Protonation of the proximal histidine ligand in heme peroxidases

The heme peroxidases have a histidine group as the axial ligand of iron. This ligand forms a hydrogen bond to an aspartate carboxylate group by the other nitrogen atom in the side chain. The aspartate is not present in the globins and it has been suggested that it gives an imidazolate character to the histidine ligand. Quantum chemical calculations have indicated that the properties of the heme site strongly depend on the position of the proton in this hydrogen bond. Therefore, we have studied the location of this proton in all intermediates in the reaction mechanism, using a set of different quantum mechanical and combined experimental and computational methods. Quantum refinements of a crystal structure of the resting Fe-III state in yeast cytochrome c peroxidase show that the geometric differences of the two states are so small that it cannot be unambiguously decided where the proton is in the crystal structure. Vacuum calculations indicate that the position of the proton is sensitive to the surroundings and to the side chains of the porphyrin ring. Combined quantum and molecular mechanics (QM/MM) calculations indicate that the proton prefers to reside on the His ligand in all states in the reaction mechanism of the peroxidases. QM/MM free energy perturbations confirm these results, but reduce the energy difference between the two states to 12-44 kJ/mol.