Substituted amino acids CompoundStructure Alanine (Ala) Serine (Ser) Proline (Pro) Valine (Val) Threonine (Thr) Cysteine (Cys) Isoleucine (Ile) Leucine (Leu) Asparagine (Asn) Aspartic acid (Asp) Glutamine (Gln) Lysine (Lys) Glutamic acid (Glu) Methionine (Met) Histidine (His) Acetylcysteine (NAC) Phenylalanine (Phe) Selenocysteine (Sec) Arginine Theanine Citrulline Tyrosine (Tyr) 3,4-Dihydroxyphenylalanine (L-DOPA) Melevodopa Methyldopa N-Acetyl-L-tyrosine (NALT) Etilevodopa Carbidopa Caramboxin Foslevodopa Saxitoxin Domoic acid Neosaxitoxin Benzylpenicillin Amoxicillin Leu-enkephalin Met-enkephalin Thyroxine (T4) α-Endorphin γ-Endorphin β-Endorphin