The role of copper in the pMMO of Methylococcus capsulatus bath: a structural vs. catalytic function

Methanotrophs convert methane to methanol by the methane monooxygenase (MMO). It is well known that two forms of the MMO can be expressed: one form is found in the cytoplasm, or in the soluble fraction (sMMO); the other is associated with the membranes, or particulate fraction (pMMO). The sMMO has been extensively examined, and much is known about its structure and mechanism of dioxygen activation. The pMMO, however, is less well understood: the enzyme has proven difficult to purify as it loses activity once the membranes become solubilized. Furthermore, although copper is known to be important for the stability and activity of the pMMO, its role is still unclear. In a recent study, we reported the use of electron paramagnetic resonance (EPR) spectroscopy to probe the nature of the copper ions in these membranes. Two EPR signals were uncovered for the highly oxidized membranes: one set of signals arises from the type 2 Cu(II) centers, and the other has been assigned to trinuclear copper (Cu(II) clusters (H.-H. Nguyen, et al., J. Biol. Chem., 269, 14995 (1994). Here, we attempt to correlate the EPR spectra of the membrane fraction of Methylococcus capsulatus Bath with the amount of copper present in the membranes and to the activity of the pMMO as measured by the production of propylene oxide from propene by the pMMO. From these studies, we conclude that the primary role of copper is in the active site of the pMMO rather than simply a structural one.