Cysteine

     Cysteine ​​(abbreviated as Cys or C) is an α-amino acid with the chemical formula HO2CCH (NH2) CH2SH. Unaminoácido is not essential, which means that can be synthesized by humans. The codons encoding cysteine ​​and are UGC UGU. The part of the chain where the cysteine ​​is the thiol is nonpolar and thus cysteine ​​are usually classified as a hydrophobic amino acid. The thiol side chain is often involved in enzymatic reactions, acting as a nucleophile. The thiol is susceptible to oxidation to yield the disulfide bridges of cysteine ​​derivatives that have an important structural role in many proteins. Cysteine ​​is also called cystine, but the latter is a dimer of two cysteines via a disulfide bridge.Content

Precursor of glutathione.

Food source

     Although it is classified as a nonessential amino acid, in some cases, the cysteine ​​may be essential for infants, elderly and people with certain metabolic diseases or suffering from malabsorption syndromes. Cysteine ​​is normally synthesized by the body within normal physiological conditions, provided that sufficient hayametionina. Cysteine ​​is potentially toxic and is catabolized in the digestive tract and blood plasma. Cysteine ​​securely travels through the gastrointestinal tract and plasma and is rapidly reduced to the two cysteine ​​molecules to enter the cell. Cysteine ​​is the most high-protein foods, such as:

     Animal Resources: pork, stuffed chicken, turkey, duck, sausage, eggs, milk, cottage cheese, yogurt.Plant resources: red peppers, garlic, onions, broccoli, Brussels sprouts, granola, wheat germ.Like other amino acids cysteine ​​has an amphoteric character.(R)-cysteine ​​(left) (S)-Cysteine ​​(right) at a neutral pH zwitterion ionic.

Industrial source
    
     L-Cysteine ​​was obtained industrially by hydrolysis of hair keratin. But now the main route of obtaining this is by fermentation using a mutant of E. Coli. Wacker Chemie introduced a route from tiazonilos substituents. By this route the L-Cysteine ​​is produced by hydrolysis of the racemic mixture of 2-amino-Δ2-tiazolin-4-carboxylico using thiazolinophilum Pseudomonas.

BiosynthesisSynthesis of cysteine.

      Cysteine ​​synthetase catalyzes the reaction beta upper and cystathionine gamma-lyase catalyzes the reaction below.In animals, the biosynthesis starts with the amino acid serine. The sulfur is derived from lametionina which is converted into homocysteine ​​via the intermediate S-adenosilmetionina.Tras that the cystathionine beta-synthase combines homocysteine ​​and serine to form cystathionine asymmetric thioether. The enzyme cystathionine gamma-lyase converts cystathionine to cysteine ​​and alpha-ketobutirato. Plant and bacterial cysteine ​​biosynthesis starts also from passing serine O-acetylserine become by action of the enzyme serine acetyltransferase (EC 2.3.1.30). The enzyme O-acetylserine (thiol)-lyase ((OAS-TL, EC 2.5.1.47), using sulfur as hydrogen sulfide, converts cysteine ​​ester by displacement of the acetate.

Biological functionsThe thiol group of cysteine ​​is nucleophilic and easily oxidized. The reactivity increases when the thiol is ionized and the cysteine ​​residues in proteins have pH values ​​near neutrality, as often found in a reactive thiols in the cell. Due to its high reactivity, the thiol group of cysteine ​​has numerous biological functions.Antioxidant glutathione precursorDue to the ability of thiols undergo redox reactions, cysteine ​​has antioxidant properties. These antioxidant properties of cysteine ​​are mainly expressed in glutathione tripeptides that occur in humans and other organisms. Systemic availability of oral glutathione (GSH) is negligible, this must be biosynthesized from the amino acids that constitute such as cysteine, glycine and glutamic acid. Glutamic acid and glycine are found abundantly in most Western diets, so the availability of cysteine ​​may be the limiting substrate.DisulfideDisulfide bridges play an important role in the assembly and stability of some proteins normally secreted proteins into the extracellular medium. Since most cell compartments are reduced means, the disulfide bridges are generally unstable in the cytosol, with certain exceptions we see below.Protein disulfide bridges are formed by oxidation of thiol groups of cysteine ​​residues. The other amino acids also contain sulfur, such as methionine can not form disulfide bridges. Very aggressive oxidants convert cysteine ​​into the corresponding acid and sulfonic acid sulfánico. Cysteine ​​residues have a valuable role in cross-linked proteins, as it increases the rigidity of proteins and confers resistance to proteolytic. Within the cell, the disulfide bridges between cysteine ​​residues support acting in the secondary structure of polypeptides. Insulin is an example of crosslinked protein with cysteines, wherein two separate peptide chains are connected by a pair of bridges disulfuros.Las protein disulfide isomerases catalyze the formation of disulfide bridges itself, the cell transfers dehydroascorbic acid into the endoplasmic reticulum. In nature, the cysteines are generally oxidized to cystine their only function is nucleophilic.Figure 2: Cystine (as neutral) is derived from two molecules of cysteine. Forming a disulfide bridge.Group precursors iron-sulfurCysteine ​​is an important source of sulfur in human metabolism. Sulfur from the iron-sulfur groups and nitrogenases is extracted from cysteine ​​and alanine passes into the process.Metal ion bindingApart from the iron-sulfur-proteins, many other metal cofactors of enzymes are unions for the substituent of the thiol of the cysteine ​​residues. Examples include zinc in zinc finger and alcohol dehydrogenase, the blue copper proteins cuprous, iron in cytochrome P450 and nickel in [NiFe]-hydrogenase. The thiol group also has high affinity to heavy metals, so that cysteine-containing proteins such as metallothionein linking metals such as [[mercury], lead and cadmiofuerte.Posttranslational modificationsApart from its oxidation to cystine, cysteine ​​participates in numerous posttranslational modifications. The nucleophilic thiol group of cysteine ​​conjugate allows other groups, such as the prenylation, ubiquitin ligases transferring ubiquitin to its pendant, proteins and caspases involved in apoptotic proteolysis in the cycle. The inteínas (protein introns) normally act to help the catalytic cysteine. These papers are typically limited to cysteine ​​intracellular environment, where the medium is reduced and cysteine ​​is not oxidized cystine.Other metabolitesThe product of decarboxylation of cysteine ​​cysteamine is a biogenic amine that is a fundamental component of coenzyme A. The product of transamination of cysteine ​​is the mercaptopyruvate, which can be degraded to pyruvate or reduced by various routes mercaptolactato possible, depending on the organism. Many microorganisms and plants cyanide anions attached by nucleophilic substitution with the sulfhydryl to give as product cyanoalanine, which can be hydrolyzed to aspartate. The sulfur of cysteine ​​can be methylated to get a homologue of methionine called S-methyl.