MT was discovered in 1957 by Vallee and Margoshe from purification of a Cd-binding protein from horse (equine) renal cortex.[3] MT plays a role in the protection against metal toxicity and oxidative stress, and is involved in zinc and copper regulation.[4] There are four main isoforms expressed in humans (family 1, see chart below): MT1 (subtypes A, B, E, F, G, H, L, M, X), MT2, MT3, and MT4. In the human body, large quantities are synthesised primarily in the liver and kidneys. Their production is dependent on availability of the dietary minerals such as zinc, copper, and selenium, as well as the amino acids histidine and cysteine.
Metallothioneins are rich in thiols, causing them to bind a number of trace metals. Metallothionein is one of the few eukaryotic proteins playing a substantial role in metal detoxification. Zinc and Cadmium are tetrahedrally coordinated to cysteine residues, and each metallothionein protein molecule may bind up to 7 atoms of Zn or Cd.[5] The biosynthesis of metallothionein appears to increase several-fold during periods of oxidative stress to shield the cells against cytotoxicity and DNA damage. Metallothionein biosynthesis can also be induced by certain hormones, pharmaceuticals, alcohols, and other compounds.[6] Metallothionein expression is upregulated during fetal development, particularly in liver tissue.[7]
The MTs from this diverse taxonomic range represent a high-heterogeneity sequence (regarding molecular weight and number and distribution of Cys residues) and do not show general homology; in spite of this, homology is found inside some taxonomic groups (such as vertebrate MTs).
From their primary structure, MTs have been classified by different methods. The first one dates from 1987, when Fowler et al., established three classes of MTs: Class I, including the MTs which show homology with horse MT, Class II, including the rest of the MTs with no homology with horse MT, and Class III, which includes phytochelatins, Cys-rich enzymatically synthesised peptides. The second classification was performed by Binz and Kagi in 2001, and takes into account taxonomic parameters and the patterns of distribution of Cys residues along the MT sequence. It results in a classification of 15 families for proteinaceous MTs. Family 15 contains the plant MTs, which in 2002 have been further classified by Cobbet and Goldsbrough into 4 Types (1, 2, 3 and 4) depending on the distribution of their Cys residues and a Cys-devoid regions (called spacers) characteristic of plant MTs.
A table including the principal aspects of the two latter classifications is included.
More data on this classification are discoverable at the Expasy metallothionein page.[8]
Secondary structure elements have been observed in several MTs SmtA from Syneccochoccus, mammalian MT3, Echinoderma SpMTA, fish Notothenia coriiceps MT, Crustacean MTH, but until this moment, the content of such structures is considered to be poor in MTs, and its functional influence is not considered.
Tertiary structure of MTs is also highly heterogeneous. While vertebrate, echinoderm and crustacean MTs show a bidominial structure with divalent metals as Zn(II) or Cd(II) (the protein is folded so as to bind metals in two functionally independent domains, with a metallic cluster each), yeast and prokaryotic MTs show a monodominial structure (one domain with a single metallic cluster). In yeast, the first 40 residues in the protein wrap around the metal by forming two large parallel loops separated by a deep cleft containing the metal cluster.[9] Although no structural data is available for molluscan, nematoda and Drosophila MTs, it is commonly assumed that the former are bidominial and the latter monodominial. No conclusive data are available for Plant MTs, but two possible structures have been proposed: 1) a bidominial structure similar to that of vertebrate MTs; 2) a codominial structure, in which two Cys-rich domains interact to form a single metallic cluster.
Quaternary structure has not been broadly considered for MTs. Dimerization and oligomerization processes have been observed and attributed to several molecular mechanisms, including intermolecular disulfide formation, bridging through metals bound by either Cys or His residues on different MTs, or inorganic phosphate-mediated interactions. Dimeric and polymeric MTs have been shown to acquire novel properties upon metal detoxification, but the physiological significance of these processes has been demonstrated only in the case of prokaryotic Synechococcus SmtA. The MT dimer produced by this organism forms structures similar to zinc fingers and has Zn-regulatory activity.
