Coupled Redox Potentials in Manganese and Iron Superoxide Dismutases from Reaction Kinetics and Density Functional/Electrostatics Calculations

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Abstract
A methodology for determining the coupled redox potentials (ΔEredox°(coupled)) of manganese and iron superoxide dismutases (Mn(Fe)SODs), from the standard redox potential of reaction (O2- + 2H+ + e- → H2O2) and the experimental kinetic rate constants of Mn(Fe)SOD proteins, has been presented for the first time. A combined density functional (DF) and electrostatic protein/reaction field (DF/electrostatics) model has also been applied to seven protein structures, to study the structural, energetic, simple redox potential, pKa, and coupled redox potential properties associated with each active site. The quantum cluster active site models, which include the metal, first shell ligands, represented by amino acid side chains and a solvent derived ligand, and the second shell H-bonding partners, were taken from the crystal structures, and geometry was optimized in four kinds of states: oxidized (III) and reduced (II) states with either a H2O molecule or a OH- group as the fifth coordinated ligand. We conclude from the calculations that the oxidized and reduced Mn(Fe)SODs are in the Mn3+(Fe3+)(OH-) and Mn2+(Fe2+)(H2O) forms, respectively; proton transfers will happen in both steps of the dismutation of superoxide anion (O2-), and the proton-transfer reactions will occur prior to or concerted with the electron transfer from O2- group to the Mn3+(Fe3+)SOD metal center. The ΔEredox°(coupled) of E. coli FeSOD calculated by the DF/electrostatics method is 0.16 V, which is very close to the experimental value of 0.25 V. The absolute values of ΔEredox°(coupled) for T. thermophilus, human wild-type, and mutant Q143N MnSODs obtained from the DF/electrostatics method are −0.25, −0.29, and −0.11 V, which present the same trend and very similar relative values to those obtained from experimental kinetic rate constants (0.40, 0.32, and 0.59 V, respectively). The order ΔEredox°(human wild-type) < ΔEredox°(T. thermophilus) < ΔEredox°(E. coli) < ΔEredox°(Q143N) for MnSOD proteins is predicted by the DF/electrostatics calculations.