Aromatic amino acids are precursors of many compounds in plants and animals. They include:

Plants - Lignin, tannins, and pigments, flavor components of spices (cinnamon oil, wintergreen oil, bitter almond, nutmeg, cayenne pepper, vanilla bean, clove, and ginger) are derived from coniferyl alcohol. Coniferyl alcohol, in turn, is derived from phenylalanine and tyrosine. Phenylalanine is also a precursor of plant pigments and related polyphenolic compounds called flavonoids. The biosynthetic scheme leads to a class of flavonoids called anthocyanins, which are common flower pigments.. An offshoot of this pathway leads to the synthesis of cocaine.

Finally, tryptophan is used for the synthesis of indole-3-acetic acid (see here), a plant hormone also known as auxin.

Animals - Animal cells do not synthesize aromatic rings. Iinstead, animal cells extensively modify amino acids with aromatic rings. Examples include the synthesis of tyrosine from phenylalanine, the synthesis of pigments and hormones from tyrosine, and the use of tyrosine, tryptophan, and histidine in synthesis of biogenic amines - compounds that serve as hormones and neurotransmitters.

Tyrosine biosynthesis (Figure 21.18) - Tyrosine is the only aromatic amino acid made in animals. The reaction is catalyzed by phenylalanine hydroxylase, a mixed-function oxidase using tetrahydrobiopterin, a pteridine co-factor.

Phenylalanine + Dihydrobiopterin + O2 <=> Tyrosine + Tetrahydrobiopterin + H2O

Deficiency of phenylalanine hydroxylase is responsible for phenylketonuria (PKU), an autosomal recessive disease that results in the accumulation of too much phenylalanine, because the synthesis of tyrosine is blocked.

Tyrosine utilization - Tyrosine serves as a precursor to thyroid hormones, melanins (biological pigments - Figure 21.20), and catecholamines (hormones and neurotransmitters). Thyroxine and triiodothyronine are thyroid hormones made by modifying tyrosine residues in the protein, thyroglobulin (Figure 21.19). Degradation of thyroglobulin yields free hormones. Synthesis occurs in the thyroid gland, which concentrates iodide from the blood. Melanin synthesis occurs in melanocytes (Figure 21.20). An individual's skin color is determined by the relative amounts of red and black melanins in the skin.

Tyrosine Catabolism - The catabolism of tyrosine to fumarate and acetoacetate is depicted in Figure 21.21. A hereditary deficiency of the enzyme homogentisic acid dioxygenase causes a disease called alkaptonuria where homogentisic acid accumulates and is excreted in large amounts in the urine. It oxidizes on standing, causing the urine to become dark.

Tryptophan - (Figure 21.22) - Tryptophan is catabolized to either glutaryl-CoA and acetoacetyl-CoA (major route) or NAD⁺. NAD⁺ can also be made from nicotinic acid (Table 11.5).

Histidine - Decarboxylation of histidine yields histamine (see here). In the stomach, histamine promotes secretion of hydrochloric acid and pepsin as digestion aids. Histamine is a potent vasodilator, released at sites of trauma, inflammation, or allergic reaction. Reddening of inflamed tissues is a result of local enlargement of blood capillaries. Antihistamines block binding of histamine to its receptors. Figure 21.23 shows that histidine is catabolized to glutamate.