[Degradation and resynthesis of the active sites of plant ferredoxins]

Mössbauer effect in Scenedesmus and spinach ferredoxins. The mechanism of electron transfer in plant-type iron-sulphur proteins

1. The Mössbauer spectra of Scenedesmus ferredoxin enriched in (57)Fe were measured and found to be identical with those of two other plant-type ferredoxins (from spinach and Euglena) that had been previously measured. Better resolved Mössbauer spectra of spinach ferredoxin are also reported from protein enriched in (57)Fe. All these iron-sulphur proteins are known to contain two iron atoms in a molecule that takes up one electron on reduction. 2. The Mössbauer spectra at 195 degrees K have electric hyperfine structure only and show that on reduction the electron goes to one of the iron atoms, the other appearing to remain unchanged. 3. In the oxidized state, both iron atoms are in a similar chemical state, which appears from the chemical shift and quadrupole splitting to be high-spin Fe(3+), but they are in slightly different environments. In the reduced state the iron atoms are different and the molecule appears to contain one high-spin Fe(2+) and one high-spin Fe(3+) atom. 4. At lower temperatures (77 and 4.2 degrees K) the spectra of both iron atoms in the reduced proteins show magnetic hyperfine structure which suggests that the iron in the oxidized state also has unpaired electrons. This provides experimental evidence for earlier suggestions that in the oxidized state there is antiferromagnetic exchange coupling, which would result in a low value for the magnetic susceptibility. 5. In a small magnetic field the spectrum of the reduced ferredoxin shows a Zeeman splitting with hyperfine field (H(n)) of 180kG at the nuclei. On application of a strong magnetic field H the spectrum splits into two spectra with effective fields H(n)+/-H, thus confirming the presence of the two antiferromagnetically coupled iron atoms. 6. These results are in agreement with the model proposed by Gibson, Hall, Thornley & Whatley (1966); in the oxidized state there are two Fe(3+) atoms (high spin) antiferromagnetically coupled and on reduction of the ferredoxin by one electron one of the ferric atoms becomes Fe(2+) (high spin).

[Ferredoxin from Bumilleriopsis filiformis Vischer]

Ferredoxin from the alga Bumilleriopsis filiformis Vischer (Xanthophyceae) is characterized by comparing some of its properties with spinach ferredoxin.It is similar to the known plant ferredoxins in the following points: a) Absorption spectrum. b) Molecular weight. c) Amino acid composition (approx. 100 amino acid residues). d) Uptake of 1 electron upon complete reduction and a 50 per cent decrease of the optical density at 420 nm. e) 2 non-heme iron atoms per molecule, which do not exchange with an iron isotope. f) SH-groups of the native protein do not react with DTNB. Bumilleriopsis ferredoxin differs from known plant ferredoxins in the following points: a) Greater stability towards atmospheric oxygen when gently heated. b) Only 1 detectable acid-labile sulfur atom per molecule. c) Dialysis against EDTA: by this procedure a part of the iron but none of the acid-labile sulfur is removed. With spinach ferredoxin a part of both elements is removed simultaneously. d) Greater reactivity with ferredoxin-NADP reductase from Bumilleriopsis than with that from spinach. With spinach ferredoxin no substantial difference between the reaction rates of the two reductases is observed (see Böger, 1969 b). The results may indicate an active center in Bumilleriopsis ferredoxin consisting of 1 atom iron and 1 atom acid-labile sulfur.