The aim of the present density functional theory (DFT) study is to investigate how the modifications in the structure of porphyrin-based complexes could change their catalytic properties, taking as examples heme and Fe and Mn porphyrins. As factors determining catalytic activity, the following parameters are chosen: the charges on the metal ion and the O-2 ligand, lengths and bond orders of the metal-oxygen and oxygen-oxygen bonds, and dioxygen-binding energies. The results indicate that the naturally occurring heme-O-2 complex should be more active than the dioxygen iron(II) porphyrin, because the bound dioxygen has more nucleophilic character and would be more susceptible to scission. The exchange of iron to manganese would improve nucleophilicity of the O-2 molecule and weaken the metal-O-2 bond. Finally, iron(II) seems to be a better choice for dioxygen binding than iron(III). (c) 2006 Elsevier B.V. All rights reserved.