Question

Which enzyme is deficient in patients with Phenylketonuria (PKU)?

• A. Tyrosinase
• B. Phenylalanine hydroxylase
• C. Amylase
• D. Lactase

Hint:

PKU Trick: Phenylalanine $\rightarrow$ Tyrosine conversion fails due to lack of Phenylalanine hydroxylase.

Answer 1

The Correct Option is B

Concept: Phenylketonuria (PKU) is a genetic metabolic disorder caused by the deficiency of the enzyme phenylalanine hydroxylase (PAH). This enzyme is responsible for converting the amino acid phenylalanine into another amino acid called tyrosine. When this enzyme is absent or deficient, phenylalanine accumulates in the body and is converted into harmful compounds called phenylketones, which can damage the brain and nervous system.

Step 1: Normal metabolic pathway. In a healthy individual: \[ \text{Phenylalanine} \xrightarrow{\text{Phenylalanine hydroxylase}} \text{Tyrosine} \] Tyrosine is then used in the synthesis of several important molecules such as melanin, dopamine, epinephrine, and norepinephrine.

Step 2: What happens in PKU. In patients with PKU:
• Phenylalanine cannot be converted to tyrosine.
• Phenylalanine accumulates in blood and tissues.
• Toxic metabolites (phenylketones) are formed. This accumulation can lead to:
• Intellectual disability
• Developmental delay
• Seizures
• Behavioral problems

Step 3: Treatment approach. PKU is usually managed by maintaining a low-phenylalanine diet, especially during early childhood, to prevent neurological damage. Therefore, the enzyme deficient in PKU is phenylalanine hydroxylase (PAH).