Mercury methylation

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Mercury methylation is the process of forming methylmercury (MeHg). The methylation of mercury can occur abiotically or biotically. Biotically, the primary methylators of mercury are sulfate-reducing and iron-reducing bacteria. [1] Three mechanisms have been proposed for the biotic methylation of mercury by sulfate-reducing bacteria. [2] Mercury methylation can be problematic as methylmercury is toxic and can be bio-magnified through the food web. [2]

Contents

Chemistry

Chemical elements on Earth cycle through atmospheric, terrestrial, and aquatic environments in a process called biogeochemical cycling. [3] Mercury goes through its own version of biogeochemical cycling named the mercury cycle where it circulates through the environment and changes between oxidation states: Hg(0), Hg(I), Hg(II). [3] [4] When mercury is present in the environment microbial organisms can uptake the elemental form of mercury. [2] This signals the transcription of the genes hgcA and hgcB are transcribed to synthesize the HgcA and HgcB proteins. [4] These proteins can then start the methylation reaction to form methylmercury. [4]  

Biochemical

Microbial

Species from all three domains of life have been found to play a role in the methylation of mercury. More species have been discovered that genetically are capable of mercury methylation due to the discovery of the hgcAB genes. [5] It is not known if the HgcA and HgcB proteins create a multienzyme complex or work sequentially. It has also been shown that deletion of either gene results in the complete loss of the ability to methylate mercury. [6]

Bacterial species currently known to methylate mercury include the major of Desulfovibrio spp. (i.e. Desulfovibrio desulfuricans). [5] [7] and Geobacter spp. (i.e. Geobacter sulfurreducens ) [5] [7] Other species with the hgcAB genes that suspected to produce MeHg include Bacteroidota , Chloroflexota , and Nitrospirota . [7]

Archaeal species known to methylate mercury include the majority of species of methanogen class Methanomicrobia , however, class Thermoplasmata has been found to carry the hgcAB genes. No other species of methanogens have been found with the ability of mercury methylation. [7]

Reactions

pH influences on mercury methylation can be variable depending on the species that are undergoing the reactions. Some findings demonstrate that an increase in the hydrogen ion concentration resulted in large increases of the Hg(II) uptake, leading to potential impacts on the actual methylation of mercury. [8] Another finding demonstrated that the decrease in pH leads to a shift in the production of methyl mercury species. Specifically, the production of dimethylmercury decreases and the production of monomethylmercury increases, but total remains essentially constant. [2]

Enough adequate studies on the temperature influences on the methylation of mercury have not been published. Mercury methylation reaches maximum activity in the summer [2] but this enhanced methylation may be due to other factors unrelated to temperature. However, it is evident that temperature affects microbial activity which will correspond to an impact on the subsequent biochemical reactions that lead to methylation of mercury.

Similar to the pH effects, different concentrations of available mercury ion lead to different products and complexes of mercury being produced. [9] In addition, the enzymes HgcA and HgcB have a very low Km and will therefore readily bind to the available mercury even at very low concentrations. [9]

Transport into cell

Before mercury can be methylated, it must be transported into the cell through the lipid membrane. Mercury ions are bound by a mercury scavenger protein, MerP. MerP transfers the mercury ion to a cytoplasmic membrane transporter, MerT, then to the active site of mercuric reductase or mercury(II) reductase in the cytoplasm. [2]

Normally mercury would be toxic to the cell, but some microorganisms are resistant to mercury ion due to an inducible mer operon. Translation of the operon results in the synthesis of mercuric reductase. Mercuric reductase will reduce the mercury ion into elemental mercury, which is volatilized from the cell. [2] If mercuric reductase is not employed, methylation of mercury can occur via three identified pathways. [2]

Biochemical pathway

Cultures of sulfate reducing bacteria grown without the presence of sulfate will not methylate mercury. It is a possibility that the respiration of these cells is coupled to mercury methylation. [2]

The Acetyl-CoA pathway for mercury methylation is done by sulfate reducing bacteria and is catalyzed by a corrinoid dependent protein. Through this pathway, the methyl group is proposed to originate from C-3 serine. A transfer of the methyl group from CH3-Tetrahydrofolate to the corrinoid protein requires the genes hgcA and hgcB . [4] The methyl group now on the corrinoid protein will then be transferred to mercury ion. [2] This activity was shown to decrease in aerobic environments, suggesting that the methylation occurs anaerobically. [2]

Acetate Metabolic pathway (methyl-transferase enzymes) is very similar to the acetyl CoA pathway, where methyltransferase enzymes involving tetrahydrofolate intermediates are utilized. [2] [10] It was shown that methylation of mercury was greater by three orders of magnitude in cells that were capable of utilizing acetate. [2]

Methylation of mercury can also occur using a cobalamin dependent methionine synthase . The cobalamin dependent process requires the use of the substrate S-adenosylmethionine, a biological methylating agent. [2] As methionine synthase was used, it is possible that the enzyme that methylates mercury is also able to transfer methyl groups from CH3-Tetrahydrofolate to thiols. [2]

Environmental impact

Animal health

Methyl mercury is a toxic substance to living organisms. The toxicity of methylmercury in humans is due to methyl mercury crossing the blood-brain barrier and causing cell lysis in the central nervous system. The cell damage is irreversible. The half-life of methylmercury in human tissue is 70 days, which allows it ample time to accumulate to toxic levels. Humans are exposed to methyl mercury from the consumption of aquatic species. As mercury bioaccumulates through the food chain, the amount of methyl mercury increases to these toxic levels. [10] [11] [9]

See also

Related Research Articles

Methylation, in the chemical sciences, is the addition of a methyl group on a substrate, or the substitution of an atom by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These terms are commonly used in chemistry, biochemistry, soil science, and biology.

