CRYSTAL-STRUCTURE OF ESCHERICHIA-COLI MANGANESE SUPEROXIDE-DISMUTASE AT 2.1-ANGSTROM RESOLUTION

The three-dimensional structure of the manganese-dependent superoxide dismutase (MnSOD) from Escherichia coli has been determined by X-ray c rystallography at 2.1 Angstrom resolution The protein crystallizes wit h two homodimers in the asymmetric unit, and a model comprising 6528 p rotein atoms (residues 1-205 of all four monomers), four manganese ion s and 415 water molecules has been refined to an R factor of 0.188 (R- free 0.218). The structure shows a high degree of similarity with othe r MnSOD and FeSOD enzymes. The Mn centres are 5-coordinate, trigonal b ipyramidal, with His26 and a solvent molecule, probably a hydroxide io n, as apical ligands, and His81, Asp167 and His171 as equatorial ligan ds. The coordinated solvent molecule is linked to a network of hydroge n bonds involving the non-coordinated carboxylate oxygen of Asp167 and a conserved glutamine residue, Gln146. The MnSOD dimer is notable for the way in which the two active sites are interconnected and a ''brid ge'' comprising His171 of one monomer and Glu170 of the other offers a route for inter-site communication. Comparison of E. coli MnSOD and F eSOD (a) reveals some differences in the dimer interface, (b) yields n o obvious explanation for their metal specificities, and (c) provides a structural basis for differences in DNA binding, where for MnSOD the groove formed by dimerization is complementary in charge and surface contour to B-DNA.