The first three-dimensional structure of a functional monomeric Cu,Zn superoxide dismutase (from Escherichin coli, E_SOD) is reported at 2.0 Angstrom resolution (R-factor = 16.8%). Compared to the homologous eukaryotic enzymes, E SOD displays a perturbed antiparallel P-barrel structure. The most striking structural features observed include extended amino acid Insertions in the surface 1,2-loop and S-S subloop, modification of the disulfide bridge connection, and loss of functional electrostatic residues, suggesting a modified control of substrate steering toward the catalytic center. The active site Cu2+ displays a distorted coordination sphere due to an unusually long bond to the metal-bridging residue His61. Inspection of the crystal packing does not show regions of extended contact indicative of a dimeric assembly. The molecular surface region involved in subunit dimerization in eukaryotic superoxide dismutases is structurally altered in E_SOD and displays a net polar nature. (C) 1997 Academic Press Limited.