The binding of dioxygen is normally a reversible process:
Under some circumstances, such as in the presence of added nucleophiles and protons, coordinated dioxygen is displaced as the superoxide anion radical, O2 -', leaving the metal center oxidized by one electron and unreactive to dioxygen :
For hemoglobin there exists a flavoprotein reductase system, comprising a reduced pyridine nucleotide (e.g., NADH), cytochrome b5 reductase, and cytochrome b5 , that reduces the ferric iron back to the ferrous state, so that it may coordinate dioxygen again. 1,51 In addition, all aerobically respiring organisms and many air-tolerant anaerobes contain a protein, superoxide dismutase, that very efficiently catalyzes the dismutation of superoxide ion to dioxygen and hydrogen peroxide:
However, the physiological effects of the superoxide moiety remain controversial. 53,54 Finally, there is a third enzyme, the hemoprotein catalase, that converts the toxic hydrogen peroxide into water and dioxygen:
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