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Alpha-amino acid with an imidazole side chain (C3H3N2); contains a nitrogen-containing aromatic ring that can exist in two tautomeric forms; pKa of imidazole group is ~6.0, making it important for enzyme catalysis at physiological pH
Precursor for histamine synthesis; essential for metal ion binding in enzymes (zinc, iron, copper); critical component of active sites in many metalloenzymes; maintains pH buffering through imidazole group
Decarboxylation to histamine by histidine decarboxylase; transamination to form imidazole pyruvate; can be converted to glutamate in the liver; involved in one-carbon metabolism via folate
Protein-rich foods: meat (especially pork, poultry), fish, dairy products, soybeans, nuts, seeds, wheat germ, and legumes; highest concentrations in hemoglobin and myoglobin
14 mg/kg body weight/day for adults; approximately 1.0-1.2 g/day for a 70-80 kg adult; no official RDA established historically but recognized as essential since 1975