metallothionein

MTs are present in a vast range of taxonomic groups, ranging from prokaryotes (such as the cyanobacteria Synechococcus sp.), protozoa (such as the ciliate Tetrahymena genera), plants (such as Pisum sativum, Triticum durum, Zea mays, or Quercus suber), yeast (such as Saccharomyces cerevisiae, Candida albicans, or Neurospora crassa), invertebrates (such as the nematode Caenorhabditis elegans, the insect Drosophila melanogaster, the mollusc Mytilus edulis, or the echinoderm Strongylocentrotus purpuratus) and vertebrates (such as the chicken Gallus gallus, or the mammalian Homo sapiens or Mus musculus).

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.{{cite web | url = https://www.uniprot.org/docs/metallo.txt | title = Metallothioneins: classification and list of entries | publisher = www.uniprot.org }}

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.{{cite journal | vauthors = Peterson CW, Narula SS, Armitage IM | title = 3D solution structure of copper and silver-substituted yeast metallothioneins | journal = FEBS Letters | volume = 379 | issue = 1 | pages = 85–93 | date = January 1996 | pmid = 8566237 | doi = 10.1016/0014-5793(95)01492-6 | doi-access = free }} 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).{{cite journal |vauthors=Dallinger R, Berger B, Hunziker P, Kägi JH | year = 1997 | title = Metallothionein in snail Cd and Cu metabolism | journal = Nature | volume = 388 | issue = 6639| pages = 237–238 | doi = 10.1038/40785 | pmid = 9230430 | bibcode = 1997Natur.388..237D | s2cid = 4404470 | doi-access = free }} 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. 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).{{cite journal | vauthors = Palacios O, Pagani A, Pérez-Rafael S, Egg M, Höckner M, Brandstätter A, Capdevila M, Atrian S, Dallinger R | title = Shaping mechanisms of metal specificity in a family of metazoan metallothioneins: evolutionary differentiation of mollusc metallothioneins | journal = BMC Biology | volume = 9 | issue = 4 | pages = 4 | date = January 2011 | pmid = 21255385 | doi = 10.1186/1741-7007-9-4 | pmc = 3033865 | doi-access = free }}

A novel functional classification of MTs as Zn- or Cu-thioneins is currently being developed based on these functional preferences.

The main biological function of metallothioneins is to maintain homeostasis of the essential metals zinc and copper, but metallothioneins also protect against metal toxicity and oxidative stress.{{cite journal | vauthors=Sekovanić A, Jurasović J, Piasek M | title=Metallothionein 2A gene polymorphisms in relation to diseases and trace element levels in humans | journal=Arhiv Za Higijenu Rada I Toksikologiju | volume=71 | issue=1 | pages=27–47 | year=2020 | doi = 10.2478/aiht-2020-71-3349 | pmc=7837243 | pmid=32597135 }}

Metallothionein has been documented to bind a wide range of metals including cadmium,{{cite book | vauthors = Freisinger E, Vašák M | title = Cadmium: From Toxicity to Essentiality | chapter = Cadmium in Metallothioneins | series = Metal Ions in Life Sciences | volume = 11 | pages = 339–71 | year = 2013 | pmid = 23430778 | doi = 10.1007/978-94-007-5179-8_11 | isbn = 978-94-007-5178-1 }} lead,{{cite book | vauthors = Wong DL, Merrifield-MacRae ME, Stillma MJ | chapter = Chapter 9. Lead(II) Binding in Metallothioneins | pages = 241–270 | publisher = de Gruyter | publication-date = 2017 | series = Metal Ions in Life Sciences | volume = 17 | title = Lead: Its Effects on Environment and Health | year = 2017 | veditors = Astrid S, Helmut S, Sigel RK | doi = 10.1515/9783110434330-009 | pmid = 28731302 }} zinc, mercury, copper, arsenic, silver, etc. Metalation of MT was previously reported to occur cooperatively {{Citation needed|date=November 2021}} but recent reports have provided strong evidence that metal-binding occurs via a sequential, noncooperative mechanism.{{cite journal | vauthors = Krezel A, Maret W | title = Dual nanomolar and picomolar Zn(II) binding properties of metallothionein | journal = Journal of the American Chemical Society | volume = 129 | issue = 35 | pages = 10911–21 | date = September 2007 | pmid = 17696343 | doi = 10.1021/ja071979s }} 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.{{cite journal | vauthors = Huang M, Krepkiy D, Hu W, Petering DH | title = Zn-, Cd-, and Pb-transcription factor IIIA: properties, DNA binding, and comparison with TFIIIA-finger 3 metal complexes | journal = Journal of Inorganic Biochemistry | volume = 98 | issue = 5 | pages = 775–85 | date = May 2004 | pmid = 15134923 | pmc = 3516448 | doi = 10.1016/j.jinorgbio.2004.01.014 }}{{cite journal | vauthors = Huang M, Shaw III CF, Petering DH | title = Interprotein metal exchange between transcription factor IIIa and apo-metallothionein | journal = Journal of Inorganic Biochemistry | volume = 98 | issue = 4 | pages = 639–48 | date = April 2004 | pmid = 15041244 | pmc = 3535305 | doi = 10.1016/j.jinorgbio.2004.02.004 }} 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.{{cite journal | vauthors=Palacios O, Atrian S, Capdevila, M | title=Zn- and Cu-thioneins: a functional classification for metallothioneins? | journal=Journal of Biological Inorganic Chemistry | volume=16 | issue=7 | pages=991–1009 | year=2011 | doi = 10.1007/s00775-011-0827-2 | pmid=21823038 | s2cid=26786966 }} 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.

