Enzymes Reduce Toxicity of Arsenic and Selenium

a metal rode has green and orange oxidation against a blurry background

Biological Strategy

Bacteria

AskNature Team

Image: meineresterampe / Pixabay / Free non-commercial use

Chemically Generate Flow of Electrons (Redox)

Electron flows, or electron transport, is a critical step in numerous biochemical pathways. For example, the flow of electrons along the electron transport chain between membrane-bound proteins of mitochondria during cellular respiration or the chloroplast during photosynthesis triggers a flow of protons across the same membrane. This flow provides the energy for cellular processes similar to the flow of water across a hydroelectric dam.

Search this Function

Modify Oxidation State

Oxidation state refers to the degree to which an element has gained or lost electrons–it can have a significant impact on the characteristics of that element. For example, hexavalent chromium with an oxidation number of +6, is a known carcinogen whereas the less oxidized (i.e., “reduced”) trivalent form, with an oxidation number of +3, is less toxic. The bacterium, Acidiphilium cryptum, can enzymatically convert chromium from the hexavalent to the trivalent form.

Search this Function

Modify Electron Transport

Electron transport is essential for helping organisms to produce energy and survive. Several species can modify the movement of electrons in different ways, allowing them to survive in a variety of conditions.

Search this Function

Bacteria

Domain Bacteria (“small staff”): L. acidophilus, Staphylococcus

From inside of our own bodies to the deepest parts of the ocean, bacteria have adapted to almost every possible environment. All bacteria are unicellular, microscopic, and lack a membrane-bound nucleus and organelles, and come in three basic shapes: spherical (cocci), rod (bacilli), and spiral (spirilla). Some strains can be deadly, like Staphylococcus aureus (MRSA) while others, like Lactobacillus acidophilus, found in our intestinal microbiome, keep us healthy. Cyanobacteria, also known as blue green algae, caused the Great Oxidation Event 2.4 billion years ago, producing Earth’s first oxygenated atmosphere.

Search this Living System

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

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.

Last Updated October 23, 2016

References

“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 metabolism

John F. Stolz, Partha Basu, Joanne M. Santini, Ronald S. Oremland

embedly preview

AskNature