Phenylalanine is a crystalline, soluble amino acid that is an essential amino acid found in most protein-rich foods. It is one of the twenty common amino acids used by terrestrial living things to form proteins, has the chemical formula HO₂CCH(NH₂)CH₂C₆H₅. The hereditary condition known as phenylketonuria (PKU) stems from an inability to metabolize phenylalanine normally.