Cysteine
Cysteine is a semiessential proteinogenic amino acid with the formula HOOC−CH(−NH2)−CH2−SH. It is named after its discovery in urine, which comes from the urinary bladder or cyst, from kystis "bladder". Cysteine is chiral, and both D and L-cysteine are found in nature, with D-cysteine having been found in developing brain.
Structure
Cysteine exists as a zwitterion, and it has L chirality in the older d/l notation based on homology to d- and l-glyceraldehyde. In the newer R/ S system of designating chirality, based on the atomic numbers of atoms near the asymmetric carbon, cysteine (and selenocysteine) have R chirality, because of the presence of sulfur (or selenium) as a second neighbor to the asymmetric carbon atom.
Chemical Reactions
Cysteine undergoes a variety of reactions, including methylation to give S-methylcysteine, treatment with formaldehyde to give the thiazolidine thioproline, and the formation of coordination complexes upon treatment with metal ions.
Safety
Cysteine is considered more toxic relative to most other amino acids.
Dietary Restrictions
The animal-originating sources of l-cysteine as a food additive are a point of contention for people following dietary restrictions such as kosher, halal, vegan, or vegetarian. To avoid this problem, l-cysteine can also be sourced from microbial or other synthetic processes.
History
In 1884, German chemist Eugen Baumann found that when cystine was treated with a reducing agent, cystine revealed itself to be a dimer of a monomer which he named "cysteïne".
Dietary Sources
Cysteinyl is a residue in high-protein foods, and some foods considered rich in cysteine include poultry, eggs, beef, and.
Biological Functions
l-Cysteine (l-Cys) is a non-essential amino acid and thus is one of the building blocks required for the synthesis of proteins[2]. It contains sulfur in the form of a thiol group (-SH) at the end of its side chain[2]. The -SH group is responsible for the high reactive capacity of the amino acid, and therefore is responsible for many of its biological functions in human beings[2]. l-Cys is the amino acid establishing disulfide bridges, a type of covalent bond that plays a fundamental role in the folding and stabilization of the tertiary structure of the proteins, thereby supporting their biological activities[2]. The presence of conserved Cys in protein motifs found in all organisms could indicate that this feature was harnessed in early evolution to support enzyme catalysis, transcriptional regulation, protein folding, and 3-dimensional structure[2].
Health Effects
Cysteine has antioxidant properties and is important for making collagen, affecting skin elasticity and texture[3]. It may help treat arthritis and hardening of the arteries, certain lung diseases, and may play a role in the normal growth rate of hair[3]. Cysteine may also help reduce the effects of aging on the skin, aid in healing after surgery or burns, and protect the skin from radiation injury[3]. However, the lack of accurate information and studies based on clinical trials hampers consensus among authors, and the debate about the role and effectiveness of supplements/drugs containing l-Cys is still open[2].
N-Acetyl Cysteine (NAC)
N-acetyl cysteine is an FDA-approved prescription drug that can cause side effects such as dry mouth, nausea, vomiting, and diarrhea[4]. It has an unpleasant odor that some people find hard to tolerate[4]. N-acetyl cysteine is also available in many dietary supplements, but according to the FDA, using N-acetyl cysteine in supplements is illegal because it is an approved drug[4]. Supplement products should not be used in place of prescription products[4].
Citations:
[1] https://en.wikipedia.org/wiki/Cysteine
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6017824/
[3] https://www.urmc.rochester.edu/encyclopedia/content.aspx?contentid=Cysteine&contenttypeid=19
[4] https://www.webmd.com/vitamins/ai/ingredientmono-1018/n-acetyl-cysteine-nac
[5] https://www.webmd.com/vitamins/ai/ingredientmono-1692/l-cysteine