Porphyrins hemes, oxygenated

In discussing the reaction cycle of the cytochrome P450s, it is useful to incorporate similar states which appear to be more or less stable in other heme-oxygen systems. The sequential steps of oxygen activation are those that follow the formation of the ferrous reduced iron-porphyrin state that is generated by electron transfer from the physiological redox donor. These steps are formally ... 

Fe" porphyrins (hemes) are air sensitive and easily oxidized to the p-oxo dimer [Fe "(Por)]20 (Scheme 17). However, the irreversible oxidation can be prevented by a large excess of the axial ligand (retards process A), at a low temperature (< — 50 C, retards process D), or by protection of the O2 binding site (prevents process C). The third approach has been intensively studied in relation to the biological oxygen carriers, myoglobin (Mb) and hemoglobin (Hb). 

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