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Mossbauer studies of adrenodoxin. The mechanism of electron transfer in a hydroxylase iron-sulphur protein.

ABSTRACT: 1. Mössbauer spectra were measured of adrenodoxin purified from porcine adrenal glands. They show similarities to the spectra of the plant ferredoxins. All of these proteins contain two atoms of iron and two of inorganic sulphide per molecule, and on reduction accept one electron. 2. As with the plant ferredoxins the adrenodoxin for these measurements was enriched with (57)Fe by reconstitution of the apo-protein, and subsequently was carefully purified and checked by a number of methods to ensure that it was in the same conformation as the native protein and contained no extraneous iron. 3. The Mössbauer spectra of oxidized adrenodoxin at temperatures from 4.2 degrees K to 197 degrees K show that the iron atoms are probably high-spin Fe(3+), and in similar environments, and experience little or no magnetic field from the electrons. 4. Mössbauer spectra of reduced adrenodoxin showed magnetic hyperfine structure at all temperatures from 1.7 degrees K to 244 degrees K, in contrast with the reduced plant ferredoxins, which showed it only at lower temperatures. This is a consequence of a longer electron-spin relaxation time in reduced adrenodoxin. 5. At 4.2 degrees K in a small magnetic field the spectrum of reduced adrenodoxin shows a sixline Zeeman pattern due to Fe(3+) superimposed upon a combined magnetic and quadrupole spectrum due to Fe(2+). 6. In a large magnetic field (30kG) each hyperfine pattern is further split into two. Analysis of these spectra at 4.2 degrees K and 1.7 degrees K shows that the effective fields at the Fe(3+) and Fe(2+) nuclei are in opposite directions. This agrees with the proposal, first made for the ferredoxins, that the iron atoms are antiferromagnetically coupled. 7. In accord with the model for the ferredoxins, it is proposed that the oxidized adrenodoxin contains two high-spin Fe(3+) atoms which are antiferromagnetically coupled; on reduction one iron atom becomes high-spin Fe(2+).

SUBMITTER: Cammack R

PROVIDER: S-EPMC1178190 | biostudies-other | 1971 Dec

REPOSITORIES: biostudies-other

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Project description: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).

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Mossbauer studies of adrenodoxin. The mechanism of electron transfer in a hydroxylase iron-sulphur protein.

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