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Methanobactin (mb) is a copper-binding chromopeptide that appears to be involved in oxidation of methane by the membrane-associated or particulate methane monooxygenase (pMMO). To examine this potential physiological role, the redox and catalytic properties of mb from three different methanotrophs were examined in the absence and presence of O₂. Metal free mb from the type II methanotroph Methylosinus trichosporium OB3b, but not from the type I methanotrophs Methylococcus capsulatus Bath or Methylomicrobium album BG8, were reduced by a variety of reductants, including NADH and duroquinol, and catalyzed the reduction of O₂ to O₂^-. Copper-containing mb (Cu-mb) from all three methanotrophs showed several interesting properties, including reductase dependent oxidase activity, dismutation of O₂^- to H₂O₂, and the reductant dependent reduction of H₂O₂ to H₂O. The superoxide dismutase …