Spectroscopic studies of ferricyanide oxidation of Azotobacter vinelandii ferredoxin I.

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Abstract
The Fe(CN)63- oxidation of the crystallographically characterized {[3Fe-3S], [4Fe-4S]} ferredoxin I of A. vinelandii was studied using absorption, circular dichroism, magnetic circular dichroism and EPR spectroscopies. A paramagnetic intermediate is observed en route to Fe-S cluster-free apoprotein, possessing an anisotropic g .apprxeq. 2 EPR signal, surviving to temperatures > 77.degree. K. This species is shown to result from 3-electron oxidation of the [4Fe-4S] cluster, without modification of the [3Fe-3S] cluster. It does not give rise to observable paramagnetic magnetic circular dichroism in the visible-near UV spectral region and is neither an oxidized HIPIP [4Fe-4S] cluster nor an oxidized [3Fe-3S] cluster. The paramagnetic species was identified as a cysteinyldisulfide radical formed on dissociation of an oxidized cysteinate and an oxidized sulfide ion from the [4Fe-4S] cluster. This conclusion is consistent with the observed reaction stoichiometry, the spectroscopic results obtained, known EPR spectra of disulfide radicals and the reconstitution of the native [4Fe-4S] cluster by dithiothreitol alone. This reaction, earlier interpreted as a HIPIP-type oxidation, is a previously uncharacterized oxidation reaction of [4Fe-4S] clusters.