Enzymes produced by Bacillus selenitireducens use molybdenum to lower the toxicity of dissolved, oxidized forms of arsenic and selenium.

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Oxidation of metals can change their properties. Solid iron metal, for example, becomes reddish and porous when oxidized. Oxidation makes some dissolved metals, such as arsenic and selenium, more toxic. Virtually all life forms, except for a few single-celled organisms, are susceptible to the multifaceted poisonous effects of these oxidized metal ions. A bacterium called Bacillus selenitireducens produces an enzyme capable of reversing the oxidation of these metal ions to their less toxic “reduced” forms. The key ingredient in these enzymes is the “transition” element, molybdenum, which grants them the ability to reduce certain unusual elements.

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“Arsenic and selenium are readily metabolized by prokaryotes, participating in a full range of metabolic functions including assimilation, methylation, detoxification, and anaerobic respiration. Arsenic speciation and mobility is affected by microbes through oxidation/reduction reactions as part of resistance and respiratory processes.” (Stolz et al. 2006:107).

C. [Chrysiogenes] arsenatis Arr [(arsenate reductase)] appears to be specific for arsenate…B. selenitireducens Arr, however, can also reduce arsenite, selenate, and selenite… In both cases, the enzymes were purified as a heterodimer, with a large catalytic subunit (ArrA) and a smaller electron transfer protein (ArrB). Metals analysis indicated the presence of iron and molybdenum.” (Stolz et al. 2006:115).

“The [Arr] group forms a cohesive clade in the DMSO reductase family of molybdenum enzymes…Arsenite oxidase (Aox) is also a member of the DMSO reductase family of molybdenum enzymes.” (Stolz et al. 2006:117)

Journal article
Arsenic and selenium in microbial metabolismJohn F. Stolz, Partha Basu, Joanne M. Santini, Ronald S. Oremland

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