Oxygen atom transfer reactions enzyme mechanisms

Oxidation Catalyzed by Metalloporphyrins. Much attention has been devoted to the metal-catalyzed oxidation of unactivated C—H bonds in the homogeneous phase. The aim of these studies is to elucidate the molecular mechanism of enzyme-catalyzed oxygen atom transfer reactions. Additionally, such studies may eventually allow the development of simple catalytic systems useful in functionalization of organic compounds, especially in the oxidation of hydrocarbons. These methods should display high efficiency and specificity under mild conditions characteristic of enzymatic oxidations. 

Such reactions are interesting as models for oxygen atom transfer in mechanisms of dioxygen activation by cytochrome P450 enzyme systems (49, 50, 51). 

Subsequently, RR was used to successfully detect structural changes between the oxidized and reduced forms of both DMSOR and BSOR that are consistent with the proposed oxygen atom transfer mechanism of the catalytic reaction (95, 97). These experiments make use of the readily measurable isotopic shifts in vibration frequency between ieO=Mo and lsO=Mo to follow the fate of the oxygen atom removed from DMSO (or BSO) by the Mo. In this way, the clean transfer of 180 from DMSlsO to Mo(IV) to yield the oxidized form of the active site as Mo(VI)=180 was directly observed as well as the substrate-bound intermediate, (DMS180)Mo(IV). Further discussion of the technique of RR applied to metal dithiolenes and dithiolene-containing enzymes is included in Chapter 4 in this volume (98). 

The steady state kinetics of arsenite oxidoreductase from A. faecalis indicate a so-called double displacement (or ping-pong ) mechanism (15) in which the enzyme cycles between oxidized and reduced forms in its reaction with arsenite and azurin (or cytochrome c). This overall kinetic scheme is common in redox-active proteins. Arsenite must bind, the oxygen atom transfer chemistry take place, and arsenate dissociate before the subsequent reaction of a second molecule of substrate. Since arsenate is not an inhibitor of arsenite oxidoreductase (43), product dissociation must be effectively irreversible. The turnover number (kcai) of 27 sec and for arsenite of 8 pM are reasonable parameters for the detoxification of arsenite, especially since A. faecalis is able to survive in at least 80 mM (1%) sodium arsenite. The considerable catalytic power of the enzyme is reflected by the kinetic parameter k JK of 3.4 X 10 M sec , which is fairly close to the diffusion-controlled maximum of 10 -10 M sec for proteins in... 

Scheme 4.3 Substituted enzyme mechanism transferred to oxygen atom transfer catalysis (M = Mo, W). In principle, this represents two coupled half-reactions with a pre-equilibrium at each half-reaction. This is the favoured mechanism due to the negative activation entropies observed for this type of reaction.

It is unknown what role is played by ligand environments in proteins and in synthetic analogues in stabilizing different species as it is also unknown which species represent active oxidants capable of transfering oxygen atoms to substrate in the enzyme systems. Moreover, it is not known how a binuclear metal active site might differ tom a mononuclear active site or if there is one type of reaction mechanism that operates in all or most of the monooxygenase enzymes or if each type of enzyme follows a different mechanism. 

Model studies clearly demonstrate that oxo transfer is a viable mechanism for many of the enzyme reactions shown in Table 2d. However, primarily because of difficulties in labeling studies, it has not yet proved possible to validate oxo transfer as a physiologically relevant enzymatic mechanism. Although it has been possible to oxidize and reduce molybdenum centers using certain oxygen atom donors or acceptors, these experiments serve only to demonstrate that such processes are possible and not that they are part of the physiologically relevant pathway [231,233],... 

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