In phenylketonuria, tyrosine becomes an essential amino acid due to deficiency in which enzyme?

• A. An Increase in Tyrosine Hydroxylase Enzyme
• B. A Deficiency in Phenylalanine Hydroxylase Enzyme
• C. An Increase in Phenylalanine Hydroxylase Enzyme
• D. A Decrease in Tyrosine Hydroxylase Enzyme

Answer: (B)

Phenylalanine hydroxylase normally converts phenylalanine to tyrosine. When this enzyme is deficient in phenylketonuria, phenylalanine cannot be effectively metabolized and tyrosine cannot be synthesized in adequate amounts. Therefore, tyrosine becomes an essential amino acid because the body can no longer make enough of it from phenylalanine. This is why the key mechanism is a deficiency of phenylalanine hydroxylase. The other scenarios involving changes in tyrosine hydroxylase or increases in phenylalanine hydroxylase do not explain why tyrosine becomes essential in PKU.