Although it is toxic in large doses,

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

is an essential micronutrient for animals. In plants, it sometimes occurs in toxic amounts as

forage Forage is a plant material (mainly plant leaves and stems) eaten by grazing livestock. Historically, the term ''forage'' has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used ...

, e.g.

locoweed Locoweed (also crazyweed and loco) is a common name in North America for any plant that produces swainsonine, a phytotoxin harmful to livestock. Worldwide, swainsonine is produced by a small number of species, most of them in three genera of the f ...

. Selenium is a component of the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s selenocysteine and

selenomethionine Selenomethionine (SeMet) is a naturally occurring amino acid. The L-selenomethionine enantiomer is the main form of selenium found in Brazil nuts, cereal grains, soybeans, and grassland legumes, while ''Se''-methylselenocysteine, or its γ-gluta ...

. In humans, selenium is a

trace element __NOTOC__ A trace element, also called minor element, is a chemical element whose concentration (or other measure of amount) is very low (a "trace amount"). They are classified into two groups: essential and non-essential. Essential trace elements ...

nutrient that functions as cofactor for

glutathione peroxidase Glutathione peroxidase (GPx) () is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage. The biochemical function of glutathione peroxidase is to reduce lipid h ...

s and certain forms of

thioredoxin reductase Thioredoxin reductases (TR, TrxR) () are enzymes that reduce thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. In bacteria TrxR also catalyzes the reduction ...

. Selenium-containing proteins are produced from inorganic selenium via the intermediacy of selenophosphate (PSeO₃³⁻).

Se-containing biomolecules

Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. Selenium exerts its biological functions through

selenoprotein In molecular biology a selenoprotein is any protein that includes a selenocysteine (Sec, U, Se-Cys) amino acid residue. Among functionally characterized selenoproteins are five glutathione peroxidases (GPX) and three thioredoxin reductases, (TrxR/TX ...

s, which contain the

selenocysteine. Twenty-five selenoproteins are encoded in the

human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...

Glutathione peroxidase

The

family of enzymes (abbreviated GSH-Px) catalyze reduction of

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

and organic

hydroperoxide Hydroperoxides or peroxols are compounds containing the hydroperoxide functional group (ROOH). If the R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. ...

s: :2GSH + H₂O₂ → GSSG + 2 H₂O The two H atoms are donated by thiols in a process that begins with oxidation of a

selenol Selenols are organic compounds that contain the functional group with the connectivity C– Se–H. Selenols are sometimes also called selenomercaptans and selenothiols. Selenols are one of the principal classes of organoselenium compounds. The be ...

side chain in GSH-Px. The

organoselenium compound Organoselenium compounds (or seleno-organic) are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and sulf ...

ebselen is a drug used to supplement the action of GSH-Px. It functions as a catalyst for the destruction of hydrogen peroxide. A related selenium-containing enzyme in some plants and in animals (

) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA precursors from RNA precursors.

Deiodinases

Selenium also plays a role in the functioning of the

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...

gland. It participates as a cofactor for the three thyroid hormone

deiodinase Deiodinase (or "Monodeiodinase") is a peroxidase enzyme that is involved in the activation or deactivation of thyroid hormones. Types Types of deiodinases include: Iodothyronine deiodinases catalyze release of iodine directly from the thyro ...

s. These enzymes activate and then deactivate various thyroid hormones and their metabolites. It may inhibit

Hashimoto's disease Hashimoto's thyroiditis, also known as chronic lymphocytic thyroiditis and Hashimoto's disease, is an autoimmune disease in which the thyroid gland is gradually destroyed. Early on, symptoms may not be noticed. Over time, the thyroid may enla ...

, an auto-immune disease in which the body's own thyroid cells are attacked by the immune system. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium.

Formate dehydrogenase

Some microorganisms utilize selenium in

formate dehydrogenase Formate dehydrogenases are a set of enzymes that catalyse the oxidation of formate to carbon dioxide, donating the electrons to a second substrate, such as NAD+ in formate:NAD+ oxidoreductase () or to a cytochrome in formate:ferricytochrome-b1 o ...

. Formate is produced in large amounts in the hepatic (liver cells) mitochondria of embryonic cells and in cancer cells by the folate cycle. Formate is reversibly oxidized by the enzyme

: :HCO₂⁻ → CO₂ + H⁺ + 2 e⁻

Thioredoxin reductase

Thioredoxin reductase Thioredoxin reductases (TR, TrxR) () are enzymes that reduce thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. In bacteria TrxR also catalyzes the reduction ...

uses a cysteine-selenocysteine pair to reduce the

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

thioredoxin Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. In humans, thioredoxins are encoded by ''TXN'' and '' TXN2'' genes. Loss-of-fu ...

. The selenocysteine is arranged in an unusual Sec-His-Glu

catalytic triad A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, li ...

, which tunes its pKa.

Indicator plants

Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium to thrive. The main selenium indicator plants are ''

Astragalus ''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...

'' species (including some

s), prince's plume ('' Stanleya'' sp.), woody asters ('' Xylorhiza'' sp.), and false goldenweed (''

Oonopsis ''Oonopsis'', or false goldenweed, is a genus of flowering plants in the family Asteraceae. ; Species

'' sp.)

Medical use of synthetic selenium compounds

The substance loosely called

selenium sulfide Selenium sulfide can refer to either of the following: * Selenium disulfide, SeS2 * Selenium hexasulfide, Se2S6 {{Short pages monitor