Metal ion reconstitution studies of yeast copper-zinc superoxide dismutase: the "phantom" subunit and the possible role of Lys7p

Using a corrected molar extinction coefficient for yeast apo copper-zinc superoxide dismutase (CuZnSOD), we have confirmed that the metal binding properties of this protein in vitro differ greatly from those of the bovine and human CuZnSOD enzymes. Thus yeast apo CuZnSOD was found to bind only one Co2+ per protein dimer under the conditions in which the bovine and human CuZnSOD apoenzymes readily bind two per dimer. The spectroscopic properties characteristic of the two Cu2+ plus two Co2+ per dimer or four Cu2+ per dimer metal-substituted bovine apo CuZnSOD derivatives were obtained for the yeast apoprotein but by the addition of only half of the appropriate metals, i.e., one Cu2+ plus one Co2+ per dimer or two Cu2+ per dimer. This half-metallated yeast CuZnSOD has been characterized by UV-visible and EPR spectroscopy as well as by native polyacrylamide gel electrophoresis. We conclude that yeast apo CuZnSOD, unlike the bovine and human apoproteins, cannot be reconstituted fully with metal ions under the same conditions. Instead, only one subunit of the homodimer, the "normal" subunit, can be remetalled in a fashion reminiscent of the well-characterized bovine protein. The other "phantom" subunit is not competent to bind metals in this fashion. Furthermore, we have shown that CuZnSOD protein isolated from Saccharomyces cerevisiae that lacks the gene coding for the copper chaperone, Lys7p, contains only one metal ion, Zn2+, per protein dimer. The possibility that yeast CuZnSOD can exist in multiple conformational states may represent an increased propensity of the yeast protein to undergo changes that can occur in all CuZnSODs, and may have implications for amyotrophic lateral sclerosis.