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Cysteine (symbol Cys or C; ) 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. The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920. It is encoded by the codons UGU and UGC.
In chemistry, a disulfide refers to a functional group with the structure R−S−S−R′. 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 biology, disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins. Persulfide usually refers to R−S−S−H compounds.
Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it (or a thioester) as a substrate. In humans, CoA biosynthesis requires cysteine, pantothenate (vitamin B5), and adenosine triphosphate (ATP).
Glutathione (GSH) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by reactive oxygen species such as free radicals, peroxides, lipid peroxides, and heavy metals. It is a tripeptide with a gamma peptide linkage between the carboxyl group of the glutamate side chain and cysteine. The carboxyl group of the cysteine residue is attached by normal peptide linkage to glycine.
Cystine is the oxidized dimer form of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is slightly soluble in water. It serves two biological functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure.
Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis. Proteins are synthesized by ribosomes translating mRNA into polypeptide chains, which may then undergo PTM to form the mature protein product. PTMs are important components in cell signaling, as for example when prohormones are converted to hormones.
Protein disulfide isomerase, or PDI, is an enzyme in the endoplasmic reticulum (ER) in eukaryotes and the periplasm of bacteria that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold. This allows proteins to quickly find the correct arrangement of disulfide bonds in their fully folded state, and therefore the enzyme acts to catalyze protein folding.
Reactive oxygen species (ROS) are highly reactive chemical molecules formed due to the electron receptivity of O2. Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The oxoglutarate dehydrogenase complex (OGDC) or α-ketoglutarate dehydrogenase complex is an enzyme complex, most commonly known for its role in the citric acid cycle.
Glutathione disulfide (GSSG) is a disulfide derived from two glutathione molecules.
Sulfur is an essential element for growth and physiological functioning of plants. However, its content strongly varies between plant species and it ranges from 0.1 to 6% of the plants' dry weight.
ER oxidoreductin 1 (Ero1) is an oxidoreductase enzyme that catalyses the formation and isomerization of protein disulfide bonds in the endoplasmic reticulum (ER) of eukaryotes. ER Oxidoreductin 1 (Ero1) is a conserved, luminal, glycoprotein that is tightly associated with the ER membrane, and is essential for the oxidation of protein dithiols. Since disulfide bond formation is an oxidative process, the major pathway of its catalysis has evolved to utilise oxidoreductases, which become reduced during the thiol-disulfide exchange reactions that oxidise the cysteine thiol groups of nascent polypeptides. Ero1 is required for the introduction of oxidising equivalents into the ER and their direct transfer to protein disulfide isomerase (PDI), thereby ensuring the correct folding and assembly of proteins that contain disulfide bonds in their native state.
Peroxiredoxins are a ubiquitous family of antioxidant enzymes that also control cytokine-induced peroxide levels and thereby mediate signal transduction in mammalian cells. The family members in humans are PRDX1, PRDX2, PRDX3, PRDX4, PRDX5, and PRDX6. The physiological importance of peroxiredoxins is illustrated by their relative abundance.
Reactive nitrogen species (RNS) are a family of antimicrobial molecules derived from nitric oxide (•NO) and superoxide (O2•−) produced via the enzymatic activity of inducible nitric oxide synthase 2 (NOS2) and NADPH oxidase respectively. NOS2 is expressed primarily in macrophages after induction by cytokines and microbial products, notably interferon-gamma (IFN-γ) and lipopolysaccharide (LPS).
In enzymology, a sulfiredoxin is an enzyme that catalyzes the chemical reaction
Peroxiredoxin-6 is a protein that in humans is encoded by the PRDX6 gene. It is a member of the peroxiredoxin family of antioxidant enzymes.
Glutaredoxin 2 (GLRX2) is an enzyme that in humans encoded by the GLRX2 gene. GLRX2, also known as GRX2, is a glutaredoxin family protein and a thiol-disulfide oxidoreductase that maintains cellular thiol homeostasis. This gene consists of four exons and three introns, spanned 10 kilobase pairs, and localized to chromosome 1q31.2–31.3.
S-Nitrosothiols, also known as thionitrites, are organic compounds or functional groups containing a nitroso group attached to the sulfur atom of a thiol. S-Nitrosothiols have the general formula RSNO, where R denotes an organic group. Originally suggested by Ignarro to serve as intermediates in the action of organic nitrates, endogenous S-nitrosothiols were discovered by Stamler and colleagues and shown to represent a main source of NO bioactivity in vivo. More recently, S-nitrosothiols have been implicated as primary mediators of protein S-nitrosylation, the oxidative modification of Cys thiol that provides ubiquitous regulation of protein function.
DsbA is a bacterial thiol disulfide oxidoreductase (TDOR). DsbA is a key component of the Dsb family of enzymes. DsbA catalyzes intrachain disulfide bond formation as peptides emerge into the cell's periplasm.
Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element in Earth's crust, and although many arsenic compounds are often considered highly toxic to most life, a wide variety of organoarsenic compounds are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organisms. This pattern is general for other related elements, including selenium, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many toxic arsenic compounds are found in some aquifers, potentially affecting many millions of people via biochemical processes.