Proton nuclear magnetic resonance investigation of adrenodoxin. Assignment of aromatic resonances and evidence for a conformational similarity with ferredoxin from Spirulina platensis.

Bovine, porcine and sheep adrenodoxin, and the trypsin-resistant form of bovine adrenodoxin have been studied by one- and two-dimensional 1H-NMR spectroscopy. Assignment of the resonances for all the aromatic amino acids with resolved aromatic resonances have been made by correlating NMR spectra with the amino acid sequences from various species. Slowly exchanging amide protons and downfield shifted alpha-protons of His10 and Phe11 suggest possible involvement in beta-sheet structure. The effects on the assigned resonances due to the specific spin-label with a nitroxide radical at Cys95 have been analyzed on a two-dimensional 1H-NMR spectrum. The present results provide evidence for a structural similarity with a model for the structure of adrenodoxin based on a sequence alignment with that of Spirulina platensis ferredoxin, for which X-ray crystallographic data is available. epsilon-Methyl groups of Met120 and Met122 have been assigned by comparing 1H-NMR spectra of adrenodoxin with those of the trypsin-resistant form of adrenodoxin which is specifically cleaved at Arg115. epsilon-Methyl groups of Met120 and Met122 have an exceptionally long longitudinal relaxation time compared with those of valyl and leucyl methyl groups, suggesting that the COOH-terminal peptide spanning over 13 amino acids rotates rather freely in the solvent.