Methanotrophic bacteria play an essential role of cycling carbon through anaerobic sediments. The chemistry behind the cycling takes a chemically inert hydrocarbon, methane, and converts it to a more active species,
methanol. Other hydrocarbons are oxidized by MMOs, so a new hydroxylation catalyst based on the understanding of MMO systems could possibly make a more efficient use of the world supply of natural gas. This is a classic monooxygenase reaction in which two reducing equivalents from
NAD(P)H are utilized to split the O-O bond of O2. One atom is reduced to water by a 2 e- reduction and the second is incorporated into the substrate to yield methanol: : CH4 + O2 + NAD(P)H + H+ → CH3OH + NAD(P)+ + H2O Two forms of MMO have been found:
soluble and
particulate. The best characterized forms of soluble MMO contains three protein components: hydroxylase, the β unit, and the reductase. Each of which is necessary for effective substrate hydroxylation and NADH oxidation. Each iron has a six coordinate octahedral environment. The dinuclear iron centers are positioned in the α subunit. Each iron atom is also coordinated to a histidine δN atom, Fe 1 to a His 147 and Fe 2 to His 246, Fe 1 is a ligated to a monodentate carboxylate, Glu 114, a semi bridging carboxylate, Glu 144, and a water molecule. However, the structure is not known. Intermediate P then converts to intermediate Q, which was proposed to contain two antiferromagnetically coupled high-spin FeIV centers. There are two mechanisms suggested for the reaction between compound Q and the alkane: radical and nonradical. The radical mechanism starts with abstraction of the hydrogen atom from the substrate to form QH (the rate determining step), hydroxyl bridged compound Q and the free alkyl radical. The nonradical mechanism implies a concerted pathway, occurring via a four-center
transition state and leading to a “hydrido-alkyl-Q” compound. As of 1999, the research suggests that the methane oxidation proceeds via a bound-radical mechanism. It was suggested that the transition state for the radical mechanism involves a torsion motion of the hydroxyl OH ligand before the methyl radical can add to the bridging hydroxyl ligand to form the alcohol. As the radical approaches, the H atom of the alkane leave the coplanar tricoordinate O environment and bends upward to create a tetrahedral tetracoordinate O environment. The final step for this reaction is the elimination of the alcohol and the regeneration of the catalysts. There are a few ways in which this can occur. It could be a stepwise mechanism that starts with the elimination of the alcohol and an intermediate Fe-O-Fe core, and the latter can eliminate the water and regenerate the enzyme through a 2e- reduction. On the other hand, it can start with a 2e- reduction process of bridging the O1 atom to give a water molecule, followed by elimination of the alcohol and regeneration of the enzyme. In addition, it is possible that there is a concerted mechanism whereby the elimination of the methanol occurs spontaneously with 2e- reduction of the bridging O1 center and regeneration of the catalyst. == See also ==