Cysteine
Overview
 
Cysteine is an α-amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 HO2CCH(NH2)CH2SH. It is a non-essential amino acid
Essential amino acid
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism , and therefore must be supplied in the diet.-Essentiality vs. conditional essentiality in humans:...

, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

, which is polar and thus cysteine is usually classified as a hydrophilic amino acid. The thiol side chain often participates in enzymatic reactions, serving as a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

. The thiol is susceptible to oxidization to give the disulfide
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...

 derivative cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...

, which serves an important structural role in many protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s.
Although classified as a non-essential amino acid, in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption
Malabsorption
Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...

 syndrome
Syndrome
In medicine and psychology, a syndrome is the association of several clinically recognizable features, signs , symptoms , phenomena or characteristics that often occur together, so that the presence of one or more features alerts the physician to the possible presence of the others...

s.
Encyclopedia
Cysteine is an α-amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 HO2CCH(NH2)CH2SH. It is a non-essential amino acid
Essential amino acid
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism , and therefore must be supplied in the diet.-Essentiality vs. conditional essentiality in humans:...

, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

, which is polar and thus cysteine is usually classified as a hydrophilic amino acid. The thiol side chain often participates in enzymatic reactions, serving as a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

. The thiol is susceptible to oxidization to give the disulfide
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...

 derivative cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...

, which serves an important structural role in many protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s.

Dietary sources

Although classified as a non-essential amino acid, in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption
Malabsorption
Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...

 syndrome
Syndrome
In medicine and psychology, a syndrome is the association of several clinically recognizable features, signs , symptoms , phenomena or characteristics that often occur together, so that the presence of one or more features alerts the physician to the possible presence of the others...

s. Cysteine can usually be synthesized by the human body under normal physiological conditions if a sufficient quantity of methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

 is available. Cysteine is catabolized in the gastrointestinal tract and blood plasma. In contrast, cystine
Cystine
Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...

 travels safely through the GI tract and blood plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 and is promptly reduced to the two cysteine molecules upon cell entry.

Cysteine is found in most high-protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 foods, including:
  • Animal sources: pork
    Pork
    Pork is the culinary name for meat from the domestic pig , which is eaten in many countries. It is one of the most commonly consumed meats worldwide, with evidence of pig husbandry dating back to 5000 BC....

    , sausage meat
    Sausage
    A sausage is a food usually made from ground meat , mixed with salt, herbs, and other spices, although vegetarian sausages are available. The word sausage is derived from Old French saussiche, from the Latin word salsus, meaning salted.Typically, a sausage is formed in a casing traditionally made...

    , chicken
    Chicken
    The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

    , turkey
    Turkey (bird)
    A turkey is a large bird in the genus Meleagris. One species, Meleagris gallopavo, commonly known as the Wild Turkey, is native to the forests of North America. The domestic turkey is a descendant of this species...

    , duck
    Duck
    Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

    , luncheon meat, eggs
    Egg (food)
    Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...

    , milk
    Milk
    Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

    , whey protein
    Whey protein
    Whey protein is a mixture of globular proteins isolated from whey, the liquid material created as a by-product of cheese production. Some preclinical studies in rodents have suggested that whey protein may possess anti-inflammatory or anti-cancer properties; however, human data is lacking...

    , ricotta
    Ricotta
    Ricotta is an Italian dairy product made from sheep milk whey left over from the production of cheese. Although typically referred to as ricotta cheese, ricotta is not properly a cheese because it is not produced by coagulation of casein...

    , cottage cheese
    Cottage cheese
    Cottage cheese is a cheese curd product with a mild flavor. It is drained, but not pressed, so some whey remains and the individual curds remain loose. The curd is usually washed to remove acidity, giving sweet curd cheese. It is not aged or colored. Different styles of cottage cheese are made from...

    , yogurt
  • Plant sources: red peppers
    Capsicum
    Capsicum is a genus of flowering plants in the nightshade family, Solanaceae. Its species are native to the Americas where they have been cultivated for thousands of years, but they are now also cultivated worldwide, used as spices, vegetables, and medicines - and have become are a key element in...

    , garlic
    Garlic
    Allium sativum, commonly known as garlic, is a species in the onion genus, Allium. Its close relatives include the onion, shallot, leek, chive, and rakkyo. Dating back over 6,000 years, garlic is native to central Asia, and has long been a staple in the Mediterranean region, as well as a frequent...

    , onion
    Onion
    The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...

    s, broccoli
    Broccoli
    Broccoli is a plant in the cabbage family, whose large flower head is used as a vegetable.-General:The word broccoli, from the Italian plural of , refers to "the flowering top of a cabbage"....

