Fig. 6.

HAO catalytic reaction cycle defined by the characterized intermediates. The time windows for observing the in crystallo reaction intermediates are highlighted. The structurally characterized intermediates include monodentate E•S complex, bidentate E•S complex, Fe-bound superoxo, alkylperoxo, ε-lactone, monodentate and then bidentate 3E,5Z,2t,4c-enol tautomer of ACMS, as well as bidentate 3E,5Z,2t,4t-enol tautomer of ACMS. The superposition of the crystal structures of the substrate-free HAO and the above seven intermediates is shown as an Inset to highlight the conformational change of loops A, B, and C during the catalytic reactions. Loops of substrate-free, bidentate E•S complex, and subsequent intermediates after dioxygen binding are colored in blue, red, and yellow, respectively.