Methane mono-oxygenases

The lower than expected yields can be explained by the nature of methane oxidation to methanol in these bacteria. This reaction, catalysed by methane mono-oxygenase, is a net consumer of reducing equivalents (NADH), which would otherwise be directed to ATP generation and biosynthesis. In simple terms the oxidation of methane to methanol consumes energy, lowering the yield. 

Colby J, DI Stirling, H Dalton (1977) The soluble methane mono-oxygenase of Methylococcus capsulatus (Bath). Its ability to oxygenate -alkanes, -alkenes, ethers, and alicyclic, aromatic and heterocyclic compounds. Biochem J 165 395-401. 

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

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. 

Once the methanotroph-containing CSR has been established, we expect it can cometabolically oxidize some of the TCE incidental to the use of the primary substrate, methane. The rate of TCE biotransformation is dictated by the effectiveness of the methane mono-oxygenase for interacting with TCE rather than methane. This enzymatic processing of TCE can be described using a Michaelis-Menten expression ... 

Basu, P. Katterle, B. Andersson, K. K. Dalton, H. The membrane-associated form of methane mono-oxygenase from Methylococcus capsulatus (Bath) is a copper/iron protein. Biochem. J. 2003, 369(2), 417 127. 

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].

N. Kitajima, H. Fukui, Y. Morooka, A model for methane mono-oxygenase Dioxygen oxidation of alkanes by use of a /e-oxo binuclear iron complex, ]. Chem. Soc. Chem. Commun. (1988) 485. 

The oxygenation of saturated hydrocarbons by methane mono-oxygenases (binuclear nonheme iron proteins, MMO) via the two-electron reduction of... 

The methane mono-oxygenase proteins (MMO, binuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons - ... 

Reaction Mimic for the Methane Mono-oxygenase Proteins Fe DPAH)2/O2lPhNHNHPh/substrate... 

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 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-... 

Particulate methane mono-oxygenase (pMMO) 2,1. 7 Methane -> Methanol ... 

Oxidation. Since some metalloenzymes such as cytochrome P-450 and methane mono-oxygenase can selectively oxidize alkanes with dioxygen, there are many approaches that mimic the reactivity of these metalloenzymes using various artificial metal complexes (1,23,24). The first approach is to combine dioxygen and reducing agents in the presence of metal complexes (eq. (12)). 

Colby, J., Stirling, D. and Dalton, H. (1977). Soluble Methane Mono-Oxygenase of Methylococcus-Capsulatus-(Bath) Ability to Oxygenate Normal-Alkanes, Normal-Alkenes, Ethers, and Alicyclic, Aromatic and Heterocyclic-Compounds, Biochem. J., 165, pp. 395-402. 

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) ... 

More recently, research concerning catalytic oxidation reactions has emerged on one side from bioinorganic chemistry with cytochrome P450 models and non-por-phyrinic methane mono-oxygenase models, and on the other side, from organic chemistry with asymmetric epoxidation and dihydroxylation. 

Thus, as in the case of alkenes in the preceding chapter, we start with the radical type of activation that is much older. Transition-metal compounds play a key role in radical activation, because they provide very strong oxidants that can oxidize hydrocarbons either by (reversible) electron transfer or H-atom transfer (more rarely by hydride transfer). Biological oxidation of hydrocarbons involves reactive metal-0X0 species in methane mono-oxygenases and many related synthetic models, and a number of simple metal-oxo complexes also work. The clear criterion of distinction between an organometallic C-H activation and a radical activation is the above selectivity in activated C-H bonds. 

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