4.10: Hazards of Life with Dioxygen

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The binding of dioxygen is normally a reversible process:

\[M + O_{2} \rightleftharpoons MO_{2} \tag{4.22}\]

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:^49,50

\[MO_{2} \rightleftharpoons M^{+} + O_{2}^{-} \tag{4.23}\]

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:^52-54

\[2O_{2}^{-} + 2H^{+} \rightarrow O_{2} + H_{2}O_{2} \tag{4.24}\]

However, the physiological effects of the superoxide moiety remain controversial.^53,54Finally, there is a third enzyme, the hemoprotein catalase, that converts the toxic hydrogen peroxide into water and dioxygen:¹

\[2H_{2}O_{2} \rightarrow O_{2} + 2H_{2}O \tag{4.25}\]

This topic is discussed further in Chapter 5.