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The 13C-resonances of the 2',6'-ring carbon atoms of both tyrosyl residues of Clostridium acidi-urici ferredoxin are shifted downfield in the oxidized and reduced protein relative to these resonance positions in free tyrosine. These results show that both tyrosyl residues in the oxidized and reduced protein are in magnetically equivalent environments, and suggest that both tyrosyl residues are close to the two iron-sulfur clusters in the reduced and oxidized proteins and that each cluster is equally accessible to one reducing electron.Ferredoxins are relatively small molecular weight iron-sulfur proteins that contain equal amounts of iron and acid-labile sulfur and show an ESR signal at g = 1.94 in the reduced state. They function as electron carriers in cell metabolism (1) and have low redox potentials (-230 mV to -430 mV) (2). The characteristic ESR resonances of reduced ferredoxins are broadened in 57Fe-and 33S-enriched ferredoxins (3). Thus, the reducing electron is associated with the iron and acidlabile sulfur atoms. One of the bacterial clostridial-type ferredoxins (a class of ferredoxins that has eight iron and eight acid-labile sulfur atoms) contains two identical iron-sulfur (Fe-S) clusters, each containing four iron and four acid-labile sulfur atoms forming two interpenetrating tetrahedra with a cysteine sulfur atom coordinated to each iron atom (4). Clostridial ferredoxins usually: contain only two residues of either phenylalanine or tyrosine or both, and the positions of these aromatic residues in five clostridial ferredoxins of known aminoacid sequence are conserved (5). This conservation suggests that the aromatic residues may be important to -the structure and function of these proteins. Mixed-valence organometallic compounds containing aromatic ligands have high electron-transfer rates and have been proposed as ferredoxin model compounds (6).For determination of the role of aromatic residues in clostridial ferredoxins, the '3C-NMR technique was used because: (i) aromatic carbon resonances can be easily assigned; (ii) carbon resonance positions, unlike those of protons, are not very sensitive to changes in protein conformation and are rarely shifted 1-2 ppm from the corresponding carbon resonance positions in free amino acids (7, 8); (iii) carbon resonances can be perturbed by nearby paramagnetic electrons, and the effect of oxidation and reduction of tyrosyl ring carbon atoms in ferredoxin can be monitored and correlated with that of tyrosyl ring protons by double resonance techniques (9). C. acidi-urici ferredoxin was chosen for this study because it contains two tyrosyl residues, and has a known sequence (10). Takano et al. (11) have observed that in cytochrome c a t In Clostridium tartarovorum ferredoxin, a histidyl and tyrosyl residue occupy the conserved aromatic positions and a histidyl residue occurs in a nonconserved position (5). tyrosyl residue changes orientation upon reduction, and have suggested that electron transfer occurs by way of a tyrosyl radical. Electron transport b...