Evidence for the presence of Mn(III) intermediates in the bacterial oxidation of Mn(II)

Abstract

Bacterial oxidation of Mn(II) to Mn(IV) is believed to drive the oxidative segment of the global biogeochemical Mn cycle and regulates the concentration of dissolved Mn(II) in the oceanic water column, where it is a critical nutrient for planktonic primary productivity. Mn(II) oxidizing activity is expressed by numerous phylogenetically diverse bacteria and fungi, suggesting that it plays a fundamental and ubiquitous role in the environment. This important redox system is believed to be driven by an enzyme or enzyme complex involving a multicopper oxidase, although the biochemical mechanism has never been conclusively demonstrated. Here, we show that Mn(II) oxidation by spores of the marine Bacillus sp. strain SG-1 is a result of two sequential one-step electron transfer processes, both requiring the putative multicopper oxidase, MnxG, in which Mn(III) is a transient intermediate. A kinetic model of the oxidation pathway is presented, which shows that the Mn(II) to Mn(III) step is the rate-limiting step. Thus, oxidation of Mn(II) appears to involve a unique multicopper oxidase system capable of the overall two-electron oxidation of its substrate. This enzyme system may serve as a source for environmental Mn(III), a strong oxidant and competitor for siderophorebound Fe(III) in nutrient-limited environments. That metabolically dormant spores catalyze an important biogeochemical process intimately linked to the C, N, Fe, and S cycles requires us to rethink the role of spores in the environment.kinetics ͉ multicopper oxidase ͉ spores ͉ x-ray absorption near-edge spectroscopy

In vitro studies indicate a quinone is involved in bacterial Mn(II) oxidation

“…The Km (204 µM) measured with cell-free extract is similar to that found for Bacillus sp. SG-1 exosporium in the presence of pyrophosphate (196 µM) (Webb et al 2005), but much higher than that determined for other Mn(II)-oxidizing proteins from vegetative cells: 7 µM with Leptothrix discophora SS-1 (Adams and Ghiorse 1987), 10 µM with Pseudomonas putida GB-1 (Okazaki et al 1997), 26 µM with Pedomicrobium sp. ACM 3067 (Larsen et al 1999).…”

“…The absorbance of the Mn (III)-pyrophosphate complex did not decrease with time ( Figure 1) as was demonstrated with Bacillus sp. SG-1 (Webb et al 2005) even when the experiment was extended to over 30 hours. In addition, when cells are grown on agar plates, Mn oxides are not confined to the colonies, but are present in a halo around the colony consistent with the production of a diffusible oxidized species such as complexed Mn(III).…”

“…Mn(III)-pyrophosphate trapping experiments were executed as described in Webb, et al (Webb et al 2005). Mn(II) oxidation by cell-free extracts was performed as described above except in a 5 ml stirred assay with 0.25 mg cell-free extract.…”

“…Mn(III) could be stabilized by chelating with organic acids in the cell-free extract. Using the pyrophosphate trapping protocol (Webb et al 2005), Mn(III)-pyrophosphate was observed to form in the Mn(II) oxidase assay, further suggesting that Mn (III) is an intermediate in the oxidation of Mn(II) to Mn(IV) oxides. The absorbance of the Mn (III)-pyrophosphate complex did not decrease with time ( Figure 1) as was demonstrated with Bacillus sp.…”

Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ

“…However, laboratory-produced bacteriogenic Mn oxides are reactive and may transform into abiotic secondary Mn oxides in the presence of the common solutes (e.g., Bargar et al, 2005;Webb et al, 2005a). The assertion that ''primary" biogenic Mn oxides dominate Mn oxide chemistry in aquatic environments thus remains open to testing.…”