Particulate methane monooxygenase (pMMO) is an integral membrane protein, found in methanotropic bacteria, that can selectively oxidize methane to produce methanol. This metalloenzyme contains three subunits, and the metal composition and exact location of its active site has been the subject of much speculation. Here it is found that the enzyme's activity is dependent on copper, and that the active site is located in the soluble domains of the pmoB subunit