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 si