Two proton-one electron coupled transfer in iron and manganese superoxide dismutases: A density functional study

A theoretical study on a simple model has been carried out to contribute to the elucidation of the reaction mechanism of iron and manganese superoxide dismutases (FeSOD and MnSOD). The presence of the water molecule in the reaction is very important since it is involved in a transfer process. In fact, this step is a complex coupled transfer process that involves two protons and one electron at the same time. This step is possible in a specific arrangement of superoxide anion in its coordination to the transition metal ion. The other possible conformations are the inactive species that evolves to the active complex in a slow way or that accepts an additional proton to produce a new species (“dead-end” complex) that is unable to evolve to hydrogen peroxide. Time-dependent formalism applied to the density functional calculations and compared to the experimental electronic spectra has allowed us to choose some possible candidates for the “dead-end” complex. Our results indicate that a hydroperoxo form (4-Mn) is the more probable candidate to it. However, other possibilities cannot be rejected.