Metallothioneins have diverse metal-binding preferences, which have been associated with functional specificity. As an example, the mammalian Mus musculus MT1 preferentially binds divalent metal ions (Zn(II), Cd(II),...), while yeast CUP1 is selective for monovalent metal ions (Cu(I), Ag(I),...). Strictly metal-selective MTs with metal-specific physiological functions were discovered by Dallinger et al. (1997) in pulmonate snails (Gastropoda, Mollusca).[10] The Roman snail (Helix pomatia), for example, possesses a Cd-selective (CdMT) and a Cu-selective isoform (CuMT) involved in Cd detoxification and Cu regulation, respectively.[10] While both isoforms contain unvaried numbers and positions of Cys residues responsible for metal ligation, metal selectivity is apparently achieved by sequence modulation of amino acid residues not directly involved in metal binding (Palacios et al. 2011).[10][11]
A novel functional classification of MTs as Zn- or Cu-thioneins is currently being developed based on these functional preferences.
Metallothionein has been documented to bind a wide range of metals including cadmium,[13] lead,[14] zinc, mercury, copper, arsenic, silver, etc. Metalation of MT was previously reported to occur cooperatively [citation needed] but recent reports have provided strong evidence that metal-binding occurs via a sequential, noncooperative mechanism.[15] The observation of partially metalated MT (that is, having some free metal binding capacity) suggest that these species are biologically important.
Metallothioneins likely participate in the uptake, transport, and regulation of zinc in biological systems. Mammalian MT binds three Zn(II) ions in its beta domain and four in the alpha domain. Cysteine is a sulfur-containing amino acid, hence the name "-thionein". However, the participation of inorganic sulfide and chloride ions has been proposed for some MT forms. In some MTs, mostly bacterial, histidine participates in zinc binding. By binding and releasing zinc, metallothioneins (MTs) may regulate zinc levels within the body. Zinc, in turn, is a key element for the activation and binding of certain transcription factors through its participation in the zinc finger region of the protein.[16][17] Metallothionein also carries zinc ions (signals) from one part of the cell to another. When zinc enters a cell, it can be picked up by thionein (which thus becomes "metallothionein") and carried to another part of the cell where it is released to another organelle or protein.[18] In this way thionein and metallothionein becomes a key component of the zinc signaling system in cells. This system is particularly important in the brain, where zinc signaling is prominent both between and within nerve cells. It also seems to be important for the regulation of the tumor suppressor protein p53.
Control of oxidative stress
Cysteine residues from MTs can capture harmful oxidant radicals like the superoxide and hydroxyl radicals.[19] In this reaction, cysteine is oxidized to cystine, and the metal ions which were bound to cysteine are liberated to the media. As explained in the Expression and regulation section, this Zn can activate the synthesis of more MTs. This mechanism has been proposed to be an important mechanism in the control of the oxidative stress by MTs. The role of MTs in reducing oxidative stress has been confirmed by MT Knockout mutants, but some experiments propose also a prooxidant role for MTs.[citation needed]
In mammalian cells, spontaneous mutagenesis is caused to a large extent by oxidative DNA damage, and the occurrence of such damage can be blocked by metallothionein.[20]
Metallothionein also plays a role in hematopoietic cell differentiation and proliferation, as well as prevention of apoptosis of early differentiated cells. Induced MT levels were adversely associated with sensitivity to etoposide-induced apoptosis, signifying that MT is a potential negative controller of apoptosis.[21]
Expression and regulation
Metallothionein gene expression is induced by a high variety of stimuli, as metal exposure, oxidative stress, glucocorticoids, Vitamin D, hydric stress, fasting, exercise, etc. Beta-hydroxylbutyration of histone proteins upregulates MT2.[22] The level of the response to these inducers depends on the MT gene. MT genes present in their promoters specific sequences for the regulation of the expression, elements as metal response elements (MRE), glucocorticoid response elements (GRE), GC-rich boxes, basal level elements (BLE), and thyroid response elements (TRE).[23][24]
Metallothionein and disease
Cancer
Because MTs play an important role in transcription factor regulation, defects in MT function or expression may lead to malignant transformation of cells and ultimately cancer.[25] Studies have found increased expression of MTs in some cancers of the breast, colon, kidney, liver, skin (melanoma), lung, nasopharynx, ovary, prostate, mouth, salivary gland, testes, thyroid and urinary bladder; they have also found lower levels of MT expression in hepatocellular carcinoma and liver adenocarcinoma.[26]
Evidence suggests that greater MT expression may cause resistance to chemotherapy.[27]
Autism
