Superoxide poisons mononuclear iron enzymes by causing mismetallation

Abstract

Superoxide (O(2)(-)) is a primary agent of intracellular oxidative stress. Genetic studies in many organisms have confirmed that excess O(2)(-) disrupts metabolism, but to date only a small family of [4Fe-4S] dehydratases have been identified as direct targets. This investigation reveals that in Escherichia coli O(2)(-) also poisons a broader cohort of non-redox enzymes that employ ferrous iron atoms as catalytic cofactors. These enzymes were inactivated by O(2)(-) both in vitro and in vivo. Although the enzymes are known targets of hydrogen peroxide, the outcome with O(2)(-) differs substantially. When purified enzymes were damaged by O(2)(-) in vitro, activity could be completely restored by iron addition, indicating that the O(2)(-) treatment generated an apoprotein without damaging the protein polypeptide. Superoxide stress inside cells caused the progressive mismetallation of these enzymes with zinc, which confers little activity. When O(2)(-) stress was terminated, cells gradually restored activity by extracting zinc from the proteins. The overloading of cells with zinc caused mismetallation even without O(2)(-) stress. These results support a model in which O(2)(-) repeatedly excises iron from these enzymes, allowing zinc to compete with iron for remetallation of their apoprotein forms. This action substantially expands the physiological imprint of O(2)(-) stress.