Comparative nuclear magnetic resonance studies of high potential iron-sulfur proteins from Chromatium vinosum and Rhodopseudomonas gelatinosa. Additional hyperfine shifted resonances and pH-dependent structural perturbations.

Proton NMR spectra and their dependence on pH are reported for the oxidized and reduced forms of the high potential iron-sulfur proteins from Chromatium vinosum and Rhodopseudomonas gelatinosa. Spectra of the protein from both species are very similar in the regions occupied by the hyperfine shifted resonances of protons located near the [Fe4S4(S-Cys)4] cluster. The oxidized proteins exhibit three new resonances that had not been previously detected, one at very low field (about 100 ppm) and two at very high field (about -30 ppm). Since only downfield hyperfine shifted peaks have been observed in all other iron-sulfur proteins, the upfield resonances may be unique to high potential 4-Fe centers and originate from protons other than those on the cysteinyl ligands to the cluster. The pH dependencies of the chemical shifts of a large number of aromatic and hyperfine-shifted resonances indicate that the ionization state of histidine-42 exerts an influence on the electronic properties of the cluster despite its being located relatively far away. The oxidation state of the cluster also affects the ionization equilibrium of the histidine; pKa values of 6.7 and 7.3 are measured in the oxidized and reduced protein, respectively. These observations support a previous proposal (Nettesheim, D. G., Johnson, W. V., and Feinberg, B. A. (1980) Biochim. Biophys. Acta 593, 371-383) based on kinetic and visible spectroscopic evidence that the ionization state of histidine-42 affects the stability and oxidation rate of the reduced cluster.