Heavy metal toxicity has been proposed as a hypothetical etiology of autism, and dysfunction of MT synthesis and activity may play a role in this. Many heavy metals, including mercury, lead, and arsenic have been linked to symptoms that resemble the neurological symptoms of autism.[28] However, MT dysfunction has not specifically been linked to autistic spectrum disorders. A 2006 study, investigating children exposed to the vaccine preservative thiomersal, found that levels of MT and antibodies to MT in autistic children did not differ significantly from non-autistic children.[29]
A low zinc to copper ratio has been seen as a biomarker for autism and suggested as an indication that the metallothionein system has been affected.[30]
Further, there is indication that the mother's zinc levels may affect the developing baby's immunological state that may lead to autism and could be again an indication that the metallothionein system has been affected.[31]
Role of Metallothionein in Cardiovascular disease
Metallothionein (MT) is an indirect redox balance regulator which regulates nuclear factor red blood cell 2-related factor 2 (Nrf2) in the body. However, MT plays an important role in the anti-injury protection of the cardiovascular system, mainly in its inhibitory effect on ischemia-reperfusion injury. Also, the MT activation of the Nrf2 mediates intermittent hypoxia (IH) cardiomyopathy protection.[32]
Transgenic mice with a deletion of any Nrf2 gene (Nrf2-KO) are highly susceptible to the cardiovascular effects of intermittent hypoxia (IH) via cardiac oxidative damage, inflammation, fibrosis, and dysfunction.[32]
Moreover, the specific overexpression in cardiomyocytes of Nrf2 (Nrf2-TG) in transgenic mice[KC1] is impervious to cardiac oxidative damage, inflammation, fibrosis, and dysfunction caused by intermittent hypoxia (IH)[KC2] . In response to IH, Nrf2 and its downstream antioxidants are strongly MT-dependent Nrf2 and may [KC3] act as a compensatory response to IH exposure by up-regulating MT (downstream antioxidant target genes) to protect the heart.[32]
Prolonged exposure to IH reduces the binding of Nrf2 factor to the MT promoter gene, thereby inhibiting MT translation and expression. Moreover, a complex PI3K/Akt/GSK3B/Fyn signaling network provides cardio protection against IH when Nrf2 or MT is overexpressed in the heart. By activating the PI3K/Akt/GSK3B/Fyn signaling pathway, MT increaseNrf2 expression and transcriptional function in response to IH exposure. Although not yet proven, these effects suggest that it is possible to activate PI3K/Akt/GSK3B/Fyn dependent signaling pathways through cardiac MT overexpression to prevent chronic IH-induced cardiomyopathy and downregulation of Nrf2.[32]
Therefore, Nrf2 or MT may be a potential treatment to avoid chronic IH-induced cardiomyopathy.
Identifiers
Symbol
Human metallothionein
PDB Code
2FJ4
Classification
Metal Binding Protein
Mechanism of Nrf2 and MT in preventing IH-induced cardiac injury
Superoxide dismutase (SOD, EC 1.15.1.1) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide (O− 2) anion radical into normal molecular oxygen (O2) and hydrogen peroxide (H 2O 2). Superoxide is produced as a by-product of oxygen metabolism and, if not regulated, causes many types of cell damage. Hydrogen peroxide is also damaging and is degraded by other enzymes such as catalase. Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen. One exception is Lactobacillus plantarum and related lactobacilli, which use intracellular manganese to prevent damage from reactive O− 2.
Zinc is a chemical element with the symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic table. In some respects, zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state (+2), and the Zn2+ and Mg2+ ions are of similar size. Zinc is the 24th most abundant element in Earth's crust and has five stable isotopes. The most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. The largest workable lodes are in Australia, Asia, and the United States. Zinc is refined by froth flotation of the ore, roasting, and final extraction using electricity (electrowinning).
Cysteine is a semiessential proteinogenic amino acid with the formula HOOC−CH(−NH2)−CH2−SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but interestingly, both D and L-cysteine are found in nature with D-cysteine having been found in developing brain. Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from kystis "bladder".
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins contain zinc-binding protein domains although there may be up to 3000 human zinc metalloproteins.
A zinc finger is a small protein structural motif that is characterized by the coordination of one or more zinc ions (Zn2+) which stabilizes the fold. It was originally coined to describe the finger-like appearance of a hypothesized structure from the African clawed frog (Xenopus laevis) transcription factor IIIA. However, it has been found to encompass a wide variety of differing protein structures in eukaryotic cells. Xenopus laevis TFIIIA was originally demonstrated to contain zinc and require the metal for function in 1983, the first such reported zinc requirement for a gene regulatory protein followed soon thereafter by the Krüppel factor in Drosophila. It often appears as a metal-binding domain in multi-domain proteins.
