Density-functional and electrostatic calculations for a model of a manganese superoxide dismutase active site in aqueous solution

We have calculated the redox potential for a solvated manganese superoxide dismutase active-site model by immersing the solute described quantum mechanically into a continuum dielectric representation of an aqueous environment. Active-site models containing 37 or 38 atoms representing the side chains of the 5 ligands-3 histidine residues, 1 aspartate, and 1 hydroxide or water molecule-and the manganese ion were constructed from X-ray crystal structure coordinates. The electronic structure of the active-site model was determined quantum mechanically using density-functional methods. The resultant ESP molecular charge distribution was then used to compute the reaction field potential between the active-site model and aqueous solvent by finite difference solutions to the Poisson-Boltzmann equation. This aqueous system was then coupled back into the molecular Hamiltonian and the process iterated to self-consistency. Electron-relaxation effects in the redox process were assessed and found to be important in the dismutation reaction of the enzyme. The final energies of the reduced and oxidized systems with the water ligand gave an ''absolute'' calculated redox potential of 0.17 eV, compared with 0.31 eV found experimentally for the E. coli protein. The calculated energetics of water deprotonation in the Mn(III) species showed strong acidity. While comparison with related physical systems supports this, the result was still probably too acidic. The redox potential arising from reduction accompanied by protonation of the Mn(II)-hydroxyl form was also analyzed.