Water, four-electron dioxygen reduction

Fukuzumi S, Okamoto K, Gros CP, Guilard R. 2004. Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins. J Am Chem Soc 126 10441. 

The four-electron reduction of dioxygen in water (Equation (70)) is an essential reaction carried out by aerobic organisms during the energy production cycle in the mitochondria. 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

D. Dolphin Collman and Anson have found with their five-atom bridged amide systems that the reduction of dioxygen is partitioned roughly one third undergoing a four-electron reduction to water and the rest a two-electron reduction to peroxide. We see the same result indeed the 5-5 systems of ours and that of Collman-Anson are indistinguishable. Neither type of cofacial dimers with chains greater than five show any four-electron reduction. 

In a subsequent study of this type (Durand, Bencosme, Collman Anson, 1983), dimers of type (143) were investigated as potential redox catalysts for the four-electron reduction of dioxygen to water (via peroxide). The Co(ii)/Co(ii) dimer is an effective catalyst for the electrochemical reduction of dioxygen once again the dioxygen binds... 

Cytochrome c oxidase. Cytochrome c oxidase is an enzyme that occurs in the inner mitochondrial membrane and, as we have seen, catalyzes the four-electron reduction of dioxygen to water as the final reaction in the... 

Clearly, the S0 state must have one Mn(II), one Mn(III) and two Mn(IV) while the S4 must have three Mn(IV) and one Mn(V)—but where they are located in the tetra-Mn centre and how their four-electron reduction is coupled with water splitting and dioxygen evolution remains to be established. (For recent reviews see Goussias et al., 2002 Ferreira et al., 2004 Rutherford and Boussac, 2004 Iverson, 2006.)... 

The hemocyanlns which cooperatively bind dioxygen are found in two invertebrate phyla arthropod and mollusc. The mollusc hemocyanlns additionally exhibit catalase activity. Tyrosinase, which also reversibly binds dioxygen and dlsmutates peroxide, is a monooxygenase, using the dloxygen to hydroxylate monophenols to ortho-diphenols and to further oxidize this product to the quinone. Finally, the multicopper oxidases (laccase, ceruloplasmin and ascorbate oxidase) also contain coupled binuclear copper sites in combination with other copper centers and these catalyze the four electron reduction of dloxygen to water. 

Electrocatalytic Reduction of Dioxygen The electrocatalytic reduction of oxygen is another multi-electron transfer reaction (four electrons are involved) with several steps and intermediate species [16]. A four-electron mechanism, leading to water, is in competition with a two-electron mechanism, giving hydrogen peroxide. The four-electron mechanism on a Pt electrode can be written as follows ... 

Only two of the substituted porphyrins caused the desired four-electron reduction, and the process took place in two distinct steps, first to produce hydrogen peroxide and then to form water. However, the use of ruthenated complexes led to the direct four-electron reduction of dioxygen. 

The discussion of porphyrin complexes in the activation of dioxygen in solution can be readily subdivided into discussions of monomeric systems and dimeric systems. The dimeric systems (cofacial metallodi-porphyrins) have recently been reviewed by Collman, Wagenknecht, and Hutchison (73). That review highlights the significant amount of research stimulated by the initial discovery by Collman and co-workers (74), confirmed later by Chang and co-workers (75), that dicobalt cofacial diporphyrins can promote the direct four-electron reduction of dioxygen to water. 

Several of these cytochromes use nitrogen compounds as electron acceptors. Cytochrome cdx from Pseudomonas aeruginosa is a greenish-brown soluble enzyme which contains one c and one chlorin dx) heme in each of two subunits of 63 000 molecular weight. The presence of four redox centres is appropriate for the efficient catalysis of the four-electron reduction of dioxygen to water. It appears, however, that its physiological role may be the reduction of nitrite to NO. Kinetic studies on the anaerobic reduction of cd, by Fe(EDTA)2- have focused attention on electron transfer between the heme groups in the enzyme.722... 

Oxidases catalyze electron transfer to dioxygen, which is reduced to superoxide, peroxide or water.1225-1226 The best characterized example of an oxidase is cytochrome oxidase, the last member of the respiratory chain, which catalyzes the four-electron reduction of dioxygen to water. 

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