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Cytochrome oxidase, also reaction mechanism

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 . 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 .
Cytochrome P450 is considered the most versatile biocatalyst known. The actual reaction mechanism is complex and has been briefly described previously (Figure 11-6). It has been shown by the use of that one atom of oxygen enters R—OH and one atom enters water. This dual fate of the oxygen accounts for the former naming of monooxygenases as mixed-function oxidases. The reaction catalyzed by cytochrome P450 can also be represented as follows ... [Pg.627]

Haem proteins that react with oxygen also utilise ferryl intermediates. Fig. 4 compares the (proposed) reaction mechanisms of cytochrome oxidase and cytochrome P-450 with those of peroxidases and catalases. As can be seen, several of the reaction intermediates have the same oxidation states (although the protonation steps and stage at which H2O is released may be different). However, in contrast to peroxidases, oxidases must react with molecular oxygen, and this requires a reaction cycle that includes Fe11. [Pg.78]

Deamination. Amine groups can be removed oxidatively via a deamination reaction, which may be catalyzed by cytochromes P-450. Other enzymes, such as monoamine oxidases, may also be involved in deamination reactions (see below). The product of deamination of a primary amine is the corresponding ketone. For example, amphetamine is metabolized in the rabbit to phenylacetone (Fig. 4.27). The mechanism probably involves oxidation of the carbon atom to yield a carbinolamine, which can rearrange to the ketone with loss of ammonia. Alternatively, the reaction may proceed via phenylacetoneoxime, which has been isolated as a metabolite and for which there are several possible routes of formation. The phenylacetoneoxime is hydrolyzed to phenylacetone. Also N-hydroxylation of amphetamine may take place and give rise to phenylacetone as a metabolite. This illustrates that there may be several routes to a particular metabolite. [Pg.92]


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Cytochrome oxidase, also

Cytochrome oxidases reaction mechanisms

Cytochrome reaction mechanism

Cytochrome reactions

Cytochromes mechanism

Oxidases reactions

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