Methane monooxygenase oxygen activation

Fig. 14. Intermediates in the activation of oxygen by binuclear iron centers. Hr, Heme-rythrin RNR, ribonucleotide reductase MMO, methane monooxygenase.

Que L Jr, Dong YH. Modeling the oxygen activation chemistry of methane monooxygenase and ribonucleotide reductase. Acc Chem Res. 1996 29 190-6. 

Methane monooxygenase consists of a catalytically active diiron center. In the presence of oxygen this enzyme oxidizes methane and other hydrocarbons, i.e. molecules without any anchor-... 

Some preparations of iron exchanged into zeolite H-MFI by vapor-phase FeCL are known to be active and selective catalysts for the reduction of NO, with hydrocarbons or ammonia in the presence of excess oxygen and water vapor (45,46). The active centers in Fe/MFI are assumed to be binuclear, oxygen-bridged iron complexes, as follows from H2-TPR, CO-TPR, and ESR data (45,47) and EXAFS and XANES results (48,49). These complexes are structurally similar to the binuclear iron centers in methane monooxygenase enzymes that are employed by methanotrophic bacteria in utilization of methane as their primary energy source (50). It is believed that molecular oxygen reacts with these centers to form peroxide as the initial step in this chemistry (50). 

Lee, S. K., and Lipscomb, J. D., 1999, Oxygen activation catalyzed by methane monooxygenase hydroxylase component proton delivery during the 060 bond cleavage steps. Biochemistry 38 4423n4432. 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The activation of dioxygen for the monooxygenation of saturated hydrocarbons by the methane monooxygenase enzyme systems (MMO hydroxylase/reductase) represents an almost unique biochemical oxygenase, especially for the transformation of methane to methanol. The basic process involves the insertion of an oxygen atom into the C-H bond of the hydrocarbon via the concerted reduction of O2 by the reductase cofactor (equation 120). 

---