A new assignment strategy for the hyperfine-shifted 13C and 15N resonances in Fe2S2 ferredoxins

Assignment of hyperfine-shifted resonances in paramagnetic metalloproteins such as Fe2S2 ferredoxins poses a major experimental challenge due to hyperfine shifts and/or severe line broadening. We have explored the possibility of using structural data from homologous proteins as part of an assignment strategy for the sequence-specific assignment of hyperfine-shifted backbone carbonyl (13C') and nitrogen resonances (15N) in Fe2S2 ferredoxins. This strategy is based on the assignment of resonances in the paramagnetic region to particular types of amino acid residues using selective isotope labeling. Reduced metal-nuclear distances are then calculated from experimentally determined T1 relaxation times for those resonances and the calculated distances aligned with the distances of nuclei at corresponding amino acid sequence positions in the crystal structure of a structurally homologous protein. The comparative assignment approach has met with success in correctly predicting the 13C' and 15N assignments in Pdx degrees from the crystal structure data of two similar and related ferredoxins, namely bovine adrenodoxin and Anabaena ferredoxin. Sequence-specific assignments made in this fashion were verified by selective 13C'{15N} decoupling experiments.