lcysteine

L-cysteine is a sulfur-containing amino acid that is among the 20 standard amino acids used to build proteins in humans. The biologically active form is the L-enantiomer; D-cysteine is not typically incorporated into human proteins.

Structurally, L-cysteine contains a reactive thiol (−SH) side chain, which enables cysteine residues to form disulfide bonds in proteins. These bonds contribute to protein folding, stability, and active-site architecture. The thiol also participates in redox reactions and serves as a precursor for other biological molecules.

Nutrition and metabolism: In humans, L-cysteine is considered non-essential because it can be synthesized from methionine via the transsulfuration pathway, with enzymes that require vitamin B6. It is, however, conditionally essential in infants, certain metabolic disorders, or under severe stress when endogenous production or intake is insufficient. Cysteine is a major precursor for glutathione, a cellular antioxidant, and for other sulfur-containing compounds such as taurine.

Uses and sources: L-cysteine occurs in high-protein foods such as meat, poultry, eggs, dairy, and legumes. It is also used as a food additive and dough conditioner to influence texture. In medicine, N-acetyl-L-cysteine (NAC) is a clinically important derivative used as a mucolytic agent and as an antidote for acetaminophen overdose; NAC acts by replenishing glutathione stores.

Summary: L-cysteine is a versatile, sulfur-containing amino acid essential for protein structure and as a key precursor to antioxidant systems, with both nutritional and therapeutic relevance.