Paramagnetic NMR Analysis of the Seven‐Iron Ferredoxin from the Hyperthermoacidophilic Archaeon Desulfurolobus ambivalens Reveals Structural Similarity to other Dicluster Ferredoxins

The seven‐iron ferredoxin from the hyperthermophilic archaeon Desulfurolobus ambivalens , has been investigated by one‐dimensional and two‐dimensional 1 H‐NMR in its oxidized and dithionite‐reduced states. All iron atoms of both the three‐iron and the four‐iron cluster are bound to cysteine residues whose hyperfine‐shifted resonances were characterized. The pattern of these resonances is similar to those from three‐iron, four‐iron and eight‐iron ferredoxins previously described in the literature, but the four‐iron cluster has a shift pattern different from that in other seven‐iron proteins. A second set of hyperfine‐shifted resonances clearly indicates sample heterogeneity, which possibly involves the four‐iron cluster. The observation of interresidue NOEs between two different cysteine residues proves the existence of close spatial proximity of the two clusters in D. ambivalens ferredoxin and therefore indicates structural homology to other dicluster ferredoxins. Moreover, this feature is crucial for the sequence‐specific assignment of the hyperfine‐shifted resonances. The Cα‐Cβ‐S‐Fe dihedral angles of the cysteine residues coordinating the four‐iron cluster could be estimated, and the electronic structure of the three‐iron cluster is discussed.