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Figure 1.
Figure 1. Mechanism of iron oxidation in heme. The heme iron
can be oxidized in two mechanisms: when the concentration of
O[2] is high (the top method) or low (the bottom method) (based
on Ref.[14]). Under high concentrations of O[2] (1), a hydronium
molecule bonds with O[2] and the ligand leaves as a neutral
superoxide radical. Water can then hydrogen bond with the distal
His. Under low concentrations of O[2] (2), a water molecule can
displace the ligand. Re-entry of O[2] can remove an electron
from the heme iron in which the coordinated water facilitates
the removal of the iron electron to O[2]. The ligand leaves the
heme pocket as superoxide anion radical. In both scenarios the
iron heme is oxidized to Fe(III).
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