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The nature of the copper ions in the membranes containing the particulate methane monooxygenase from Methylococcus capsulatus (Bath).

It is shown that the particulate methane monooxygenase (pMMO) has an obligate requirement for copper. The MMO activity in the particulate fractions obtained from Methylococcus capsulatus (Bath) cells is found to increase with increasing copper content of the membranes. The enzyme activity from membranes obtained from cells grown at low copper levels can be stimulated further by the addition of Cu(II) ions to the assay medium. The membrane-bound copper ions can exist in both Cu(II) and Cu(I) forms. EPR and magnetic susceptibility characterizations of the membranes indicate the presence of an exchange-coupled trinuclear Cu(II) cluster when the bulk of the copper ions is oxidized. However, the functional form of the enzyme is the reduced or partially reduced form. The copper ions in the membrane fractions as isolated often exhibit a high level of reduction. An EPR spectrum with one unpaired electron spin delocalized over three copper nuclei has been observed for the two-electron reduced trinuclear copper cluster. The high correlation between the copper level in the membranes and enzymatic activity as well as the high reactivity of the reduced copper clusters toward dioxygen strongly indicate that the membrane-bound copper ions constitute the active sites of the pMMO.