Figure 9.

Proposed reaction mechanism for Compound 0, Compound I, and Compound II formation in B-class DyPs. Hydrogen peroxide enters the narrow distal heme access channel, binds (k₁), and is deprotonated by Asp-143 (Compound 0 formation). Heterolytic cleavage of the O–O bond (k₃) forms Compound I (oxoiron(IV) porphyrin radical) (k₃), which is reduced by either two-electron donors like thiocyanate (⁻SCN) directly to the ferric resting state, thereby producing hypothiocyanite (HOSCN) (k₄), or by one-electron donors like serotonin (AH) to Compound II (oxoiron(IV) species) (k₅), thereby producing the corresponding radical (A^•). Finally, a second one-electron donor (AH) reduces Compound II to the ferric resting state (k₆).