Investigation of the structure of bovine erythrocyte superoxide dismutase by proton nuclear magnetic resonance spectroscopy

The 270-MHz 1H nuclear magnetic resonance spectra of the apo, copper(I)-zinc(II), and copper(II)-zinc(II) forms of bovine erythrocyte superoxide dismutase (EC 1.15.1.1) are reported, and assignments of resonances to ten amino acid residues are proposed. The data require that at least four and probably six histidine residues serve as ligands to the metals in each subunit of the enzyme, consistent with x-ray diffraction results. The remaining assigned resonances are associated with His-19, His-41, Tyr-108, and the N-terminal N-acetyl group. The imidazole C(2)H of His-41 exchanges readily at pH greater than 8. The structural implications of the effect of the paramagnetic Cu(II) in the holoenzyme on the proton relaxation times are in reasonable accord with the data from x-ray diffraction studies.