The Copper- and Zinc-Containing Superoxide Dismutase fromEscherichia coli:Molecular Weight and Stability

The periplasmic Cu,Zn superoxide dismutase (Cu,ZnSOD) fromEscherichia colihas been shown by sedimentation equilibrium to be a monomer with a molecular weight of ∼17,000. The enzyme suffered a reversible inactivation when heated to 70°C. This was minimized by added Cu(II) or Zn(II). Heat lability was greater in phosphate than in Tris buffer. The enzyme exhibited a time-dependent inactivation by Hg(II) and this too was greater in phosphate than in Tris. This behavior can be explained by a modest affinity of the enzyme for Cu(II) and Zn(II) which results in a dissociation/association equilibrium. Elevation of the temperature shifts this equilibrium toward dissociation and phosphate sequesters the released metals making them less available for reinsertion at the active site. Hg(II) competes for occupancy of the active site and there were more unoccupied sites in phosphate than in Tris. A parallel was drawn between theE. coliCu,ZnSOD and FALS varients of human Cu,ZnSOD, which are also relatively unstable and exhibit low affinity for Cu(II).