<span class="mw-page-title-main">DNA methylation</span> Biological process

DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis.

<span class="mw-page-title-main">Methylmercury</span> Toxic chemical compound

Methylmercury (sometimes methyl mercury) is an organometallic cation with the formula [CH3Hg]+. It is the simplest organomercury compound. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a bioaccumulative environmental toxicant.

<span class="mw-page-title-main">Sulfate-reducing microorganism</span> Microorganisms that "breathe" sulfates

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO2−
4) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.

<span class="mw-page-title-main">Dimethyl sulfate</span> Chemical compound

Dimethyl sulfate (DMS) is a chemical compound with formula (CH3O)2SO2. As the diester of methanol and sulfuric acid, its formula is often written as (CH3)2SO4 or Me2SO4, where CH3 or Me is methyl. Me2SO4 is mainly used as a methylating agent in organic synthesis.

Histone methylation is a process by which methyl groups are transferred to amino acids of histone proteins that make up nucleosomes, which the DNA double helix wraps around to form chromosomes. Methylation of histones can either increase or decrease transcription of genes, depending on which amino acids in the histones are methylated, and how many methyl groups are attached. Methylation events that weaken chemical attractions between histone tails and DNA increase transcription because they enable the DNA to uncoil from nucleosomes so that transcription factor proteins and RNA polymerase can access the DNA. This process is critical for the regulation of gene expression that allows different cells to express different genes.

<span class="mw-page-title-main">Sulfur assimilation</span> Incorporation of sulfur into living organisms

Sulfur assimilation is the process by which living organisms incorporate sulfur into their biological molecules. In plants, sulfate is absorbed by the roots and then be transported to the chloroplasts by the transipration stream where the sulfur are reduced to sulfide with the help of a series of enzymatic reactions. Furthermore, the reduced sulfur is incorporated into cysteine, an amino acid that is a precursor to many other sulfur-containing compounds. In animals, sulfur assimilation occurs primarily through the diet, as animals cannot produce sulfur-containing compounds directly. Sulfur is incorporated into amino acids such as cysteine and methionine, which are used to build proteins and other important molecules. Besides, With the rapid development of economy, the increase emission of sulfur results in environmental issues, such as acid rain and hydrogen sulfilde.

<span class="mw-page-title-main">Methyltransferase</span> Group of methylating enzymes

Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossmann fold for binding S-Adenosyl methionine (SAM). Class II methyltransferases contain a SET domain, which are exemplified by SET domain histone methyltransferases, and class III methyltransferases, which are membrane associated. Methyltransferases can also be grouped as different types utilizing different substrates in methyl transfer reactions. These types include protein methyltransferases, DNA/RNA methyltransferases, natural product methyltransferases, and non-SAM dependent methyltransferases. SAM is the classical methyl donor for methyltransferases, however, examples of other methyl donors are seen in nature. The general mechanism for methyl transfer is a SN2-like nucleophilic attack where the methionine sulfur serves as the leaving group and the methyl group attached to it acts as the electrophile that transfers the methyl group to the enzyme substrate. SAM is converted to S-Adenosyl homocysteine (SAH) during this process. The breaking of the SAM-methyl bond and the formation of the substrate-methyl bond happen nearly simultaneously. These enzymatic reactions are found in many pathways and are implicated in genetic diseases, cancer, and metabolic diseases. Another type of methyl transfer is the radical S-Adenosyl methionine (SAM) which is the methylation of unactivated carbon atoms in primary metabolites, proteins, lipids, and RNA.

<span class="mw-page-title-main">MTRR (gene)</span> Protein-coding gene in the species Homo sapiens

Methionine synthase reductase, also known as MSR, is an enzyme that in humans is encoded by the MTRR gene.

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.

<i>Desulfovibrio vulgaris</i> Species of bacterium

Desulfovibrio vulgaris is a species of Gram-negative sulfate-reducing bacteria in the Desulfovibrionaceae family. It is also an anaerobic sulfate-reducing bacterium that is an important organism involved in the bioremediation of heavy metals in the environment. Desulfovibrio vulgaris is often used as a model organism for sulfur-reducing bacteria and was the first of such bacteria to have its genome sequenced. It is ubiquitous in nature and has also been implicated in a variety of human bacterial infections, although it may only be an opportunistic pathogen. This microbe also has the ability to endure high salinity environments, which is done through the utilization of osmoprotectants and efflux systems.

Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element in Earth's crust, and although many arsenic compounds are often considered highly toxic to most life, a wide variety of organoarsenic compounds are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organisms. This pattern is general for other related elements, including selenium, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many toxic arsenic compounds are found in some aquifers, potentially affecting many millions of people via biochemical processes.

<span class="mw-page-title-main">Mercury transporter</span>

The mercury transporter superfamily is a family of transmembrane bacterial transporters of mercury ions. The common origin of all Mer superfamily members has been established. The common elements between family members are included in TMSs 1-2. A representative list of the subfamilies and proteins that belong to those subfamilies is available in the Transporter Classification Database.

(Methyl-Co methylamine-specific corrinoid protein):coenzyme M methyltransferase is an enzyme with systematic name methylated monomethylamine-specific corrinoid protein:coenzyme M methyltransferase. This enzyme catalyses the following chemical reaction

5-methyltetrahydrofolate:corrinoid/iron-sulfur protein Co-methyltransferase is an enzyme with systematic name 5-methyltetrahydrofolate:corrinoid/iron-sulfur protein methyltransferase. This enzyme catalyses the following chemical reaction

Syntrophobacter wolinii is a non-motile, gram-negative and rod-shaped species of bacteria that was originally isolated from a wastewater digester. This species is able to perform propionate degradation and sulfate reduction. S. wolinii can be grown in co-culture or pure culture. 16s rRNA analysis shows its close relation to other sulfate reducers.

Arsenate-reducing bacteria are bacteria which reduce arsenates. Arsenate-reducing bacteria are ubiquitous in arsenic-contaminated groundwater (aqueous environment). Arsenates are salts or esters of arsenic acid (H3AsO4), consisting of the ion AsO43−. They are moderate oxidizers that can be reduced to arsenites and to arsine. Arsenate can serve as a respiratory electron acceptor for oxidation of organic substrates and H2S or H2. Arsenates occur naturally in minerals such as adamite, alarsite, legrandite, and erythrite, and as hydrated or anhydrous arsenates. Arsenates are similar to phosphates since arsenic (As) and phosphorus (P) occur in group 15 (or VA) of the periodic table. Unlike phosphates, arsenates are not readily lost from minerals due to weathering. They are the predominant form of inorganic arsenic in aqueous aerobic environments. On the other hand, arsenite is more common in anaerobic environments, more mobile, and more toxic than arsenate. Arsenite is 25–60 times more toxic and more mobile than arsenate under most environmental conditions. Arsenate can lead to poisoning, since it can replace inorganic phosphate in the glyceraldehyde-3-phosphate --> 1,3-biphosphoglycerate step of glycolysis, producing 1-arseno-3-phosphoglycerate instead. Although glycolysis continues, 1 ATP molecule is lost. Thus, arsenate is toxic due to its ability to uncouple glycolysis. Arsenate can also inhibit pyruvate conversion into acetyl-CoA, thereby blocking the TCA cycle, resulting in additional loss of ATP.

<span class="mw-page-title-main">Mercury(II) reductase</span>

Mercury(II) reductase (EC 1.16.1.1), commonly known as MerA, is an oxidoreductase enzyme and flavoprotein that catalyzes the reduction of Hg2+ to Hg0. Mercury(II) reductase is found in the cytoplasm of many eubacteria in both aerobic and anaerobic environments and serves to convert toxic mercury ions into relatively inert elemental mercury.

<span class="mw-page-title-main">Mercury pollution in the ocean</span> Mercury contamination in sea and sediments

Mercury is a heavy metal that cycles through the atmosphere, water, and soil in various forms to different parts of the world. Due to this natural mercury cycle, irrespective of which part of the world releases mercury it could affect an entirely different part of the world making mercury pollution a global concern. Mercury pollution is now identified as a global problem and awareness has been raised on an international action plan to minimize anthropogenic mercury emissions and clean up mercury pollution. The 2002 Global Mercury Assessment concluded that “International actions to address the global mercury problem should not be delayed”. Among many environments that are under the impact of mercury pollution, the ocean is one which cannot be neglected as it has the ability to act as a “storage closet” for mercury. According to a recent model study the total anthropogenic mercury released into the ocean is estimated to be around 80,000 to 45,000 metric tons and two-thirds of this amount is estimated to be found in waters shallower than 1000m level where much consumable fish live. Mercury can get bio-accumulated in marine food chains in the form of highly toxic methyl mercury which can cause health risks to human seafood consumers. According to statistics, about 66% of global fish consumption comes from the ocean. Therefore, it is important to monitor and regulate oceanic mercury levels to prevent more and more mercury from reaching the human population through seafood consumption.

Desulfovibrio desulfuricans is a Gram-negative sulfate-reducing bacteria. It is generally found in soil, water, and the stools of animals, although in rare cases it has been found to cause infection in humans. It is particularly noted for its ability to produce methyl mercury. The reductive glycine pathway, a seventh route for organisms to capture CO2, was discovered in this species. Since these bacteria are killed by exposure to atmospheric oxygen, the environmental niches most frequently occupied by these bacteria are anaerobic. Desulfovibrio desulfuricans 27774 was reported to produce gene transfer agents.

References

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