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Cysteine (symbol Cys or C; [5] / ˈ s ɪ s t ɪ iː n /) [6] is a semiessential [7] proteinogenic amino acid with the formula HOOC−CH(−NH 2)−CH 2 −SH. The thiol side chain in cysteine enables the formation of disulfide bonds, and often participates in enzymatic reactions as a nucleophile. Cysteine is chiral, but both D and L-cysteine ...
Acetylcysteine is the N-acetyl derivative of the amino acid L-cysteine, and is a precursor in the formation of the antioxidant glutathione in the body. The thiol (sulfhydryl) group confers antioxidant effects and is able to reduce free radicals .
Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH 2 CH(NH 2)CO 2 H) 2.It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure.
It is an intermediate in the biosynthesis of the common amino acid cysteine in bacteria and plants. O-Acetylserine is biosynthesized by acetylation of the serine by the enzyme serine transacetylase. The enzyme O-acetylserine (thiol)-lyase, using sulfide sources, converts this ester into cysteine, releasing acetate: [1]
L-cysteine production pathways; Reactants → Enzyme Cofactors Notes O-acetyl-L-serine/hydrogen sulfide → cysteine synthase [9] pyridoxal phosphate not present in humans L-cystine/2 glutathione → glutathione-cystine transhydrogenase [10] cystathionine: → cystathionine γ-lyase [4] pyridoxal phosphate 3-mercapto-pyruvate: → cysteine ...
Its acetyl-coenzyme A form is the primary input in the citric acid cycle and is obtained from glycolysis, amino acid metabolism, and fatty acid beta oxidation. This process is the body's primary catabolic pathway and is essential in breaking down the building blocks of the cell such as carbohydrates , amino acids , and lipids .
In IUPAC nomenclature, an acetyl group is called an ethanoyl group. An acetyl group contains a methyl group (−CH 3) that is single-bonded to a carbonyl (C=O), making it an acyl group. The carbonyl center of an acyl radical has one non-bonded electron with which it forms a chemical bond to the remainder (denoted with the letter R) of the molecule.
Unlike cysteine and serine, threonine is a secondary hydroxyl (i.e. has a methyl group). This methyl group greatly restricts the possible orientations of triad and substrate as the methyl clashes with either the enzyme backbone or histidine base. [ 2 ]