A glutamate bridge is essential for dimer stability and metal selectivity in manganese superoxide dismutase

In Escherichia coli manganese superoxide dismutase (MnSOD), the absolutely conserved Glu¹⁷⁰ of one monomer is hydrogen-bonded to the Mn ligand His¹⁷¹ of the other monomer, forming a double bridge at the dimer interface. Point mutation of Glu¹⁷⁰ → Ala destabilizes the dimer structure, and the mutant protein occurs as a mixture of dimer and monomer species. The purified E170A MnSOD contains exclusively Fe and is devoid of superoxide dismutase activity. E170A Fe₂-MnSOD closely resembles authentic FeSOD in terms of spectroscopic properties, anion interactions and pH titration behavior. Reconstitution of E170A Fe₂-MnSOD with Mn(II) salts does not restore superoxide dismutase activity despite the spectroscopic similarity between E170A Mn₂-MnSOD and wild type Mn₂-MnSOD. Growth of sodA⁺ and sodA^- E. coli containing the mutant plasmid pDT1-5(E170A) is impaired, suggesting that expression of mutant protein is toxic to the host cells.