Phenylalanine

Phenylalanine is an essential aromatic amino acid required for protein synthesis in humans and many other organisms. It features a benzyl side chain attached to the α-carbon, giving it a nonpolar, hydrophobic character. Its chemical formula is C9H11NO2, and the molecule exists as two enantiomers, L-phenylalanine and D-phenylalanine. In humans, only the L-form is incorporated into proteins.

Phenylalanine is obtained from the diet. Rich sources include meat, fish, eggs, dairy products, legumes and nuts. After absorption, phenylalanine is metabolized through several pathways; the primary route converts it to tyrosine via the enzyme phenylalanine hydroxylase, with tetrahydrobiopterin as a cofactor. Tyrosine then serves as a precursor for catecholamines (dopamine, norepinephrine, epinephrine), melanin, and thyroid hormones.

Clinical significance: Phenylalanine metabolism disorders include phenylketonuria (PKU), caused by deficiency of phenylalanine hydroxylase or related enzymes. Elevated phenylalanine can lead to intellectual disability and other neurologic problems if untreated. PKU is detected by newborn screening, and management relies on a lifelong diet restricting phenylalanine intake with regular monitoring of blood levels. People with PKU must limit phenylalanine-containing foods; products containing aspartame are labeled to indicate phenylalanine content.

Physiologically and industrially, phenylalanine also serves as a precursor to various bioactive compounds and is used in some pharmaceutical and flavor applications, though its role as a protein constituent remains its most relevant biological function.