Direct assignment of the cysteinyl, the slowly exchangeable, and the aromatic ring 1H nuclear magnetic resonances in clostridial-type ferredoxins.

Abstract

We have directly assigned the 1H NMR corresponding to the cysteinyl protons, the slowly exchangeable protons, and the aromatic ring protons in the 1H NMR spectrum of Clostridium acidi-urici ferredoxin by isotopic labeling and 13C NMR decoupling techniques. We also show that the resonance pattern in the 8- to 20-ppm (from 2,2-dimethyl-2-sialapentanesulfonic acid) region of the 1H NMR spectra of oxidized Clostridium acidi-urici, Clostridium pasteurianum, Clostridium perfringens, and Peptococcus aerogenes ferredoxins are very similar, and we assign the resonances in this region by analogy with the spectrum of C. acidi-urici ferredoxin. The 1H NMR spectra of the beta protons of the cysteinyl residues of these ferredoxins differ, however, from the 1H NMR spectra of equivalent beta protons of the methylene carbon atoms bonded via a sulfur atom to [4Fe-4S] clusters in synthetic inorganic analogues. In the spectra of the synthetic compounds, the beta protons appear as a single resonance shifted 10 ppm from its unbonded reference position. In the spectra of oxidized clostridial ferredoxins, the cysteinyl beta protons appear as a series of at least eight resolved resonances with shifts that range from 6 to 14 ppm, relative to the free amino acid resonance position. This difference in the spectra of the protein and the synthetic compounds probably results from the fact that the equivalent beta protons of the synthetic compounds are not constrained and are free to rotate and thus assume the same average orientation with respect to the [4Fe-4S] cluster. The shift pattern in the 9- to 14-ppm region is identical in three different clostridial ferredoxins. This suggests that the molecular environments of the corresponding cysteinyl residues are identical. Significant differences in the resonance positions occur, however, in the 14- to 18-ppm region, suggesting that the physical environments of these cysteinyl residues differ. This may reflect differences in the orientation of the corresponding cysteinyl residues relative to the [4Fe-4S] clusters or differences in charge density at the cysteinyl beta protons or both. The slowly exchangeable protons were identified by comparing the 1H NMR spectra of ferredoxins reconstituted in H2O and 2H2O. The remaining resonances in the 8- to 20-ppm region were assigned to each of the 2 tyrosyl residues in C. acidi-urici ferredoxin. This was done by comparing the 1H NMR spectra of C. acidi-urici [(3',5'-2H2)Tyr]ferredoxin and C. acidi-urici [PHE2]ferredoxin with that of C. acidi-urici native ferredoxin.