The nine essential amino acids cannot be synthesized by the human body and must be obtained from the diet. Histidine contains an imidazole side chain with a pKa near 6.0, allowing it to act as both an acid and base at physiological pH; this unique chemistry makes it critical in metalloenzyme active sites (zinc, iron, copper binding) and in proton shuttling during enzyme catalysis, including in serine proteases such as chymotrypsin and in carbonic anhydrase. Histidine also serves as the precursor for histamine through decarboxylation by histidine decarboxylase (a vitamin B6-dependent reaction), with dietary sources concentrated in pork, poultry, fish, dairy, soybeans, and hemoglobin-rich foods. The estimated requirement is approximately 14 mg/kg body weight daily, and deficiency produces anemia, eczema, immune compromise, and in severe cases cataracts and hearing deficits.
The aromatic amino acids phenylalanine and tryptophan share the distinction of being precursors to critical neurotransmitters. Phenylalanine is hydroxylated by phenylalanine hydroxylase (requiring tetrahydrobiopterin, BH4) to tyrosine, which in turn becomes L-DOPA and then dopamine, norepinephrine, and epinephrine. Phenylalanine also contributes to thyroid hormone synthesis (T3 and T4) and melanin production. The genetic disorder phenylketonuria (PKU) results from phenylalanine hydroxylase deficiency, requiring lifelong restriction to prevent brain damage. Tryptophan is the least abundant amino acid in proteins but is the limiting precursor for serotonin and melatonin synthesis via the rate-limiting tryptophan hydroxylase step. Both amino acids compete with other large neutral amino acids (LNAAs) for transport across the blood-brain barrier via the LAT1 transporter, a competition that significantly affects neurotransmitter production.
The sulfur-containing amino acids methionine and cysteine work together in transsulfuration. Methionine is converted to S-adenosylmethionine (SAMe), the body's universal methyl donor that regulates DNA methylation, phosphatidylcholine synthesis, neurotransmitter production, and creatine synthesis. After donating a methyl group, SAMe becomes S-adenosylhomocysteine and then homocysteine, which must be remethylated to methionine using folate and vitamin B12, or converted to cysteine via the B6-dependent transsulfuration pathway. Cysteine is the rate-limiting substrate for glutathione synthesis, the body's master antioxidant, and it forms the disulfide bonds that give keratin and other proteins their structural strength. Methionine is found at highest concentrations in eggs, fish, and meat, while cysteine is abundant in poultry, pork, and egg whites, and is commonly supplemented as N-acetylcysteine (NAC).
Lysine, threonine, and the branched-chain amino acids (isoleucine, leucine, and valine) round out the essential amino acids. Lysine is required for collagen cross-linking (via hydroxylation to hydroxylysine), carnitine synthesis (transporting fatty acids into mitochondria), and calcium absorption, while competing with arginine for cellular uptake, a relationship exploited clinically for herpes virus management. Threonine contributes to connective tissue, mucin proteins that protect the gastrointestinal lining, and immune function through antibody production. The branched-chain amino acids share a unique catabolic fate in skeletal muscle rather than the liver, making them central to exercise physiology and muscle protein synthesis, a topic covered in detail in a dedicated chapter.