Manganese Based Superoxide Dismutases from Escherichia coli
Superoxide dismutases catalyze the fast conversion of the toxic superoxide ion via disproportionation, O2-, which is formed during oxygen metabolism:
2 O2- + 4 H+ → O2 + 2 H2O2
The thus formed (and also toxic) product hydrogen peroxide is converted by catalases to water and O2.
The active site of superoxide dismutases can contain a variety of metals. Examples of superoxide dismutases with iron, zinc, copper, and manganese centers are known.
The manganese containing enzyme in Escherichia coli is a tetramer; each of its subunits contains a manganese ion. The metal atom is coordinated by three histidine residues, one asparagine residue, and a hydroxo ligand.
Coordination sphere of the Mn atoms
In the same bacterium, the destruction of superoxide is also carried out by enzymes containing iron, zinc and copper. This underlines the enormous significance of this reaction.
The coordination geometry of the formed complexes is almost independent of the metal ions in their centers. This can be seen in a comparison of their structures.