THE FE4S4 CENTERS IN FERREDOXINS STUDIED THROUGH PROTON AND CARBON HYPERFINE COUPLING - SEQUENCE-SPECIFIC ASSIGNMENTS OF CYSTEINES IN FERREDOXINS FROM CLOSTRIDIUM-ACIDI-URICI AND CLOSTRIDIUM-PASTEURIANUM

Oxidized ferredoxin from Clostridium acidi urici, containing two [Fe4S4]2+ clusters, has been investigated through H-1 NOESY and TOCSY spectroscopies. The protons of coordinated cysteines have been identified and assigned to each cluster with use of a procedure based on the assignment of two spatially close betaCH2 pairs and on the shift ratios of each betaCH2 Proton in oxidized, half-reduced, and reduced forms; each cysteine proton has been then sequence-specifically and stereospecifically assigned by looking for dipolar connectivities with amino acid residues in the vicinity of the cluster. By comparing the present data with the available spectra of the analogous protein from Clostridium pasteurianum, the sequence-specific and stereospecific assignments of cysteine protons have been obtained also for the latter protein. The natural abundance C-13 signals of the cysteine protons have been also sequence-specifically assigned. By taking advantage of the X-ray structure of a similar protein, the H-1 and C-13 hyperfine shifts have been related to the dihedral angle between the iron-sulfur-beta-carbon plane and the sulfur-beta-carbon-beta-proton or sulfur-beta-carbon-alpha-carbon planes. A parametric equation is proposed. The spin delocalization mechanism has been found to be largely dependent on unpaired spin density on the p(z) orbital of the sulfur atom. Through EXSY spectroscopy, the proton signals of the [Fe4S4]+ clusters in the reduced protein have been assigned. Their temperature dependence is compared with that of the [Fe4S4]3+ clusters present in oxidized HiPIPs and discussed in terms of the Heisenberg model for the magnetic exchange coupling within the clusters.