MODELING THE 3-DIMENSIONAL STRUCTURE AND ELECTROSTATIC POTENTIAL-FIELD OF THE 2 CU,ZN SUPEROXIDE-DISMUTASE VARIANTS FROM XENOPUS-LAEVIS

The crystallographic structure of bovine superoxide dismutase has been used as a template for the graphic reconstruction of the three-dimensional structures of the two Xenopus laevis variants (Schinina', M.E. et al. Arch. Biochem. Biophys. 272:507-515, 1989). In these models the structure-essential residues maintain their position and their structural role, and the interactions between the subunits and the close packing within the beta-barrel are maintained with conservative substitutions and even increased with "aromatic pairs". Because of the same topological motif and surface location of charges, arising from the model building of the two variants with respect to the bovine enzyme, we have calculated the electrostatic potential fields around the models of the two Xenopus laevis variants by numerically solving the Poisson-Boltzmann equation. We show that conservation of a specific space-relationship of charges maintains the potential field pattern already observed in the bovine enzyme, where a negative potential field surrounds the protein surface and specific positive regions wrap up the copper center active site. This electrostatic potential field distribution supports the idea that electrostatic interactions control, like in the bovine enzyme, the mechanism of enzyme-substrate recognition in the Xenopus laevis Cu,Zn superoxide dismutases, suggesting that coordinated mutation of charged residues has occurred in the evolution of this enzyme.