Oxygen atom transfer enzymes

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

The redox properties of Mo also make it useful in enzymes that catalyze reactions involving two-electron or oxygen-atom transfer (Frausto da Silva and Williams 2001). Such enzymes include nitrate reductase, sulfite oxidase, formate dehydrogenase and aldehyde oxidase (Hille 1996 Stiefel 1997 Kroneck and Abt 2002). Hence, while Mo is rarely a terminal electron... 

Thermodynamic considerations indicate that the oxygen atom transfer reactions between Mo02(R2dtc)2 and enzyme substrates should be largely irreversible.175 For example, the hypothetical reaction of equation (17) has AH = — 28.5 5.5 kcalmol-1 (= —119 23.0 kJ mol1). The reaction should have a small AS and hence should have a negative AG and proceed spontaneously.175 There are, however, complicating factors. 

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). 

Cytochrome P-450 enzymes have been isolated from a variety of mammalian tissues, insects, plants, yeasts and bacteria. The P-450 cytochromes (Gunter and Turner, 1991) are membrane bound mono-oxygenase enzymes which catalyse oxygen atom transfer to entrapped non-polar substrates. The binding of carbon monoxide to the enzyme produces a split in the 420 nm Soret band to give bands at 364 and 450 nm. The absorption at 450 nm distinguishes the hemoprotein from all others and hence provides... 

Figure 1. Postulated oxygen atom transfer mechanism for the enzymic reduction of nitrate by Mo(H20)63+, assuming NADPH, H+ to be the substrate for reducing Mo02(H20) to Mo(H20)63+...

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). 

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