept or discharge electrons, the effect being change in the oxidation states (+2 or +3) of the iron atoms. This way, ferredoxin acts as electron transfer agents in biological redox reactions.

Other bioinorganic electron transport systems include rubredoxins, cytochromes, blue copper proteins, and the structurally related Rieske proteins.

Ferredoxins can be classified according to the nature of their iron-sulfur clusters and by sequence similarity.