Methane mono-oxygenase enzymes

GAC Granular activated carbon MBAS Methyl blue active substances MMO Methane mono-oxygenase enzyme MSW Municipal solid waste... 

The activation of dioxygen for the mono-oxygenation of saturated hydrocarbons by the methane mono-oxygenase enzyme systems (MMO hydroxylase/reductase) represents an almost unique biochemical oxygenase, especially for the transformation of methane to methanol. l 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... 

The photo-excited states of some inorganic complexes are able to abstract an H atom from alkanes. Finally, high-oxidation-state late transition-metal oxo complexes snch as the active Fe =0 or Fe -0 species of cytochrome P450 methane mono-oxygenase enzyme, are also able to abstract an H atom from alkanes, which then leads to their hydroxylation (see Chap. 18). It is also possible, in some cases, to remove an H atom from Hj (see Chap. 15) ... 

Copper enzymes are involved in reactions with a large number of other, mostly inorganic substrates. In addition to its role in oxygen and superoxide activation described above, copper is also involved in enzymes that activate methane, nitrite and nitrous oxide. The structure of particulate methane mono-oxygenase from the methanotrophic bacteria Methylococcus capsulatus has been determined at a resolution of 2.8 A. It is a trimer with an a3P33 polypeptide arrangement. Two metal centres, modelled as mononuclear and dinuclear copper, are located in the soluble part of each P-subunit, which resembles CcOx subunit II. A third metal centre, occupied by Zn in the crystal, is located within the membrane. 

Fig. 5. Mechanism of action of dinuclear non-haem iron enzymes utilising ferryl intermediates. Mechanisms for ribonucleotide reductase and methane mono-oxygenase adapted from that of Que [72]. Compound I and compound II define intermediates at the same oxidation state as the equivalent peroxidase intermediate (see Fig. 2). X is an unknown group suggested to bridge between the two iron atoms and form a cation radical. The nature of the electron required for the reduction of ribonucleotide reductase compound II is not clear - it is possible that this intermediate can also oxidise tyrosine [72].

The utilization of a di-iron center for oxidation reactions is also explored in the chapters by Lipscomb et al. (Chapter 19) and by Stahl and Lippard (Chapter 18). In Chapter 19, a fascinating chemical approach to enzyme catalysis is taken in order to elucidate the mode of action of methane mono-oxygenase (MMO). Surprisingly, new structures are revealed for the di-iron active center such as the existence of a stable p-oxo bridged Fe(IV)-Fe(IV) intermediate. In Chapter 18, an elegant paral-... 

This mechanism represents a shortcut in biological mono-oxygenase enzyme systems that use O2 as oxygen atom sources and whose mechanism is much more complex because of the requirement to cleave O2. Non-porphyrinic binuclear methane mono-oxygenase model complexes are also able to activate methane in the same way (see Chap. 19). When S = RH, Groves originally proposed the well-known rebound mechanism in which the Fe =0 species removes an H atom from RH, then transfers OH to produce the alcohol ROH ... 

Since the discovery of an oxo-bridged binuclear iron center in hemerythrin , a similar structural feature has been implicated in proteins such as ribonucleotide reductase , methane mono-oxygenase , ferritin, and the purple acid phosphatases. As noted earlier (Sect. III.B.2.), the presence of an oxo-bridged site in purple, oxidized uteroferrin and a hydroxo-bridged structure in the pink, reduced form of the enzyme is likely. 

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