Phenylketonuria (PKU) is a rare genetic condition that causes an amino acid called phenylalanine to build up in the body. Amino acids are the building blocks of protein. Phenylalanine is found in all proteins and some artificial sweeteners. Your body uses an enzyme called phenylalanine hydroxylase to convert phenylalanine into tyrosine, a nonessential amino acid. Your body needs tyrosine to create neurotransmitters, such as epinephrine, norepinephrine, and dopamine.