nounIntermediate
Definition
Sulfur-containing amino acid (Cys, C) whose thiol side chain (–SH) forms disulfide bonds, chelates metals, and serves as the catalytic residue in many enzymes; critical for peptide structure and redox biology.

Detailed Explanation

Cysteine is unique among the 20 standard amino acids for its reactive thiol (sulfhydryl) group. Two cysteines can oxidize to form a disulfide bond (–S–S–), a covalent cross-link that stabilizes peptide and protein three-dimensional structures. Insulin has three disulfide bonds, oxytocin has one. Cysteine is also the key residue in glutathione (Glu-Cys-Gly), the body's master antioxidant, where its thiol group donates electrons to neutralize reactive oxygen species. In peptide synthesis, cysteine's reactivity requires careful protecting group strategies to prevent unwanted oxidation. Selenocysteine, the '21st amino acid,' replaces the sulfur with selenium and is found in 25 human selenoproteins.

Key Facts

  • Cysteine is unique among the 20 standard amino acids for its reactive thiol (sulfhydryl) group
  • Two cysteines can oxidize to form a disulfide bond (–S–S–), a covalent cross-link that stabilizes peptide and protein three-dimensional structures
  • Insulin has three disulfide bonds, oxytocin has one
  • Cysteine is also the key residue in glutathione (Glu-Cys-Gly), the body's master antioxidant, where its thiol group donates electrons to neutralize reactive oxygen species
  • In peptide synthesis, cysteine's reactivity requires careful protecting group strategies to prevent unwanted oxidation
Related Terms Disulfide Bond Glutathione Amino Acid Keratin

Part of the PeptideBond.com education network