Figure 8.

Proposed mechanism of Fe⁺² oxidation at the FOC of EncB. In step 1, the 2 Fe⁺² atoms in the “high-affinity” binding state (state 1) interact with O₂ to form a peroxodiferric intermediate. In step 2, the two conserved Y35 move towards the peroxodiferric intermediate to donate e− + H⁺ to the Fe⁺² atoms forming two tyrosyl radicals and two hypothetical Fe⁺⁴ intermediates with an unknown oxidation state. In step 3, one of the two Fe⁺⁴ is reduced to Fe⁺³ through an electro-transfer chain formed by the site C Fe⁺² and H61 releasing a H₂O molecule. The oxidized site C Fe⁺³ leaves the site and is replaced by a second Fe⁺² which in turn reduces the second Fe⁺⁴ releasing a second H₂O. In step 4 the tyrosyl radical is reduced by an unidentified redox partner followed by the addition of a proton. Finally, Fe⁺³ atoms at the FOC are displaced by the incoming Fe⁺². See also Figure S10.