Figure 1 - Oxygen activation by extradiol dioxygenases.

This type of activation involves simultaneous activation of O₂ and substrate, once both are bound to the active site Fe(II). It proceeds through formation of an alkylperoxo intermediate in which activated dioxygen attacks the substrate before O-O bond cleavage. The R groups of the substrates considered here are CH₂COO (homoprotocatechnic acid, HPCA, 1) and NO₂ (4-nitrocatechol, 4NC, 2). The structures shown are the Brevibacterium fuscum HPCD 4-nitrocatechol-semiquinone-Fe(II)-[O₂]^– (top) and Fe(II)- alkylperoxo (bottom) intermediates (Protein Data Bank (PDB) code 2IGA)²⁹. Atom color code: gray, carbon (enzyme residues); yellow, carbon (substrate); blue, nitrogen; red, oxygen; cyan, iron. Red and gray dashed lines show hydrogen bonds and potential bonds to iron, respectively.