Global cycling of environmental manganese requires catalysis by bacteria and fungi for MnO(2) formation, since abiotic Mn(II) oxidation is slow under ambient conditions. Genetic evidence from several bacteria indicates that multicopper oxidases (MCOs) are required for MnO(2) formation. However, MCOs catalyze one-electron oxidations, whereas the conversion of Mn(II) to MnO(2) is a two-electron process. Trapping experiments with pyrophosphate (PP), a Mn(III) chelator, have demonstrated that Mn(III) is an intermediate in Mn(II) oxidation when mediated by exosporium from the Mn-oxidizing bacterium Bacillus SG-1. The reaction of Mn(II) depends on O(2) and is inhibited by azide, consistent with MCO catalysis. We show that the subsequent conversion of Mn(III) to MnO(2) also depends on O(2) and is inhibited by azide. Thus, both oxidation steps appear to be MCO-mediated, likely by the same enzyme, which is indicated by genetic evidence to be the MnxG gene product. We propose a model of how the manganese oxidase active site may be organized to couple successive electron transfers to the formation of polynuclear Mn(IV) complexes as precursors to MnO(2) formation.
