Manganese Ions Do An Enzyme's Job

To guard against reactive superoxide radicals (O₂???^-), many organisms rely on superoxide dismutases (SODs), which are enzymes that convert superoxide into O₂ and H₂O₂. Some bacteria lack SODs, but they have a high internal concentration of manganese(II) ions, which provide antioxidant protection by a mechanism that is not entirely clear. A team led by Joan Selverstone Valentine at UCLA and Diane E. Cabelli at Brookhaven National Laboratory has shown that, in the presence of phosphate anions, Mn²⁺ catalyzes the destruction of superoxide radicals, but by a mechanism distinct from that of SODs (J. Am. Chem. Soc., DOI: 10.1021/ja710162n). Superoxide is known to rapidly react with Mn²⁺ to form a short-lived MnO₂⁺ adduct. The researchers found that, within a certain range of phosphate concentrations that mimic levels found inside cells, two of the adduct molecules participate in a disproportionation reaction—simultaneous oxidation and reduction—to regenerate Mn²⁺ ions. This process differs from SOD mechanisms, which involve separate reduction and oxidation steps at the enzyme's metal center.