Scheme 3. Two-pathway reaction mechanism of EFE with insights provided by the enzyme structures^a.

^aEFE•Fe(II) binds 2OG predominantly in two monodentate conformations, with a small amount of chelate coordination to the metal. The nearby binding of L-Arg leads to a change in orientation of D191 (shown in cyan) as it forms a hydrogen bond with this substrate and the 2OG switches to bidentate coordination. Dioxygen binds to the metallocenter opposite of L-Arg to form the Fe(III)-superoxo species that transforms to a cyclic peroxide-Fe(IV) intermediate. The mechanism diverges at this intermediate, with the predominant pathway invoking a decarboxylative fragmentation to release ethylene with oxalic acid transiently bound to the ferryl species. Subsequent reactivity of this intermediate yields CO₂ and bicarbonate or two molecules of CO₂ (Scheme 2) and the recycled EFE•Fe(II). The second pathway depicts oxidative decarboxylation of 2OG to yield succinate and a ferryl species pointing away from the L-Arg substrate. A ferryl flip repositions the ferryl group to point toward C5 of L-Arg, with F283 (and the repositioned D191) partly hindering this process. Hydrogen atom abstraction, hydroxyl radical rebound, elimination of guanidine from 5-hydroxy L-Arg, and ring formation complete this cycle.