Fe ligandation in the structure of APD in complex with 3-iminooxepin-2(3H)-one and 2-aminomuconic 6-semialdehyde. (a) Lactone-intermediate complex (subunit D) fitting the electron density. (b) Stick model of the lactone intermediate and superoxide binding in the Fe(II) coordination sphere in subunit D. (c) Product complex (subunit B) fitting the electron density. (d) Stick model of product and a solvent molecule binding in the Fe(II) coordination sphere in subunit B. The grey 2Fobs − Fcalc maps are contoured at 1.0σ and the green Fobs − Fcalc ligand-omit maps are contoured at 3.5σ. Element colour indices: carbon, grey (protein) or yellow (ligand); nitrogen, blue; oxygen, red; iron, magenta. Hydrogen bonds and 2AP–Fe2+ bonds are represented by dashed lines. Numbers indicate bond distances in Å.