An oxygenase is the name given to an enzyme that can take O2 and attach it to something. There are two main types of oxygenases, the first is a monooxygenase which oxidizes a substrate with one atom of oxygen. The second type is a dioxygenase which attaches both atoms of oxygen in the O2 to the target.
Methane monooxygenase (MMO) is a metalloenzyme with the capability to oxidize alkanes into primary alcohols. There are two well known forms of MMO known as both the soluble, or sMMO, and the particulate type better known as pMMO. All methanotrophs generate pMMO, but in a copper limited environment sMMO is also formed. The pMMO enzyme mechanisms are still highly debatable whereas the understanding of the metallo-center in sMMO is detailed and replicable. This article will focus on the enzyme characteristics associated with sMMO.
The sMMO enzyme contains a two iron core. Each of the iron cores is arranged in an octahedral six ligand geometry. Each of the iron atoms in the diiron core is similar in symmetry when considering only the immediate ligands The irons are each independently bonded to five oxygens and a nitrogen as can be seen in the first image on Figure 1.
Figure 1: The resting, oxidized, and reduced state of the diiron core. <http://en.wikipedia.org/wiki/File:MMOstates.JPG>
This arrangement yields a non-linear metal complex with a C4 principle axis. The single nitrogen ligand does not allow for perpendicular C2 axes nor a σh plane. The single nitrogen does however allow for 4 σv planes through the ligand bond vectors providing an overall C4v symmetry point group on each iron on the sMMO active site.
The sMMO enzyme as a whole is a three subunit dimmer with approximate dimensions of 60x100x120 Å. The enzyme is non-linear with no principle axis of rotation. There is no discernible mirror plane or center of inversion, and having no overall symmetry it belongs to the C1 symmetry point group.
Studies performed by the Stephen Lippard Research Group at the MIT Laser Research Facility have produced Raman data for the sMMO enzyme intermediate during oxygenation. Raman spectroscopy of intermediates where Fe-O bonds are formed show two distinct peaks occurring around 844 and 885 wave numbers for oxygenation by both 16-O2 and 18-O2. More information can be found at the Lippard Group Website listed.
There are hydrophobic packets near the iron active sites to which methane can be held until oxygenation begins. Firstly, a carboxylate ligand of the iron1 atom becomes reduced which triggers a 1,2 carboxylate shift allowing the oxygen to become a bridge between the two irons in the active site. Iron1 essentially has a CN=5 at this point allowing for the activation of O2. Now that both irons in the active site are oxidized to Fe(IV) they align to a high spin antiferromagnetic configuration.
In the next step, the diiron center reacts with O2 to become a peroxide intermediate as supported by spectroscopy. The peroxide intermediate flexes into a diamond configuration which will allow for the interaction between the activated O2 and the alkane.
Next, a hydrogen is abstracted from the alkane by the active site and a free alkyl-radical is formed. In the final step, the diiron core eliminates a hydroxyl to form an Fe-O-Fe intermediate which then further oxidize to return the catalyst. In this manner the free-alkyl radical and hydroxyl will favorably bond to form free methanol and reset the cycle described as pictures in Figure 2 below.
Methane itself is fairly unreactive. Although incredibly abundant, this straight alkyl chain doesn’t offer many options as far as synthesis. On the other hand, methanol is incredibly reactive, but the conversion of methane into methanol requires large activation energy. The sMMO allows for a much lower energy conversion of methane to methanol and may prove helpful in increasing energy yields from vast supplies of methane.
Methane on its own is often times used as a fuel. Its combustion as natural gas is used to power machines and provide heat to billions of people worldwide. It is one of the cleanest fuels to burn because it provides the least CO2 emission per unit of heat released when compared to other hydrocarbon fuels. It is also used industrially to convert water into hydrogen gas at high temperatures using a nickel catalyst.
Methanol is most often used as a chemical to make other chemicals. It is a fantastic solvent and much of it is converted to formaldehyde for use in other synthetic processes. Methanol gained massive popularity in the 1970’s because of its use in the Mobil process to make gasoline for automobiles. In the 1990’s huge quantities of ethanol were used to make Methyl-tert-butyl-ether (MTBE) as an additive for gasoline which promised to stabilize fuels and provide effectively higher octane ratingsl. That program was abandoned once it was discovered that MTBE was a environmental health concern.
Natural occurrences of the MMO enzyme are found in bacteria called methanotrophs. Methanotrophs are bacteria that can survive based purely on the metabolism of methane. Using carbon as their only source of energy they can benefit from the release of energy due to the conversion of methane directly to methanol. Every methanotroph must contain methane mononoxygenase, and the gene sequences attributed to the production of the mmo enzyme are found in all of these organisms. This process of oxidizing methane is not only important to the bacterium itself but in occurrences where these methanotrophs are involved in symbiotic environments, they can use oxygen and methane waste to produce the methanol required for other organisms to survive.
- Housecroft, Catherine, and Alan G. Sharpe. Inorganic Chemistry. 3rd ed. Essex: Pearson Education Limited, 2008. Figure 4-10. Print.