Particulate Methane Monooxygenase

Particulate methane monooxygenase (pMMO) catalyzes the oxidation of methane to methanol in methanotrophic bacteria. The 300-kDa pMMO enzyme is a trimeric integral membrane protein, comprising three copies each of three subunits: pmoB, pmoA, and pmoC. Purified pMMO contains both copper and iron, although some of the iron may be due to heme contamination. The copper stoichiometry ranges from 2 to 15 copper ions per 100-kDa pMMO protomer, depending on the enzyme source. EPR spectra of whole cells, membrane-bound, and purified pMMO consistently indicate the presence of type 2 Cu(II). According to XAS data, both Cu(I) and Cu(II) are present. EXAFS data reveal the presence of oxygen/nitrogen ligands and a short Cu[BOND]Cu interaction at 2.5 Å that increases to 2.6 Å upon chemical reduction. Crystal structures of pMMO from two organisms have been reported. A soluble region consisting of six cupredoxin-like β-barrel structures is supported by 15 transmembrane helices. Three different metal centers have been detected crystallographically: a highly conserved dinuclear copper center, a nonconserved mononuclear copper center, and a site that can be occupied by zinc or copper. Although the identity of the pMMO active site remains unknown, some information regarding the mechanism has been obtained from substrate studies and computational work.