phenylalanines

Phenylalanine is one of the twenty standard amino acids used by cells to synthesize proteins. In humans, the L-enantiomer (L-phenylalanine) is incorporated into proteins, while the D-enantiomer (D-phenylalanine) is rare in nature but occurs in some microorganisms and certain synthetic peptides. It is an essential amino acid, meaning it must be obtained from the diet.

Chemically, phenylalanine has a nonpolar, aromatic benzyl side chain that contributes to hydrophobic interactions in proteins. As a component of proteins, it helps define three-dimensional structure and function, and it is typically consumed as part of dietary protein.

Biologically, phenylalanine is converted to tyrosine by the enzyme phenylalanine hydroxylase, with tetrahydrobiopterin as a cofactor. Tyrosine then serves as a precursor for several important compounds, including the neurotransmitters dopamine, norepinephrine, and epinephrine; the pigment melanin; and thyroid hormones.

Clinical relevance includes phenylketonuria (PKU), a metabolic disorder caused by impaired phenylalanine hydroxylase activity or cofactor deficiency. Elevated phenylalanine can lead to neurodevelopmental problems if not managed. Management typically involves lifelong dietary restriction of phenylalanine and regular monitoring of levels.

In nutrition and food science, phenylalanine occurs in proteins across plants and animals. It is also a component of the artificial sweetener aspartame, the methyl ester of the dipeptide formed from aspartic acid and phenylalanine. Individuals with PKU must limit phenylalanine intake from all sources, including aspartame-containing products. Phenylalanine residues are widely represented in proteins, underscoring the amino acid’s role in structure and function.