Proton magnetic resonance studies of Azotobacter vinelandii ferredoxin I. Evidence for a difference in coordination of the 3Fe centers in azotobacter vinelandii ferredoxin I and desulfovibrio gigas ferredoxin II.

Proton magnetic resonance studies have been made of Azotobacter vinelandii ferredoxin I. This protein contains a low potential 3Fe-3S center (Emp = -424 mV) and a high potential 4Fe-4S center (Emp = +320 mV). A series of five single proton resonances are visible downfield of 11 ppm in the isolated form of the protein. On reduction of the protein the three most downfield resonances are no longer visible and no new resonances are observed. These resonances are assigned to alpha-CH cysteinyl protons on residues 8, 20, and 49 which coordinate the 3Fe center. The two remaining downfield resonances are altered on oxidation of the protein, and are assigned to beta-CH2 cysteinyl protons on residues bound to the high potential 4Fe center. Comparison of the reported NMR spectrum of Desulfovibrio gigas ferredoxin II (Moura, J. J. G., Xavier, A. V., Bruschi, M., and Le Gall, J. (1977) Biochim. Biophys. Acta 459, 278-289) to that of A. vinelandii ferredoxin I is made. The 3Fe centers found in D. gigas ferredoxin Ii exhibit a reduction potential almost 300 mV more positive than the 3Fe center in A. vinelandii ferredoxin I. Evidence is presented that the 3Fe centers in the two proteins are not co-ordinated identically, and arguments are made which suggest that a small noncysteinyl ligand, modeled as a nonprotein oxygen atom in the x-ray structure A. vinelandii ferredoxin I, may be replaced in D. gigas ferredoxin II by a glutamyl epsilon-oxygen linkage to an iron atom. Further, it is noted that such a change could be responsible for the significant difference in reduction potential observed between the 3Fe centers in these two proteins.