pMMO

pMMO, or particulate methane monooxygenase, is a copper-containing enzyme complex that catalyzes the first and rate-limiting step of aerobic methane oxidation in methanotrophic bacteria. It is membrane-associated, localized in intracytoplasmic membranes, and formed by the pmoCAB gene cluster encoding the PmoA, PmoB, and PmoC subunits that assemble into a functional oligomer, typically a trimer of heterotrimers.

The enzyme converts methane to methanol using molecular oxygen, with electrons supplied from the electron transport chain. The overall stoichiometry is CH4 + O2 + 2 e− + 2 H+ → CH3OH + H2O. Methanol is subsequently oxidized by methanol dehydrogenase and carbon is assimilated via formaldehyde pathways.

pMMO is copper-dependent; the precise active site arrangement has been the subject of ongoing investigation. Structural and spectroscopic data support a copper-containing center within the PmoB subunit, and many models invoke a dicopper or more elaborate multi-copper cluster bridged by histidines. The enzyme functions in the membrane environment, and oxygen activation occurs at this site to form a reactive species capable of hydroxylating methane.

Methanotrophic bacteria that express pMMO include species such as Methylococcus capsulatus, Methylosinus trichosporium, and Methylocystis species. Expression is Copper-responsive: high copper favors pMMO expression and repression of soluble MMO (sMMO); copper limitation favors sMMO.

Understanding pMMO has implications for methane mitigation and biocatalysis. Challenges include the membrane-bound nature of the enzyme, which has historically hindered detailed structural characterization, though advances in cryo-electron microscopy and spectroscopy continue to illuminate its mechanism.