Bound dioxygen reactions

In accord with this mechanism, free peroxyl radical of the reaction product hydroperoxide activates the inactive ferrous form of enzyme (Reaction (1)). Then, active ferric enzyme oxidizes substrate to form a bound substrate radical, which reacts with dioxygen (Reaction (4)). The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

Scheme 16. Reaction of coordinated carbon monoxide with bound dioxygen at an iridium center.

The reaction of C02 with Ir(CH3)CO(02)[P(p-tolyl)3]2 also results in the formation of a peroxycarbonate complex (191) via external attack by carbon dioxide. In this case, however, only gaseous carbon dioxide is required, rather than the more strenuous conditions of liquid C02. This same complex reacts with gaseous carbon monoxide to form the carbonate complex. Labeling experiments demonstrate that the coordinated CO does not participate in the reaction External attack by the added CO is responsible for the reaction (191). Coordinated CO has been shown to react with bound dioxygen, as is seen in Scheme 16. In this case, the chelating triphos ligand obviously has a significant effect on the reactivity (189). 

The presently accepted model1555 for the reaction of bleomycin with DNA involves the binding of an Fen-bleomycin complex to DNA, and the reduction of iron-bound dioxygen. Cleavage of DNA is known to occur by the action of the 1,10-phenanthroline complex of Cu1 in the presence of hydrogen peroxide, and this reaction is sustained by the presence of NADH.1556 This provides a useful parallel for the bleomycin reaction. 

In the final phase of the dioxygen reaction, cytochrome a donates an electron to F with the concomitant uptake of a proton [60] (Figure 7), and the fully oxidized enzyme is formed [52b, 60b,d]. This has at least one OH bound at the binuclear center [61, 62]. Like the previous step (P —> F), the rate of formation of O is sug-... 

Fig. 3.3. Tentative mechanism of reduction of dioxygen. The scheme shows some of the more significant reaction steps at the haem iron-Cug centre of cytochrome oxidase. The reaction may be initiated by delivery of dioxygen to the reduced enzyme (in anaerobiosis top of figure). An initially formed oxy intermediate is normally extremely short-lived, but can be stabilised and identified in artificial conditions (see Refs. 92, 99,129, 134). Concerted transfer of two electrons from Fe and Cu to bound dioxygen yields a peroxy intermediate. This, or its electronic analogue, is stabilised in the absence of electron donors (ferrocytochrome a and/or reduced Cu ), and has been termed Compound C [129,130,132). It may also be observed at room temperature, and is then probably generated from the oxidised state by partial oxidation of water in the active site, in an energy-linked reversed electron transfer reaction [29] (see also Refs. 92, 99). Also the ferryl intermediate [92,99,100] has been tentatively observed in such conditions [29]. In aerobic steady states the reaction is thought to involve the cycle of intermediates in the centre of the figure (dark frames). The irreversible step is probably the conversion of g = 6 (see Refs. 98, 133) to peroxy .

Sjogren, T. and J. Hajdu (2001). Structure of the bound dioxygen species in the cytochrome oxidase reaction of cytochrome cdl nitrite reductase. 

At least one phosphate group is considered bound to the molbydenum(v). H2O2 reacts about 20 times faster than O2 with [Mo 2(cys)2] and addition of peroxide to Mo i yields the same product as that of the dioxygen reaction. If the metal complex is present in excess, however, there is evidence for adduct formation ... 

The reaction likely proceeds through an electron transfer between ferric chloride and amine producing a radical cation and iron (II) chloride. Hydrogen abstraction from the radical cation is proposed to lead to iminium species either directly or via iron(II)-bound dioxygen. ... 

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