Figure 4.

Peroxidase and oxygenase activities of cytochrome c. Reaction model merging the proposal from Kagan and collaborators [149] and the adapted catalytic model of cyclooxygenases as reviewed by Marnett [150]. Blue arrows correspond to the canonical peroxidase cycle [146]. Heterolytic cleavage of a peroxide substrate—preferentially for Cc, a lipid hydroperoxide—yields the corresponding hydroxyl derivative (or water when the substrate is H₂O₂) and Compound I, which is reduced back to the resting ferric state in two sequential single-electron transfers from A substrate. Red and green arrows indicate the reactions purportedly leading to oxygenase activity according to the literature. Spin trap experiments have detected Y48 radicals [151]. Dimers of tyrosines 67 and 74 and oxidation products of Y48 are detectable even in the absence of H₂O₂ [152]. The tyrosyl radical sequesters a hydrogen atom from an unsaturated fatty acid. Finally, O₂ reacts with the alkyl radical to form an alkyl peroxide radical as an initial product undergoing further reactions.