Methane monooxygenase

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Methane monooxygenase (MMO) is an enzyme used by certain bacteria to turn methane into methanol, the first step in using methane as food or fuel. It does this by adding one oxygen atom from oxygen gas to methane, with electrons supplied by NADH or NADPH.

MMO comes in two forms:
- Soluble MMO (sMMO): This form is soluble in the cell fluid and works with three protein parts: a hydroxylase (the active site where methane is oxidized), a reductase (which donates electrons), and a small beta subunit. Its active site contains a diiron (Fe-Fe) cluster that activates O2 for the reaction.
- Particulate MMO (pMMO): This form is membrane-bound and uses copper at its active site. It is structurally different from sMMO, and the exact details of how pMMO works are still under active study. pMMO is related to enzymes like ammonia monooxygenase and often forms multi-subunit membrane complexes.

Where MMO is found:
- MMO is produced by methanotrophic bacteria, such as Methylococcus capsulatus (Bath), which live at the edge of oxygen-rich and oxygen-poor environments. Some anaerobic methane-oxidizing organisms may have MMO-like enzymes, but detecting them is more challenging.

How it works (simplified):
- In sMMO, electrons are transferred from NADH to the diiron center. Oxygen binds to the iron cluster and is activated to form a reactive species that can hydroxylate methane, turning it into methanol. The process uses two reducing equivalents and ultimately regenerates the enzyme for another cycle.
- In pMMO, a copper-based active site carries out a similar oxidation, though the exact mechanism and the arrangement of its copper centers are still being researched.

Why MMO matters:
- MMO plays a key role in the global methane cycle, helping bacteria convert methane in the environment into more usable chemicals.
- Understanding MMO could inspire new catalysts for efficiently converting methane and other hydrocarbons into useful products, potentially improving natural gas utilization and pollution remediation.