Enzymic Reactions of Dioxygen

There are a number of metalloenzymes, EM, that bring about reactions of dioxygen and its derivatives, hydrogen peroxide and the superoxide ion. They may act as reversible oxygen carriers, such as hemoglobin and myoglobin in the following reaction  

The enzyme may introduce either one or two oxygens into the substrate, as shown in the following reactions  

Other enzymes protect the system from reactions with superoxide or hydroxy radicals by catalyzing the disproportionation of Oj and the decomposition of hydrogen peroxide, as in reactions (8.30) and (8.31), respectively. 

The oxidases and peroxidases have been succinctly reviewed. The following sections discuss two examples of these metalloenzymes. 

Takano reported the structure of metmyoglobin, which is the Fe(III) complex with axial water and imidazole ligands. The same structural features also are found in the model system described by Collman and co-workers, where the displacement of iron depends on the steric requirements of the axial ligand, 0.086 A with 2-methylimidazole and 0.03 A with 1-methylimidazole. 

---