The copper clusters in the particulate methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath)

The particulate methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath) is a multicopper protein that hydroxylates methane and other small n-alkanes to their corresponding alcohols with high regiospecificity and stereos electivity. The copper ions appear to be arranged into similar to5 trinuclear copper clusters. Two of these clusters are thought to be involved in the dioxygen chemistry and alkane hydroxylation mediated by the enzyme. Accordingly, these catalytic clusters (C-clusters) are typically oxidized, as the protein is isolated. The remaining copper ions are normally reduced, and it has been suggested that they provide a reservoir of reducing equivalents needed for the turnover of the enzyme. In this study, we have oxidized the protein to different levels of oxidation of the copper ions using dioxygen, hydrogen peroxide, ferricyanide, and dioxygen in the presence of the suicide substrate acetylene, and have characterized the oxidized copper centers using low temperature electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy. The results are consistent with the grouping of the copper ions into catalytic clusters and electron transfer clusters (E-clusters). Quantification indicates that there are, indeed, two C-clusters. A model in which the two C-clusters are activated by dioxygen starting from their fully reduced states and mediate the hydroxylation chemistry of methane will be presented and discussed.