Spinach ferredoxin reduction potential

The 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

The advantage of ferredoxins over rubredoxins in terms of redox chemistry is that by combining several Fe centres in close proximity, it is possible to access a greater range of reduction potentials. Different conformations of the protein pockets which surround the Fe S clusters affect the detailed structural features of the cluster cores and, thus, their reduction potentials, e.g. —420 mV for spinach [2Fe-2S] ferredoxin, and —270 mV for adrenal [2Fe-2S] ferredoxin. A [2Fe-2S] ferredoxin acts as a one-electron transfer centre, going from an Fe(II)/Fe(II) state in the reduced form to an Fe(II)/Fe(III) state when oxidized and vice versa. Evidence for the localized, mixed valence species comes from EPR spectroscopic data. 

Rieske protein is the electron-transfer site in the oxidation of plastoquinol (a hydroquinone) to plastosemi-quinone, during which protons are released. Rieske protein has a positive reduction potential (4-290 mV) for that isolated from spinach chloroplasts, contrasting with negative values for [2Fe-2S] ferredoxins. The difference must be attributed to the His versus Cys coordination of one Fe centre. 

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