An S-to-N acyl shift occurs in a reaction involving an amine group on this molecule and a thioester, called native chemical ligation. The remarkable stability of cyclotides is due to a motif involving six instances of this molecule that shares its structure with a rotaxane, its namesake “knot.” An N-acetyl derivative of this amino acid is used to treat Tylenol overdose because it is converted (15[1])to glutathione in the liver. Papain and (*) caspases belong to a class of proteases named for this non-alcohol-containing amino acid. Beta-mercaptoethanol denatures proteins by cleaving bonds formed between two instances of this amino acid, which are an important part of tertiary structure. For 10 points, disulfide bridges form between two instances of what amino acid whose one-letter code is C? ■END■ (10[1]0[1])