Versatility of biological non-heme Fe(II) centers in oxygen activation reactions

Oxidase and oxygenase enzymes allow the use of relatively unreactive O ₂ in biochemical reactions. Many of the mechanistic strategies used in nature for this key reaction are represented within the 2-histidine-1- carboxylate facial triad family of non-heme Fe(II)-containing enzymes. The open face of the metal coordination sphere opposite the three endogenous ligands participates directly in the reaction chemistry. Here, data from several studies are presented showing that reductive O₂ activation within this family is initiated by substrate (and in some cases cosubstrate or cofactor) binding, which then allows coordination of O₂ to the metal. From this starting point, the O₂ activation process and the reactions with substrates diverge broadly. The reactive species formed in these reactions have been proposed to encompass four oxidation states of iron and all forms of reduced O₂ as well as several of the reactive oxygen species that derive from O-O bond cleavage.