Methane monoxygenase

In a shallow aquifer, the oxidation of TCE and other chloroalkenes was compared under conditions of phenol or methane amendments (Hopkins, Semprini McCarty, 1993). The objective was to stimulate the growth and metabolism of bacteria comparable to P. cepacia G4 and M. trichosporium OB3b, respectively. It is possible, however, that the methane-stimulated bacteria expressed particulate (membrane-bound) methane monoxygenase, which is known to oxidize TCE much more slowly than sMMO (DiSpirito et al., 1992). Under the conditions used, the phenol stimulation led to greater removal of TCE and dichloroethylenes. Ammonia addition, thought to stimulate ammonia-oxidizing bacteria that oxidize TCE (Vanelli et al., 1990), showed the lowest extent of TCE oxidation in groundwater microcosm tests (Hopkins et al., 1993). 

One particular feature of this mechanism is that the two metals in the active site can provide three electrons, which means that a fourth electron has to come from a different source, in this case a redox active tyrosine residue. A different solution to the same problem has been found in the enzyme methane monoxygenase, which involves a binuclear iron center and will be discussed subsequently. 

Electrophilic C-H activations can also be effected in water. At first glance, water would appear to be particularly unpromising as a solvent for such reactions. Because of their extremely poor coordinating ability (no fully characterized alkane complex is known [33]) alkanes should not be able to compete with water for coordination sites. Moreover, the intermediate metal-alkyl species would be prone to hydrolytic decomposition. In one respect, however, water is almost an ideal medium for C-H functionalization the O-H bond energy exceeds the corresponding C-H bond energy of even methane. Indeed, the selective oxidation of methane to methanol is carried out by methane monoxygenase in aqueous medium [17]. 

Rates of Fe binding/ oxidation by recombinant H and L ferritins differ over a 1000-fold. The L type of ferritin protein forms polynuclear complexes, as soon as the iron is oxidized, that are indistinguishable from the mineral (B. H. Huynh and E. C. Theil, unpublished results), whereas the H type ferritin proteins form a series of ferric intermediates that include a diferric-peroxo as the first product. The di-ferric peroxo species is similar to complexes that form in methane monoxygenase and ribonucleotide reductase (see Chapter 16). Thus, the Fe - - O2 inorganic chemistry... 

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