    , brussels sprout
    Brussels sprout
    The Brussels sprout is a cultivar of wild cabbage grown for its edible buds. The leafy green vegetables are typically 2.5–4 cm in diameter and look like miniature cabbages. The sprout is Brassica oleracea, in the "gemmifera" group of the family Brassicaceae...

    , oat
    Oat
    The common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...

    s, granola
    Granola
    Granola is a breakfast food and snack food, popular in North America, consisting of rolled oats, nuts, honey, and sometimes rice, that is usually baked until crisp. During the baking process the mixture is stirred to maintain a loose, breakfast cereal-type consistency...

    , wheat germ, lentil
    Lentil
    The lentil is an edible pulse. It is a bushy annual plant of the legume family, grown for its lens-shaped seeds...



As other amino acids, cysteine has an amphoteric
Amphoterism
In chemistry, an amphoteric species is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word amphoteroi meaning "both"...

 character.

Industrial sources

The majority of L-Cysteine was once obtained industrially by hydrolysis of human hair, but in recent years 80% is produced from duck feathers. Due to marketing restraints with Jewish Kosher and Muslim Halal however, it is now possible to get synthetically produced material, albeit at a higher price. The synthetic route involves fermentation utilizing a mutant of E. coli. Wacker Chemie
Wacker Chemie
Wacker Chemie AG is a worldwide operating company in the chemical business, founded 1914. The company is controlled by the Wacker-family holding more than 50 percent of the shares. The corporation is operating more than 25 production sites in Europe, Asia, and the Americas...

 introduced a route from substituted thiazoline
Thiazoline
Thiazoline is a heterocyclic compound containing both sulfur and nitrogen in the ring. Although thiazoline itself is rarely encountered, its derivatives are often bioactive. For example, in a common post-translational modification, cysteine residues are converted into...

s. Following this technology, L-cysteine is produced by the hydrolysis of racemic 2-amino-Δ2-thiazoline-4-carboxylic acid using Pseudomonas thiazolinophilum.

Biosynthesis

In animals, biosynthesis begins with the amino acid serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

. The sulfur is derived from methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

, which is converted to homocysteine
Homocysteine
Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...

 through the intermediate S-adenosylmethionine. Cystathionine beta-synthase then combines homocysteine and serine to form the asymmetrical thioether cystathionine
Cystathionine
Cystathionine is an intermediate in the synthesis of cysteine.It is generated from homocysteine and serine by cystathionine beta synthase.It is cleaved into cysteine and α-ketobutyrate by cystathionine gamma-lyase....

. The enzyme cystathionine gamma-lyase
Cystathionine gamma-lyase
Cystathionine gamma-lyase is an enzyme which breaks down cystathionine into cysteine and α-ketobutyrate. Pyridoxal phosphate is a prosthetic group of this enzyme....

 converts the cystathionine into cysteine and alpha-ketobutyrate. In plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, cysteine biosynthesis again starts from serine, which is converted to O-acetylserine by the enzyme serine transacetylase. The enzyme O-acetylserine (thiol)-lyase, using sulfide sources, converts this ester into cysteine, releasing acetate.

Biological functions

The cysteine thiol group is nucleophilic
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

 and easily oxidized. The reactivity is enhanced when the thiol is ionized, and cysteine residues in proteins have pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

 values close to neutrality, so are often in their reactive thiolate form in the cell. Because of its high reactivity, the thiol group of cysteine has numerous biological functions.

Precursor to the antioxidant glutathione

Due to the ability of thiols to undergo redox reactions, cysteine has antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 properties. Cysteine's antioxidant properties are typically expressed in the tripeptide glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...

, which occurs in humans as well as other organisms. The systemic availability of oral glutathione (GSH) is negligible; so it must be biosynthesized from its constituent amino acids, cysteine, glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

, and glutamic acid
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...

. Glutamic acid and glycine are readily available in most Western diets, but the availability of cysteine can be the limiting substrate
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

.

Disulfide bonds

Disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...

s play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium. Since most cellular compartments are reducing environments, disulfide bonds are generally unstable in the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

 with some exceptions as noted below.

Disulfide bonds in proteins are formed by oxidation of the thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

 groups of cysteine residues. The other sulfur-containing amino acid, methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

, cannot form disulfide bonds. More aggressive oxidants convert cysteine to the corresponding sulfinic acid and sulfonic acid
Sulfonic acid
Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...

