Oxygen, activated, iron catalysis

The catalysis by iron of the generation of OH, a highly reactive and dangerous species of activated oxygen, has been well established ... 

Active Sites Iron Proteins with Dinuclear Active Sites Manganese The Oxygen-evolving Complex Models Oxidation Catalysis by Transition Metal Complexes Oxygen Inorganic Chentistry. 

A potentially harmful chemical property of iron and its compounds is the ability to catalytically generate potent hydroxyl radicals that are oxidatively toxic to cells [3,4]. This catalysis (Haber-Weiss and Fenton reactions) occurs with activated oxygen species (superoxide and hydrogen peroxide, O3 and H2O2) as shown below. Superoxide is formed by the univalent reduction of oxygen (xanthine plus xanthine oxidase). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

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