Rubredoxin analogues

The structural parameters for the oxidized rubredoxin analogues are very similar to those of the oxidized Rd iron site. The reduced complexes reveal a lengthening of the average Fe-S bond from 2.27 to 2.36 A, consistent with the change in oxidation state from ferric to ferrous. The addition of an electron has a more profound structural effect in this single-iron center than in some of the multiiron clusters, where electrons are more delocalized. 

The rate of reduction of cytochrome c by the cobalt-substituted analogue of rubredoxin is a factor of 2.25 lower than the rate with rubredoxin itself. Both proteins mediate the reduction of cytochrome c in the presence of NADH and the decrease in the rate is attributed to the decreased efficiency in oxidation of cobalt(ii) compared with iron(ii). Reduction of cytochrome c by NADPH is catalysed by an adrenodoxin reductase-adrenodoxin complex in which the rate-determining step is electron transfer from the flavin (FAD) of the reductase to the FeaS2 centre of adrenodoxin. The pH dependence of the rate shows a pATa of 6.75, with the high-pH form 27.5 times more reactive than the low-pH form. Both NADPH reduction of the complex and cytochrome c oxidation of the complex were faster than the catalytic rate. Catalytic roles of four iron-sulphur centres in trimethylene dihydrogenase and ferredoxin nitrite reductase have also been examined. Synthetic analogues of four iron ferredoxins have also attracted much attention. - ... 

Although there are three types of active sites in non-heme iron proteins, namely containing one, two, and four iron atoms per center, in the discussion above only tetrameric Fe-S species have been considered. This topic has been treated with some detail because these compounds meet properly the concept of cluster and are therefore clearly within the scope of this book. That is not the case for the di-iron species which form the 2Fe-2S active sites in the ferredoxins nor for the mononuclear center in the rubredoxins. However, in order to get a better understanding of the chemistry of the iron-sulfur proteins, an overview of the chemistry of the 2Fe-2S analogues is also outlined in this Section. 

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