An S-to-N acyl shift occurs in a reaction involving an amine group on this molecule and a thioester, called native chemical ligation. (-5[1])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 (15[1]-5[2])of this (15[1])amino acid (-5[1])is used to treat Tylenol (15[1]-5[1])overdose (15[1])because it is converted (15[1])to glutathione in the liver. Papain and (*) caspases belong to a class of proteases (10[1])named for (-5[1])this (-5[2])non-alcohol-containing (10[1]-5[2])amino acid. Beta-mercaptoethanol (10[2])denatures proteins by cleaving bonds (10[1])formed between (10[1])two instances (10[1])of this amino acid, (-5[1])which (10[1])are an important part (10[1])of tertiary structure. (10[1])For (10[1])10 points, disulfide (10[1])bridges (10[2])form (10[3])between two instances (10[1])of (10[1])what amino (10[1])acid (10[1])whose one-letter code is C? (10[2])■END■ (10[15]0[8])