Zinc finger inhibitors, or zinc ejectors, are substances or compounds that interact adversely with zinc fingers and cause them to release their zinc from its binding site, disrupting the conformation of the polypeptide chain and rendering the zinc fingers ineffective, thereby preventing them from performing their associated cellular functions. This is typically accomplished through chelation of the zinc binding site. As zinc fingers are known to be involved in m-RNA regulation, reverse transcription, protection of synthesized viral DNA, transcription inhibition, and initial integration processes, prevention of zinc finger function can have drastic effects on the function of the cell or virus.
Copper fist is an N-terminal domain involved in copper-dependent DNA binding. It is named for its resemblance to a fist closed around a penny. Functionally, the "penny" is a collection of copper ions and the "knuckles" of the fist are proteins that interact with the promoter of the metallothionein gene, enhancing its transcription by creating a more stable binding site for RNA polymerase during transcription, an essential step in DNA replication.
Copper proteins are proteins that contain one or more copper ions as prosthetic groups. Copper proteins are found in all forms of air-breathing life. These proteins are usually associated with electron-transfer with or without the involvement of oxygen (O2). Some organisms even use copper proteins to carry oxygen instead of iron proteins. A prominent copper protein in humans is in cytochrome c oxidase (cco). This enzyme cco mediates the controlled combustion that produces ATP. Other copper proteins include some superoxide dismutases used in defense against free radicals, peptidyl-α-monooxygenase for the production of hormones, and tyrosinase, which affects skin pigmentation.
Nitrite reductase refers to any of several classes of enzymes that catalyze the reduction of nitrite. There are two classes of NIR's. A multi haem enzyme reduces NO2− to a variety of products. Copper containing enzymes carry out a single electron transfer to produce nitric oxide.
Nuclear factor erythroid 2-related factor 2 (NRF2), also known as nuclear factor erythroid-derived 2-like 2, is a transcription factor that in humans is encoded by the NFE2L2 gene. NRF2 is a basic leucine zipper (bZIP) protein that may regulate the expression of antioxidant proteins that protect against oxidative damage triggered by injury and inflammation, according to preliminary research. In vitro, NRF2 binds to antioxidant response elements (AREs) in the promoter regions of genes encoding cytoprotective proteins. NRF2 induces the expression of heme oxygenase 1 in vitro leading to an increase in phase II enzymes. NRF2 also inhibits the NLRP3 inflammasome.
In enzymology, a sulfiredoxin is an enzyme that catalyzes the chemical reaction
Metallothionein-1A is a protein that in humans is encoded by the MT1A gene.
Metal regulatory transcription factor 1 is a protein that in humans is encoded by the MTF1 gene.
Zinc compounds are chemical compounds containing the element zinc which is a member of the group 12 of the periodic table. The oxidation state of zinc in most compounds is the group oxidation state of +2. Zinc may be classified as a post-transition main group element with zinc(II). Zinc compounds are noteworthy for their nondescript appearance and behavior: they are generally colorless, do not readily engage in redox reactions, and generally adopt symmetrical structures.
In molecular biology the ZZ-type zinc finger domain is a type of protein domain that was named because of its ability to bind two zinc ions. These domains contain 4-6 Cys residues that participate in zinc binding, including a Cys-X2-Cys motif found in other zinc finger domains. These zinc fingers are thought to be involved in protein-protein interactions. The structure of the ZZ domain shows that it belongs to the family of cross-brace zinc finger motifs that include the PHD, RING, and FYVE domains. ZZ-type zinc finger domains are found in:
Wilson disease protein (WND), also known as ATP7B protein, is a copper-transporting P-type ATPase which is encoded by the ATP7B gene. The ATP7B protein is located in the trans-Golgi network of the liver and brain and balances the copper level in the body by excreting excess copper into bile and plasma. Genetic disorder of the ATP7B gene may cause Wilson's disease, a disease in which copper accumulates in tissues, leading to neurological or psychiatric issues and liver diseases.
The carbonic anhydrases form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid. The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.
Evolution of metal ions in biological systems refers to the incorporation of metallic ions into living organisms and how it has changed over time. Metal ions have been associated with biological systems for billions of years, but only in the last century have scientists began to truly appreciate the scale of their influence. Major and minor metal ions have become aligned with living organisms through the interplay of biogeochemical weathering and metabolic pathways involving the products of that weathering. The associated complexes have evolved over time.
Metal-binding proteins are proteins or protein domains that chelate a metal ion.
