Enzymes Detoxify Mercury Compounds

Biological Strategy

Bacteria

AskNature Team

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Protect From Chemicals

Chemicals are everywhere in the bodies of living organisms and their external environments. While most chemicals are valuable or benign, some are toxic, including those used for defense (such as the mucus that protects clownfish from an anemone’s stinging tentacles). Even naturally-occurring chemicals, such as arsenic, must be managed to reduce their impact. Some living systems have strategies to break down harmful chemicals, alter them into less toxic forms, physically prevent chemicals from harming sensitive tissues, and more. For example, some herbivorous mammals can digest toxic compounds in plants because they have a particular enzyme that helps them process poisonous plant compounds.

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Cleave Heavy Metals From Organic Compounds

Organisms tend to use the chemical resources available in their immediate surroundings. However, when it comes to heavy metal elements, such as cadmium, lead, and mercury, organisms tend to sequester them to avoid toxicity rather than incorporate them into biomolecules. However, when industrial chemical wastes that contain heavy metal-based organic compounds, such as some dyes, are dumped on land, local microorganisms such as fungi can sometimes adapt to their environment by developing strategies to cleave and sequester the heavy metal element.

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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.

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The enzymatic system of aerobic bacteria detoxifies mercury compounds such as methyl-mercury via the enzymes organomercurial lyase (MerB) and mercuric ion reductase.

“Mercury is well known for its toxicity to living organisms. Inorganic mercuric compounds (HgX2) and organomercurials (R-Hg-X), in which the Hg is formally in the +2 oxidation state, are primarily responsible for the toxicity…Elemental mercury itself (Hg0) has little affinity for cellular ligands and is toxic only if it becomes oxidized to the +2 state in the cell…aerobic bacteria have evolved the ingenious strategy of eliminating mercuric and organomercurial compounds from their environment through reduction of Hg2+ to Hg0. To accomplish this, they couple the activity of two enzymes: organomercurial lyase (MerB) and mercuric ion reductase.” (Miller 2007:537)

“Accumulation of extremely toxic methylmercury in the environment—particularly in fish—has triggered an effort by scientists to unravel the process by which a set of bacterial enzymes capture and then detoxify the compound. In a new development, Jonathan G. Melnick and Gerard Parkin of Columbia University report a synthetic mercury complex that provides insight into how one of these enzymes catalyzes cleavage of the Hg-C bond (Science 2007, 317, 225). The finding is expected to boost efforts to genetically modify plants to sequester HgCH3+ for environmental cleanup. In nature, microbes synthesize HgCH3+ from naturally occurring Hg2+, as well as from mercury released in the emissions of coal-fired power plants. Organomercury compounds are toxic because the metal has a high affinity for sulfur, in particular the sulfur of thiol (-SH) groups in cysteine units of proteins. Once the mercury binds, the normal function of the proteins is disrupted. Bacteria resistant to HgCH3+ toxicity produce an enzyme named MerB, which has three cysteine residues in its active site that are known to be crucial for cleaving the Hg-C bond. But the exact way in which MerB coordinates to HgCH3+ and the ‘intimate details of the reaction mechanism’ have been a mystery, Parkin says. (A second enzyme, MerA, reduces the resulting Hg2+ to less toxic elemental mercury.) Melnick and Parkin thus set out to decipher the mechanism of action of MerB. Melnick and Parkin ‘provide an elegant atomic-level description for the facile cleavage of a carbon-mercury bond,’ notes James G. Omichinski of the University of Montreal in a Science commentary. Their observations provide valuable insight into the basic mechanism of MerB’s activity, he adds. Considerable work remains to be done, but understanding this mechanism ‘is essential to efforts to reengineer MerB to improve its catalytic efficiency for the bioremediation of methylmercury,’ Omichinski writes.” (Ritter 2007:10)