. Cysteine residues play a valuable role by crosslinking proteins, which increases the rigidity of proteins and also functions to confer proteolytic resistance (since protein export is a costly process, minimizing its necessity is advantageous). Inside the cell, disulfide bridges between cysteine residues within a polypeptide support the protein's tertiary structure. Insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

 is an example of a protein with cystine crosslinking, wherein two separate peptide chains are connected by a pair of disulfide bonds.

Protein disulfide isomerase
Protein disulfide isomerase
Protein disulfide isomerase or PDI is an enzyme in the endoplasmic reticulum in eukaryotes that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold...

s catalyze the proper formation of disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...

s; the cell transfers dehydroascorbic acid
Dehydroascorbic acid
Dehydroascorbic acid is an oxidized form of ascorbic acid. It is actively imported into the endoplasmic reticulum of cells via glucose transporters. It is trapped therein by reduction back to ascorbate by glutathione and other thiols. Therefore, L-dehydroascorbic acid is a vitamin C compound much...

 to the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

, which oxidises the environment. In this environment, cysteines are, in general, oxidized to cystine and are no longer functional as a nucleophiles.

Precursor to iron-sulfur clusters

Cysteine is an important source of sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 in human metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. The sulfide in iron-sulfur cluster
Iron-sulfur cluster
For biological Fe-S clusters, see iron-sulfur proteins.Iron-sulfur clusters are ensembles of iron and sulfide centres. Fe-S clusters are most often discussed in the context of the biological role for iron-sulfur proteins. Many Fe-S clusters are known in the area of organometallic chemistry and as...

s and in nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...

 is extracted from cysteine, which is converted to alanine
Alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...

 in the process.

Metal ion binding

Beyond the iron-sulfur proteins, many other metal cofactors in enzymes are bound to the thiolate substituent of cysteinyl residues. Examples include zinc in zinc finger
Zinc finger
Zinc fingers are small protein structural motifs that can coordinate one or more zinc ions to help stabilize their folds. They can be classified into several different structural families and typically function as interaction modules that bind DNA, RNA, proteins, or small molecules...

s and alcohol dehydrogenase
Alcohol dehydrogenase
Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide...

, copper in the blue copper protein
Plastocyanin
Plastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants...

s, iron in cytochrome P450, and nickel in the [NiFe]-hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

s. The thiol group also has a high affinity for heavy metals, so that proteins containing cysteine, such as metallothionein
Metallothionein
Metallothionein is a family of cysteine-rich, low molecular weight proteins. They are localized to the membrane of the Golgi apparatus...

, will bind
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 metals such as mercury, lead, and cadmium tightly.

Post-translational modifications

Aside from its oxidation to cystine, cysteine participates in numerous posttranslational modification
Posttranslational modification
Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....

s. The nucleophilic thiol group allows cysteine to conjugate to other groups, e.g., in prenylation
Prenylation
Prenylation, or isoprenylation, or lipidation is the addition of hydrophobic molecules to a protein. It is usually assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchor like the GPI anchor, though direct evidence is missing...

. Ubiquitin
Ubiquitin
Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...

 ligase
Ligase
In biochemistry, ligase is an enzyme that can catalyse the joining of two large molecules by forming a new chemical bond, usually with accompanying hydrolysis of a small chemical group dependent to one of the larger molecules...

s transfer ubiquitin to its pendant, proteins, and caspase
Caspase
Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....

s, which engage in proteolysis in the apoptotic cycle. Intein
Intein
An intein is a segment of a protein that is able to excise itself and rejoin the remaining portions with a peptide bond. Inteins have also been called "protein introns"....

s often function with the help of a catalytic cysteine. These roles are typically limited to the intracellular milieu, where the environment is reducing, and cysteine is not oxidized to cystine.

Applications

Cysteine, mainly the L-enantiomer, is a precursor in the food, pharmaceutical, and personal care industries. One of the largest applications is the production of flavors. For example, the reaction of cysteine with sugars in a Maillard reaction
Maillard reaction
The Maillard reaction is a form of nonenzymatic browning similar to caramelization. It results from a chemical reaction between an amino acid and a reducing sugar, usually requiring heat....

 yields meat flavors. L-cysteine is also used as a processing aid for baking.

In the field of personal care, cysteine is used for permanent wave
Permanent wave
A permanent wave, commonly called a perm, involves the use of chemicals to break and reform the bonds of the hair. The hair is washed and wrapped on a perm rod and waving lotion is applied with a base. This solution creates a chemical reaction that softens the inner structure of the hair by...

 applications predominantly in Asia. Again the cysteine is used for breaking up the disulfide bonds in the hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

's keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

.