Zinc is an essential trace element for humans and other animals, for plants and for microorganisms. Zinc is required for the function of over 300 enzymes and 1000 transcription factors, and is stored and transferred in metallothioneins. It is the second most abundant trace metal in humans after iron and it is the only metal which appears in all enzyme classes.
References
↑ PDB: 2KAK; Peroza EA, Schmucki R, Güntert P, Freisinger E, Zerbe O (March 2009). "The beta(E)-domain of wheat E(c)-1 metallothionein: a metal-binding domain with a distinctive structure". Journal of Molecular Biology. 387 (1): 207–18. doi:10.1016/j.jmb.2009.01.035. PMID19361445.
↑ Sigel H, Sigel A, eds. (2009). Metallothioneins and Related Chelators (Metal Ions in Life Sciences). Vol.5. Cambridge, England: Royal Society of Chemistry. ISBN978-1-84755-899-2.
↑ Margoshes M, Vallee BL (1957). "A cadmium protein from equine kidney cortex". Journal of the American Chemical Society. 79 (17): 4813–4814. doi:10.1021/ja01574a064.
↑ Felizola SJ, Nakamura Y, Arata Y, Ise K, Satoh F, Rainey WE, Midorikawa S, Suzuki S, Sasano H (September 2014). "Metallothionein-3 (MT-3) in the human adrenal cortex and its disorders". Endocrine Pathology. 25 (3): 229–35. doi:10.1007/s12022-013-9280-9. PMID24242700. S2CID39871076.
↑ Wong DL, Merrifield-MacRae ME, Stillma MJ (2017). "Chapter 9. Lead(II) Binding in Metallothioneins". In Astrid S, Helmut S, Sigel RK (eds.). Lead: Its Effects on Environment and Health. Metal Ions in Life Sciences. Vol.17. de Gruyter. pp.241–270. doi:10.1515/9783110434330-009. PMID28731302.
↑ Krezel A, Maret W (September 2007). "Dual nanomolar and picomolar Zn(II) binding properties of metallothionein". Journal of the American Chemical Society. 129 (35): 10911–21. doi:10.1021/ja071979s. PMID17696343.
↑ Kumari MV, Hiramatsu M, Ebadi M (August 1998). "Free radical scavenging actions of metallothionein isoforms I and II". Free Radical Research. 29 (2): 93–101. doi:10.1080/10715769800300111. PMID9790511.
↑ Rossman TG, Goncharova EI. Spontaneous mutagenesis in mammalian cells is caused mainly by oxidative events and can be blocked by antioxidants and metallothionein. Mutat Res. 1998 Jun 18;402(1-2):103-10. doi: 10.1016/s0027-5107(97)00287-x. PMID 9675254
↑ Klaassen CD, Liu J, Choudhuri S (1999). "Metallothionein: an intracellular protein to protect against cadmium toxicity". Annual Review of Pharmacology and Toxicology. 39: 267–94. doi:10.1146/annurev.pharmtox.39.1.267. PMID10331085.
↑ Krizkova S, Fabrik I, Adam V, Hrabeta J, Eckschlager T, Kizek R (2009). "Metallothionein--a promising tool for cancer diagnostics". Bratislavske Lekarske Listy. 110 (2): 93–7. PMID19408840.
↑ Cherian MG, Jayasurya A, Bay BH (December 2003). "Metallothioneins in human tumors and potential roles in carcinogenesis". Mutation Research. 533 (1–2): 201–9. doi:10.1016/j.mrfmmm.2003.07.013. PMID14643421.
↑ Singh VK, Hanson J (June 2006). "Assessment of metallothionein and antibodies to metallothionein in normal and autistic children having exposure to vaccine-derived thimerosal". Pediatric Allergy and Immunology. 17 (4): 291–6. doi:10.1111/j.1399-3038.2005.00348.x. PMID16771783. S2CID2843402.
↑ Faber S, Zinn GM, Kern JC, Kingston HM (May 2009). "The plasma zinc/serum copper ratio as a biomarker in children with autism spectrum disorders". Biomarkers. 14 (3): 171–80. doi:10.1080/13547500902783747. PMID19280374. S2CID205770002.
Cherian MG, Jayasurya A, Bay BH (December 2003). "Metallothioneins in human tumors and potential roles in carcinogenesis". Mutation Research. 533 (1–2): 201–9. doi:10.1016/j.mrfmmm.2003.07.013. PMID14643421.
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