The iron(II) superoxide dismutase (FeSOD) from Escherichia coli exhibits relatively sharp well-resolved paramagnetically shifted NMR signals. These signals can be associated with the endogenous ligands characterized by X-ray crystallography for the oxidized form FeIIISOD. Our results demonstrate that the active site remains intact upon reduction of the Fe(III) site, retaining the same coordination modes for the three N,-coordinated His residues (presumably His-26, His-75, and His-162) and the Asp residue (presumably Asp-158). The N-H resonances of the coordinated histidines are found at 88, 43, and 37 ppm, while the signals at 24.5 (E), 19 (G), and 15 (J) ppm are assigned to the Asp residue by the observation of nuclear Overhauser effects (NOE) and bond correlations among the resonances. Distances of 1.9 and 2.4 Å can be estimated between the protons E and G and the protons E and J, respectively, attributable to the -CβH2-CαH< spin system of the Asp residue. The Asp assignments are corroborated by the observation of interresidue interactions between the CαH's of Asp-158 and Trp-160. The resonances of the latter residue are identified by NOE and scalar connectivities, the most critical assignment being the NαH at 22 ppm (F). According to the crystal structure of the corresponding enzyme from Psedumonas ovalis (Stoddard, B. L.; Howell, P. L.; Ringe, D.; Petsko, G. A. Biochemistry 1990, 29, 8885-8893), this proton is hydrogen-bonded to the free carboxylate oxygen of Asp-158. This interaction thus rationalizes the paramagnetic shift observed for signal F and its very slow exchange with solvent which occurs only upon long-term standing.