Studies using myoglobins reconstituted with a variety of chemically modified heme cofactors revealed that the oxygen affinity and autoxidation reaction rate of the proteins are highly correlated to each other, both decreasing with decreasing the electron density of the heme iron atom. An Fe3+-O-2(-)-like species has been expected for the Fe2+-O-2 bond in the protein, and the electron density of the heme iron atom influences the resonance process between the two forms. A shift of the resonance toward the Fe2+-O-2 form results in lowering of the O-2 affinity due to an increase in the O-2 dissociation rate. On the other hand, a shift of the resonance toward the Fe3+-O-2(-)-like species results in acceleration of the autoxidation through increasing H+ affinity of the bound ligand.