Atomic Structures of Wild-Type and Thermostable Mutant Recombinant Human Cu,Zn Superoxide Dismutase

Superoxide dismutase enzymes protect aerobic organisms from oxygen-mediated free-radical damage. Crystallographic structures of recombinant human Cu,Zn superoxide dismutase have been determined, refined, and analyzed at 2.5 Å resolution for wild-type and a designed thermostable double-mutant enzyme (Cys-6→Ala, Cys-111→Ser). The 10 subunits (five dimers) in the crystallographic asymmetric unit form an unusual stable open lattice with 80-Å-diameter channels. The 10 independently fit and refined subunits provide high accuracy, error analysis, and insights on loop conformations. There is a helix dipole interaction with the Zn site, and 14 residues form two or more structurally conserved side-chain to main-chain hydrogen bonds that appear critical to active-site architecture, loop conformation, and the increased stability resulting from the Cys-111→Ser mutation.