Cysteine

Cysteine (symbol Cys or C) is a semi-essential (conditionally essential or nonessential in some classifications) proteinogenic amino acid—one of the 20 standard amino acids used to build proteins in living organisms.

Its chemical formula is \( \mathrm{C_3H_7NO_2S} \) (or more specifically \( \mathrm{HS{-}CH_2{-}CH(NH_2){-}COOH} \)), and it is a sulfur-containing amino acid distinguished by its thiol (-SH) side chain. This reactive thiol group enables key chemical behaviors: it readily forms disulfide bonds (oxidizing to cystine, the dimer of two cysteines linked by an S–S bond), acts as a nucleophile in enzymatic reactions, and participates in redox processes.

Key Properties and Structure

• It exists as a zwitterion in solution and is chiral; the L-form is the one incorporated into proteins across organisms, while D-cysteine occurs in some signaling roles.
• Molecular mass is approximately 121.16 g/mol.
• It is encoded by the codons UGU and UGC.
• In the older D/L system it is designated L; in the R/S system it is R due to the sulfur atom.
  
  
  

Biological Roles

• Protein structure: Disulfide bridges stabilize the three-dimensional shape of many proteins, including structural ones like keratin (in hair, skin, and nails) and proteins in connective tissues.
• Antioxidant and detoxification: It is a rate-limiting precursor for glutathione (often called the body’s master antioxidant), supporting cellular protection against oxidative stress and aiding detoxification.
• Metabolism and other functions: Involved in various enzymatic reactions, collagen production, and sulfur metabolism. The body synthesizes it from the essential amino acid methionine via the transsulfuration pathway (requiring adequate methionine, vitamin B6, etc.), but dietary intake or supplementation can become important under stress, illness, or limited methionine availability.

Sources and Uses

Dietary sources include high-protein foods such as meat, poultry, fish, eggs, dairy, soybeans, legumes, seeds, and certain grains. As a food additive it has the E number E920. Supplements often appear as N-acetylcysteine (NAC), a more stable form used for various purposes (e.g., supporting glutathione levels).

In short, cysteine’s unique sulfur chemistry makes it disproportionately important for protein folding/stability and cellular antioxidant defense relative to its abundance.