Scheme 2. Proposed mechanisms consistent with the available data^†.

^†A. Formation of Fe(IV)=O(coproporphyrin■+) (coproheme compound I) (E = enzyme; Fe(III) in red and Fe(IV) in purple). B. Oxidative decarboxylation of propionate 2 requires Tyr145. The neutral Tyr145-O■ radical is depicted abstracting hydrogen (H■) from the propionate C_β, though this is hypothetical. Transfer of an electron and proton to the ferryl of Compound II then leads to the ferric-OH complex of harderoheme III, from which water can ultimately depart. C. Following another round of H₂O₂-activation analogous to (A), a second Fe(IV)=O(harderoporphyrin■+) complex forms. Three possible redox-active amino acids are connected to propionate 4 via hydrogen bonding through water molecules, though only Lys149 appeared to play a role decarboxylating propionate 4. The identity of the mediator for proton coupled electron transfer from the propionate (designated E+■ or E■) is unclear. Net uptake of H■ by the enzyme is designated via E(H).