1 H Nuclear‐Magnetic‐Resonance Investigation of Oxidized Fe 4 S 4 Ferredoxin from Thermotoga maritima

The oxidized Fe 4 S 4 ferredoxin from the hyperthermophilic bacterium Thermotoga maritima has been investigated by one‐ and two‐dimensional NMR in order to characterize its hyperfine‐shifted resonances originating from the cysteinyl cluster ligands and to assign its resonances in the diamagnetic shift range. The chemical shift and relaxation time pattern of the hyperfine‐shifted signals is very similar to other oxidized Fe 4 S 4 ferredoxins. A tentative sequence‐specific assignment of these resonances according to a general pattern of chemical shift of cysteine protons versus sequence position of cluster ligand is presented. Furthermore, sequence‐specific assignments for 85% of the amino acid residues that were obtained without any guidance by known X‐ray structures of ferredoxins are given. They reveal the formation of at least two elements of secondary structure by the polypeptide chain of T. maritima ferredoxin: an α‐helix comprising residues C43–D49 and a double–stranded antiparallel β‐sheet consisting of the N‐and C‐terminal parts of the protein. This folding pattern is very similar to that of the crystallographically characterized ferredoxin from the mesophile Desulfovibrio gigas [Kissinger, C. R., Sieker, L. C., Adman E. T. & Jensen, L. H. (1991) J. Mol. Biol. 219 , 693–715] and therefore suggesting different mechanisms of stabilization for T. maritima ferredoxin and the ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus that was recently investigated by NMR [Teng, Q., Zhou, Z. H., Smith, E. T., Busse, S. C., Howard, J. B., Adams M. W. W. & La Mar, G. N. (1994) Biochemistry 33 , 6316–6326].