Sulfur (also spelled sulphur in British English) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with the chemical formula S8. Elemental sulfur is a bright yellow, crystalline solid at room temperature.
Cysteine 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. Cysteine is chiral, but interestingly, both D and L-cysteine are found in nature with D-cysteine having been found in developing brain. Cysteine is named after its discovery in urine, which comes from the urinary bladder or cyst, from kystis "bladder".
Methionine is an essential amino acid in humans.
Taurine, or 2-aminoethanesulfonic acid, is a non-proteinogenic amino sulfonic acid that is widely distributed in animal tissues. It is a major constituent of bile and can be found in the large intestine, and accounts for up to 0.1% of total human body weight.
Sulfonucleotide reductases are a class of enzymes involved in reductive sulfur assimilation. This reaction consists of a conversion from activated sulfate to sulfite.. The sulfite is used in essential biomolecules such as cysteine. The sulfonucleotide reductases are through to have all evolved from a common ancestor.
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.
Sulfite oxidase is an enzyme in the mitochondria of all eukaryotes, with exception of the yeasts. It oxidizes sulfite to sulfate and, via cytochrome c, transfers the electrons produced to the electron transport chain, allowing generation of ATP in oxidative phosphorylation. This is the last step in the metabolism of sulfur-containing compounds and the sulfate is excreted.
Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossmann fold for binding S-Adenosyl methionine (SAM). Class II methyltransferases contain a SET domain, which are exemplified by SET domain histone methyltransferases, and class III methyltransferases, which are membrane associated. Methyltransferases can also be grouped as different types utilizing different substrates in methyl transfer reactions. These types include protein methyltransferases, DNA/RNA methyltransferases, natural product methyltransferases, and non-SAM dependent methyltransferases. SAM is the classical methyl donor for methyltransferases, however, examples of other methyl donors are seen in nature. The general mechanism for methyl transfer is a SN2-like nucleophilic attack where the methionine sulfur serves as the leaving group and the methyl group attached to it acts as the electrophile that transfers the methyl group to the enzyme substrate. SAM is converted to S-Adenosyl homocysteine (SAH) during this process. The breaking of the SAM-methyl bond and the formation of the substrate-methyl bond happen nearly simultaneously. These enzymatic reactions are found in many pathways and are implicated in genetic diseases, cancer, and metabolic diseases. Another type of methyl transfer is the radical S-Adenosyl methionine (SAM) which is the methylation of unactivated carbon atoms in primary metabolites, proteins, lipids, and RNA.
Sulfinic acids are oxoacids of sulfur with the structure RSO(OH). In these organosulfur compounds, sulfur is pyramidal.
Cystathionine-β-synthase, also known as CBS, is an enzyme (EC 4.2.1.22) that in humans is encoded by the CBS gene. It catalyzes the first step of the transsulfuration pathway, from homocysteine to cystathionine:
Cystathionine is an intermediate in the synthesis of cysteine from homocysteine. It is produced by the transsulfuration pathway and is converted into cysteine by cystathionine gamma-lyase (CTH).
In enzymology, a taurine dioxygenase (EC 1.14.11.17) is an enzyme that catalyzes the chemical reaction.
In enzymology, a cysteamine dioxygenase (EC 1.13.11.19) is an enzyme that catalyzes the chemical reaction
Dioxygenases are oxidoreductase enzymes. Aerobic life, from simple single-celled bacteria species to complex eukaryotic organisms, has evolved to depend on the oxidizing power of dioxygen in various metabolic pathways. From energetic adenosine triphosphate (ATP) generation to xenobiotic degradation, the use of dioxygen as a biological oxidant is widespread and varied in the exact mechanism of its use. Enzymes employ many different schemes to use dioxygen, and this largely depends on the substrate and reaction at hand.
Cysteine sulfinic acid is the organic compound with the nominal formula HO2SCH2CH(NH2)CO2H. It is a rare example of an amino acid bearing a sulfinic acid functional group. It is a white solid that is soluble in water. Like most natural amino acids, it is chiral, only the d-enantiomer occurs in nature, and it exists as the zwitterion at neutral pH. It is an intermediate in cysteine metabolism. It is not a coded amino acid, but is produced post-translationally. Peptides containing the cysteine sulfinic acid residue are substrates for cysteine sulfinic acid reductase.
Hypotaurine is a sulfinic acid that is an intermediate in the biosynthesis of taurine. Like taurine, it also acts as an endogenous neurotransmitter via action on the glycine receptors. It is an osmolyte with antioxidant properties.
In molecular biology, TauD refers to a protein domain that in many enteric bacteria is used to break down taurine as a source of sulfur under stress conditions. In essence, they are domains found in enzymes that provide bacteria with an important nutrient.
Transition metal thiolate complexes are metal complexes containing thiolate ligands. Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as opposed to those that behave as hard Lewis acids. Most complexes contain other ligands in addition to thiolate, but many homoleptic complexes are known with only thiolate ligands. The amino acid cysteine has a thiol functional group, consequently many cofactors in proteins and enzymes feature cysteinate-metal cofactors.
Glutamate decarboxylase like 1 (GADL1) is the enzyme responsible for decarboxylating aspartate (Asp) to β-alanine and cysteine sulfinic acid (CSA) to hypotaurine. GADL1 is a Pyridoxal 5’-phosphate (PLP)-dependent enzyme. By decarboxylating Asp to β-alanine, GADL1 consequently plays a role in the production of carnosine. Carnosine and taurine have multiple biological functions such as calcium regulation, pH buffering, metal chelation, and antioxidant effects. β-Alanine also plays a role as neurotransmitter or neuromodulator in the central nervous system (CNS) and olfactory bulbs.
Isethionate sulfite-lyase is a glycyl radical enzyme that catalyzes the degradation of isethionate into acetaldehyde and sulfite through the cleavage of a carbon-sulfur bond. This conversion is a necessary step for taurine catabolism in anaerobic bacteria like Bilophila wadsworthia. IslA is activated by the enzyme IslB which uses S-adenoslymethionine (SAM) as the initial radical donor.
