Normal phenylalanine catabolism: At the first committed step of phenylalanine breakdown in humans, phenylalanine is initially converted to which compound?

Introduction / Context:
Understanding the normal pathway of phenylalanine catabolism clarifies multiple inherited disorders, including PKU and tyrosinemias. The very first step sets the stage for subsequent ring-cleavage reactions that eventually yield both glucogenic and ketogenic end products.

Given Data / Assumptions:

• Phenylalanine hydroxylase in the liver converts phenylalanine to tyrosine.
• This step requires the cofactor tetrahydrobiopterin, molecular oxygen, and iron.
• Downstream, tyrosine is further degraded toward fumarate (glucogenic) and acetoacetate (ketogenic).

Concept / Approach:
Identify the immediate product of the PAH-catalyzed reaction. The initial hydroxylation adds a hydroxyl group to the phenyl ring, producing tyrosine (p-hydroxyphenylalanine). Only afterward do transamination and ring-cleavage steps generate keto-acids and TCA intermediates. Therefore, tyrosine is the correct first product, not fumarate or phenylpyruvate.

Step-by-Step Solution:

Verification / Alternative check:
Deficiency of phenylalanine hydroxylase leads to PKU with elevated phenylalanine and low/normal tyrosine, reinforcing that tyrosine is the normal first product in phenylalanine breakdown.

Why Other Options Are Wrong:

• Fumarate: a later product after multiple steps.
• Phenylpyruvate: an abnormal side product seen in PKU, not the normal first step.
• Lysine and acetoacetate: not immediate products of the initial reaction.

Common Pitfalls:
Confusing the normal initial step with the alternative pathway active in PKU; remember that phenylpyruvate appears only when hydroxylation fails.

Final Answer:
Tyrosine