Fig. 5: Proposed mechanistic cycle of Fe²⁺ oxidation by FtMt.

Following binding of two Fe²⁺ ions to the apo FC (top of the main cycle), O₂ binds and is activated by transfer of a single electron from Tyr34 to yield a tyrosyl radical and a superoxo adduct of the di-Fe²⁺ FC. The radical is quenched by electron transfer from a remote FC, and electron transfer from Fe²⁺ at the site with superoxide bound yields the peroxide product of O₂ reduction and the MVFC intermediate. Binding of O₂ and its activation by the MVFC is rapid in comparison to that occurring at the di-Fe²⁺ FC form, and hence, the MVFC intermediate does not accumulate under continuous turnover. This reaction results in the DFP intermediate, which is hydrolyzed to the metastable di-Fe³⁺ form. In the presence of excess O₂, the metastable di-Fe³⁺ FC breaks down to release Fe³⁺ ions to the FtMt cavity, and the reaction cycle begins again (marked as (i)). Under limiting O₂, where both di-Fe²⁺ and di-Fe³⁺ FCs are present, a disproportionation reaction occurs via long-range electron transfer, resulting in two MVFCs (marked as (ii) on the smaller left-hand cycle). When O₂ is available again, these are oxidized to the di-Fe³⁺ form (marked as (iii)). The overall stoichiometry of the reaction is identical to that catalyzed by cytosolic ferritins, with each center coupling the oxidation of two equivalents of Fe²⁺ to Fe³⁺ with the reduction of O₂ to peroxide. Fe²⁺ ions are indicated in cyan, Fe³⁺ are in brown, and the DFP is in royal blue.