Hydroxyl radical oxidation-reduction cycle

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

The next step of the water reduction cycle, i.e., the formation of the native intermediate, is radically different in the two schemes. Solomon s mechanism requires a second two-electron reduction step—deemed necessary in order to overcome a large thermodynamic barrier for a one-electron process. This step completes the reduction of O2 to FI2O and leads to the formation of an all-bridged intermediate (see Figure 13). This intermediate is proposed to undergo reduction by the substrate (by way of the T1 center) to start a new catalytic cycle. Messerschmidt s proposal involves a sequence of two single-electron reduction steps. The first leads to the formation of an oxyl or hydroxyl radical bound to a reduced T3 copper center (in accordance with the earlier interpretations of the EPR signal), and with concomitant release of a water molecule. Further oxidation of this reduced copper ion and the release of a second molecule of water leads to the formation of the catalytically relevant all-oxidized intermediate which once fully re-reduced can commence a new catalytic cycle. 

---