Methane monooxygenase catalytic activity

Metalloenzymes with non-heme di-iron centers in which the two irons are bridged by an oxide (or a hydroxide) and carboxylate ligands (glutamate or aspartate) constitute an important class of enzymes. Two of these enzymes, methane monooxygenase (MMO) and ribonucleotide reductase (RNR) have very similar di-iron active sites, located in the subunits MMOH and R2 respectively. Despite their structural similarity, these metal centers catalyze very different chemical reactions. We have studied the enzymatic mechanisms of these enzymes to understand what determines their catalytic activity [24, 25, 39-41]. 

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-... 

Shteinman, A. A., 1995, The mechanism of methane and dioxygen activation in the catalytic cycle of methane monooxygenase, FEES Lett. 362 5n9. 

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). ... 

Methane is oxidized under aerobic conditions by a group of bacteria called methanotrophs. These widespread bacteria play an important role in the global cycling of methane. Two types of methane oxidation systems are known, a ubiquitous particulate methane monooxygenase (pMMO) and a cytoplasmic soluble methane monooxygenase (sMMO) found in only a few strains. These enzymes have different catalytic characteristics, and so it is important to know the conditions under which each is expressed. In those strains containing both sMMO and pMMO, the available copper concentration controls which enzyme is expressed. However, the activity of the pMMO is also affected by copper. Data on methane oxidation in natural samples suggest that methanotrophs are not copper-limited in nature and express the pMMO predominantly. 

Various iron salts and mononuclear Fe or binuclear Fe complexes with a N,0 environment, biomimetic to methane monooxygenase complexes, have been applied to the oxidation of cyclohexane with various oxidants [6u,v,7a-g], but their catalytic activity is usually modest, with the exception of a hexanuclear Fe(III) compound derived from p-nitrobenzoic acid, which gives the highest total yield to Ol/One of about 30% [7a]. Moreover, most of these complexes are often unstable and very expensive. A hexanuclear heterotrimetallic Fe/Cu/Co complex bearing two Cu(p-0)2Co(p-0)2Fe cores, prepared by self-assembly, oxidizes cyclohexane with aqueous HP, with a maximum yield to Ol/One of 45%, virtually total selectivity to the two compounds, and preferred formation of cyclohexanol [7hj. The remarkable activity of the Fe/Cu/Co cluster was associated with the synergic effect of the three metals. 

A report has highlighted the delicate interplay between Fe "Fe distances in such systems and the chemistry that results with the point made that this distance has been found to be particularly short at 2.53 A in the peroxo intermediate of ferroxidase activity as deduced from X-ray absorption studies.Although other techniques have been applied and compared to support this result, caution in placing too much reliance on metrical data from EXAFS should be exercised. Nonetheless, the hypothesis that this short distance and the strain that is introduced into the system might account for the fact that in ferroxidase activity the peroxo intermediate is a substrate intermediate rather than a catalytic cofactor as is the case in the related diferric enzymes such as methane monooxygenase. 

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