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 to glutathione in the liver. Papain and (*) caspases belong to a class of proteases named for this non-alcohol-containing (-5[1])amino acid. Beta-mercaptoethanol (10[1])denatures proteins by cleaving bonds formed between two instances of this amino acid, which are an important part of tertiary structure. (10[1])For 10 points, disulfide bridges form between two instances of what amino acid whose one-letter code is C? ■END■ (10[1])