Cysteine is a very popular target for site-directed labeling experiments to investigate biomolecular structure and dynamics. Maleimide
Maleimide
Maleimide is the chemical compound with the formula H2C22NH . This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -CNHC- functional group...

s will selectively attach to cysteine using a covalent Michael addition. Site-directed spin labeling
Site-directed spin labeling
Site-directed spin labeling is a technique for investigating protein local dynamics using electron spin resonance. The theory of SDSL is based on the specific reaction of spin labels with amino acids. A spin label's built-in protein structure can be detected by EPR spectroscopy...

 for EPR or paramagnetic relaxation enhanced NMR also uses cysteine extensively.

In a 1994 report released by five top cigarette
Cigarette
A cigarette is a small roll of finely cut tobacco leaves wrapped in a cylinder of thin paper for smoking. The cigarette is ignited at one end and allowed to smoulder; its smoke is inhaled from the other end, which is held in or to the mouth and in some cases a cigarette holder may be used as well...

 companies, cysteine is one of the 599 additives to cigarettes. Like most cigarette additives, however, its use or purpose is unknown. Its inclusion in cigarettes could offer two benefits: Acting as an expectorant, since smoking increases mucus production in the lungs; and increasing the beneficial antioxidant glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...

 (which is diminished in smokers).

Sheep

Cysteine is required by sheep in order to produce wool: It is an essential amino acid that must be taken in as food from grass. As a consequence, during drought conditions, sheep stop producing wool; however, transgenic sheep that can make their own cysteine have been developed.

Reducing toxic effects of alcohol

Cysteine has been proposed as a preventative or antidote for some of the negative effects of alcohol, including liver damage and hangover
Hangover
A hangover describes the sum of unpleasant physiological effects following heavy consumption of alcoholic beverages. The most commonly reported characteristics of a hangover include headache, nausea, sensitivity to light and noise, lethargy, dysphoria, diarrhea and thirst, typically after the...

. It counteracts the poisonous effects of acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...

, which is the major by-product of alcohol metabolism and is responsible for most of the negative aftereffects and long-term damage associated with alcohol use (but not the immediate effects of drunkenness
Drunkenness
Alcohol intoxication is a physiological state that occurs when a person has a high level of ethanol in his or her blood....

). Cysteine supports the next step in metabolism, which turns acetaldehyde into the relatively harmless acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

. In a rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

 study, test animals received an dose of acetaldehyde (the amount that normally kills half of all animals). Those that received cysteine had an 80% survival rate; when both cysteine and thiamine
Thiamine
Thiamine or thiamin or vitamin B1 , named as the "thio-vitamine" is a water-soluble vitamin of the B complex. First named aneurin for the detrimental neurological effects if not present in the diet, it was eventually assigned the generic descriptor name vitamin B1. Its phosphate derivatives are...

 were administered, all animals survived. There is not yet direct evidence for or against its effectiveness in humans who consume alcohol at normal levels.

N-acetylcysteine (NAC)

N-acetyl-L-cysteine (NAC) is a derivative of cysteine wherein an acetyl group is attached to the nitrogen atom. This compound is sold as a dietary supplement commonly claiming antioxidant and liver-protecting effects. NAC is often used as a cough medicine because it breaks up the disulfide bonds in the mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 and thus liquefies it, making it easier to cough up. It is also this action of breaking disulfide bonds that makes it useful in thinning the abnormally thick mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 in Cystic Fibrosis
Cystic fibrosis
Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...

 patients. NAC is also used as a specific antidote
Antidote
An antidote is a substance which can counteract a form of poisoning. The term ultimately derives from the Greek αντιδιδοναι antididonai, "given against"....

 in cases of acetaminophen overdose, and obsessive compulsive disorders like Trichotillomania
Trichotillomania
Trichotillomania, which is classified as an impulse control disorder by DSM-IV, is the compulsive urge to pull out one's own hair leading to noticeable hair loss, distress, and social or functional impairment. It is often chronic and difficult to treat....

.

See also

  • Selenocysteine
    Selenocysteine
    Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

  • Amino acid
    Amino acid
    Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

    s
  • Thiol
    Thiol
    In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

    s
  • Cysteine metabolism
    Cysteine metabolism
    Cysteine metabolism refers to the biological pathways that consume or create cysteine. The pathways of different amino acids and other metabolites interweave and overlap to creating complex systems.-Human cysteine metabolism:...

  • Cystinuria
    Cystinuria
    Cystinuria is an inherited autosomal recessive disease that is characterized by the formation of cystine stones in the kidneys, ureter, and bladder.-Signs and symptoms:Cystinuria is a cause of persistent kidney stones...


External links

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