Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes (). There are two forms of MMO: the well-studied soluble form (sMMO) and the particulate form (pMMO). The active site in sMMO contains a di-iron center bridged by an oxygen atom (Fe-O-Fe), whereas the active site in pMMO utilizes copper. Structures of both proteins have been determined by X-ray crystallography; however, the location and mechanism of the active site in pMMO is still poorly understood and is an area of active research. The particulate methane monooxygenase and related ammonia monooxygenase are integral membrane proteins, occurring in methanotrophs and ammonia oxidisers, respectively, which are thought to be related. These enzymes have a relatively wide substrate specificity and can catalyse the oxidation of a range of substrates including ammonia, methane, halogenated hydrocarbons, and aromatic molecules. These enzymes are homotrimers composed of 3 subunits - A (), B () and C () and most contain two monocopper centers. The A subunit from Methylococcus capsulatus (Bath) resides primarily within the membrane and consists of 7 transmembrane helices and a beta-hairpin, which interacts with the soluble region of the B subunit. A conserved glutamate residue is thought to contribute to a metal center. Methane monooxygenases are found in methanotrophic bacteria, a class of bacteria that exist at the interface of aerobic (oxygen-containing) and anaerobic (oxygen-devoid) environments. One of the more widely studied bacteria of this type is Methylococcus capsulatus (Bath). This bacterium was discovered in the hot springs of Bath, England. Notably, strictly anaerobic methanotrophs may also harbour methane monooxygenases, although there are critical mismatches in the gene which prevent common methanotroph-seeking primers from matching. Soluble methane monooxygenase systems 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: