The manganese and iron SODs (superoxide dismutases) form a superfamily of closely related antioxidant defence metalloenzymes. MnSOD requires Mn (not Fe) for activity. However, when MnSOD is expressed in Escherichia coli grown in medium supplemented with ferrous salts, Fe substitutes for Mn in the active site, reflecting relatively indiscriminate uptake of either Mn or Fe and a surprisingly low selectivity for the identity of the bound metal ion. X-ray crystallographic studies on Fe-substituted MnSOD show that the substrate-access channel is blocked by solvent (hydroxide), providing a structural explanation for the observed metal specificity of the catalytic activity. The mechanism of metal binding has been investigated in vitro using recombinant thermophilic SODs. The thermophilic Thermus thermophilus MnSOD expressed in E. coli was isolated as the metal-free apoprotein when heat treatment was eliminated from the purification procedure. While incubation of the purified MnSOD apoprotein with metal salts at ambient temperatures did not restore SOD activity, re-activation could be achieved by heating the protein with Mn salts at elevated temperatures. This in vitro thermally triggered metal uptake is non-specific for the metal ion; both Mn and Fe bind, but only Mn restores catalytic activity. Formation of the metal complex is essentially irreversible under these conditions. The metallation process is strongly temperature- dependent, suggesting that there are substantial activation barriers to metal uptake at ambient temperatures that are overcome by a transition in the apoprotein structure under physiological conditions. Two mechanisms may be proposed for SOD metallation: one involving subunit dissociation and another involving domain separation. Thermally triggered metal binding by thermophilic SODs is providing new insight into the metallation mechanism of the SOD apoprotein, which is likely to be conserved over this family of enzymes.
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