Cysteine residues from MTs can capture harmful oxidant radicals like the superoxide and hydroxyl radicals.{{cite journal | vauthors = Kumari MV, Hiramatsu M, Ebadi M | title = Free radical scavenging actions of metallothionein isoforms I and II | journal = Free Radical Research | volume = 29 | issue = 2 | pages = 93–101 | date = August 1998 | pmid = 9790511 | doi = 10.1080/10715769800300111 }} 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|reason=Reliable source needed for the whole sentence|date=August 2016}}

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.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

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.{{cite journal | vauthors = Takahashi S | title = Molecular functions of metallothionein and its role in hematological malignancies | language = En | journal = Journal of Hematology & Oncology | volume = 5 | issue = 1 | pages = 41 | date = July 2012 | pmid = 22839501 | pmc = 3419633 | doi = 10.1186/1756-8722-5-41 | doi-access = free }}

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.{{cite journal | vauthors=Stubbs BJ, Koutnik AP, Volek JS, Newman JC | title=From bedside to battlefield: intersection of ketone body mechanisms in geroscience with military resilience | journal=GeroScience | volume=43 | issue=3 | pages=1071–1081 | year=2021 | doi = 10.1007/s11357-020-00277-y | pmc=8190215 | pmid=33006708 }} 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).{{cite journal | vauthors = Klaassen CD, Liu J, Choudhuri S | title = Metallothionein: an intracellular protein to protect against cadmium toxicity | journal = Annual Review of Pharmacology and Toxicology | volume = 39 | pages = 267–94 | date = 1999 | pmid = 10331085 | doi = 10.1146/annurev.pharmtox.39.1.267 }}{{cite journal | vauthors = Mostafa WZ, Hegazy RA | title = Vitamin D and the skin: Focus on a complex relationship: A review | journal = Journal of Advanced Research | volume = 6 | issue = 6 | pages = 793–804 | date = November 2015 | pmid = 26644915 | pmc = 4642156 | doi = 10.1016/j.jare.2014.01.011 }}

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.{{cite journal | vauthors = Krizkova S, Fabrik I, Adam V, Hrabeta J, Eckschlager T, Kizek R | title = Metallothionein--a promising tool for cancer diagnostics | journal = Bratislavske Lekarske Listy | volume = 110 | issue = 2 | pages = 93–7 | year = 2009 | pmid = 19408840 }} 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.{{cite journal | vauthors = Cherian MG, Jayasurya A, Bay BH | title = Metallothioneins in human tumors and potential roles in carcinogenesis | journal = Mutation Research | volume = 533 | issue = 1–2 | pages = 201–9 | date = December 2003 | pmid = 14643421 | doi = 10.1016/j.mrfmmm.2003.07.013 }}

Evidence suggests that greater MT expression may cause resistance to chemotherapy.{{cite journal | vauthors = Basu A, Krishnamurthy S | title = Cellular responses to Cisplatin-induced DNA damage | journal = Journal of Nucleic Acids | volume = 2010 | pages = 1–16 | date = August 2010 | pmid = 20811617| pmc =2929606 | doi = 10.4061/2010/201367 | doi-access = free }}

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.{{cite journal | vauthors = Drum DA | title = Are toxic biometals destroying your children's future? | journal = Biometals | volume = 22 | issue = 5 | pages = 697–700 | date = October 2009 | pmid = 19205900 | doi = 10.1007/s10534-009-9212-9 | s2cid = 31579963 }} 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.{{cite journal | vauthors = Singh VK, Hanson J | title = Assessment of metallothionein and antibodies to metallothionein in normal and autistic children having exposure to vaccine-derived thimerosal | journal = Pediatric Allergy and Immunology | volume = 17 | issue = 4 | pages = 291–6 | date = June 2006 | pmid = 16771783 | doi = 10.1111/j.1399-3038.2005.00348.x | s2cid = 2843402 }}

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.{{cite journal | vauthors = Faber S, Zinn GM, Kern JC, Kingston HM | title = The plasma zinc/serum copper ratio as a biomarker in children with autism spectrum disorders | journal = Biomarkers | volume = 14 | issue = 3 | pages = 171–80 | date = May 2009 | pmid = 19280374 | doi = 10.1080/13547500902783747 | s2cid = 205770002 }}

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.{{cite journal | vauthors = Vela G, Stark P, Socha M, Sauer AK, Hagmeyer S, Grabrucker AM | title = Zinc in gut-brain interaction in autism and neurological disorders | journal = Neural Plasticity | volume = 2015 | pages = 972791 | year = 2015 | pmid = 25878905 | pmc = 4386645 | doi = 10.1155/2015/972791 | doi-access = free }}

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.{{cite journal | vauthors = Zhou S, Yin X, Jin J, Tan Y, Conklin DJ, Xin Y, Zhang Z, Sun W, Cui T, Cai J, Zheng Y, Cai L | display-authors = 6 | title = Intermittent hypoxia-induced cardiomyopathy and its prevention by Nrf2 and metallothionein | journal = Free Radical Biology & Medicine | volume = 112 | pages = 224–239 | date = November 2017 | pmid = 28778483 | pmc = 7453314 | doi = 10.1016/j.freeradbiomed.2017.07.031 }}

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.

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.

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.

Therefore, Nrf2 or MT may be a potential treatment to avoid chronic IH-induced cardiomyopathy.

File:Mechanism of Nrf2 and MT in preventing IH-induced cardiac injury..jpg

• Phytochelatin

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• {{cite journal | vauthors = Cherian MG, Jayasurya A, Bay BH | title = Metallothioneins in human tumors and potential roles in carcinogenesis | journal = Mutation Research | volume = 533 | issue = 1–2 | pages = 201–9 | date = December 2003 | pmid = 14643421 | doi = 10.1016/j.mrfmmm.2003.07.013 }}

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• [https://www.uniprot.org/docs/metallo.txt Expasy metallothionein page]

Category:Enzymes