Abstract
Bacterial manganese (Mn) oxidation is catalyzed by a diverse group of microbes and can affect the fate of other elements in the environment. Yet, we understand little about the enzymes that catalyze this reaction. The Mn oxidizing protein MopA, from Erythrobacter sp. strain SD-21, is a heme peroxidase capable of Mn(II) oxidation. Unlike Mn oxidizing multicopper oxidase enzymes, an understanding of MopA is very limited. Sequence analysis indicates that MopA contains an N-terminal heme peroxidase domain and a C-terminal calcium binding domain. Heterologous expression and nickel affinity chromatography purification of the N-terminal peroxidase domain (MopA-hp) from Erythrobacter sp. strain SD-21 led to partial purification. MopA-hp is a heme binding protein that requires heme, NAD+, and calcium (Ca2+) for activity. Mn oxidation is also stimulated by the presence of pyrroloquinoline quinone. MopA-hp has a KM for Mn(II) of 154 ± 46 μM and kcat = 1.6 min−1. Although oxygen requiring MopA-hp is homologous to peroxidases based on sequence, addition of hydrogen peroxide and hydrogen peroxide scavengers had little effect on Mn oxidation, suggesting this is not the oxidizing agent. These studies provide insight into the mechanism by which MopA oxidizes Mn.
Highlights
Manganese (Mn) oxidizing bacteria and fungi catalyze the oxidation of Mn(II) to Mn(III, IV) leading to the formation of Mn(III, IV) oxides, which can affect the fate of Mn, iron, other metals, carbon, and sulfur due to the adsorptive and reactive properties of the Mn(III, IV) oxides (Tebo et al, 2004, 2005; Miyata et al, 2007; Gao et al, 2015; Butterfield et al, 2016)
SD21 consists of an N-terminal heme peroxidase domain and a C-terminal calcium-binding domain based on sequence analysis
NAD+, and calcium were required for activity and pyrroloquinoline quinone (PQQ) stimulated activity (Table 2 and described below)
Summary
Manganese (Mn) oxidizing bacteria and fungi catalyze the oxidation of Mn(II) to Mn(III, IV) leading to the formation of Mn(III, IV) oxides, which can affect the fate of Mn, iron, other metals, carbon, and sulfur due to the adsorptive and reactive properties of the Mn(III, IV) oxides (Tebo et al, 2004, 2005; Miyata et al, 2007; Gao et al, 2015; Butterfield et al, 2016). The importance of Mn oxidizing bacteria and fungi are well-appreciated, as they can significantly increase the rate of Mn oxidation over abiotic processes (Tebo et al, 2004; Morgan, 2005). The diverse molecular mechanisms of Mn oxidation, especially in bacteria, remain to be resolved. Bacteria and fungi use both direct and indirect means to oxidize Mn. Fungi oxidize Mn as part of lignin degradation using both Mn peroxidases and multicopper oxidases (Makela et al, 2016). Multicopper oxidases and superoxide production are involved in the accumulation of Mn oxides by fungi (Miyata et al, 2004; Hansel et al, 2012; Tang et al, 2013). Direct Mn-oxidation in bacteria occurs by two different enzymes – multicopper oxidases and peroxidase cyclooxygenases
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