NMR studies of Azotobacter vinelandii and Pseudomonas putida seven-iron ferredoxins. Direct assignment of beta-cysteinyl carbon NMR resonances and further proton NMR assignments of cysteinyl and aromatic resonances.
Ferredoxins are proteins which contain iron and inorganic sulfide and are capable of electron transport. They are found in a wide range of organisms, from anaerobic bacteria, to plants and mammals. Although NMR spectroscopy has been used to study ferredoxins since the 1970s, little important structural or biochemical information has resulted from these investigations. The major difficulty has been the effect of the paramagnetic iron-sulfur clusters on the peptide resonances, hindering nuclear Overhauser effect (NOE) studies and causing broad line widths. These effects are most pronounced on resonances arising from the nuclei closest to the iron-sulfur center. Unfortunately, these are likely to be the most interesting nuclei, as they report the events and geometry in the vicinity of the active sites. In this paper, the first direct assignment of beta-cysteinyl 13C resonances for any iron-sulfur protein is reported for the spectrum of Pseudomonas putida ferredoxin. These resonances are of special significance, as they arise from the atoms on the protein closest to the iron centers, with the exception of the directly bound cysteinyl sulfur atoms. In addition, cysteinyl and ring system 1H NMR resonance assignments are made for the spectra of P. putida ferredoxin and Azotobacter vinelandii ferredoxin I.