Biochemistry · Molecule
Histidine !
Histidine is an essential amino acid with an imidazole ring that gains or loses protons at physiological pH: it buffers and takes part in catalysis.
Updated 2026-11-01
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Key facts
- A basic, essential amino acid; its side chain (imidazole) has a pKa ≈ 6, the only one close to blood pH.
- Haemoglobin histidines buffer H⁺ in the red cell; the proximal histidine (F8) binds the haem iron.
- Part of many active sites: the Ser–His–Asp catalytic triad of serine proteases (trypsin, elastase, thrombin).
- Decarboxylation (histidine decarboxylase, with vitamin B6) gives histamine.
- Also called
- L-histidine, His, H
- Formula
- C6H9N3O2
- PubChem
- CID 6274
Structure
- An amino acid backbone (α carbon, amino, carboxyl) with a –CH₂– linked to a five-membered imidazole ring containing two nitrogens.
- The imidazole can be protonated (positive) or neutral depending on pH, and binds metals such as iron, zinc and copper.
Clinical notes
- In folate deficiency, histidine breakdown stops at FIGLU (formiminoglutamate), which is lost in urine.
- Histamine derived from it mediates allergy, gastric acid secretion (H2 receptors) and neurotransmission.
- It is essential in the diet, especially for infants.
Related conditions
Related
- Amino acids: structure, charge and pKa
- Hemoglobin and 2,3-BPG
- Oxygen binding: haemoglobin versus myoglobin
- Molecules derived from amino acids
- Folate (vitamin B9)
Test yourself
Think of the answer, then tap the question to see it.
Why is histidine a good buffer at pH 7.4?
Its imidazole pKa (≈ 6) is close to physiological pH, so it can accept or give up protons.
Which molecule is formed by decarboxylating histidine?
Histamine, by vitamin B6–dependent histidine decarboxylase.
Which histidine metabolite builds up in folate deficiency?
FIGLU (formiminoglutamate), because its conversion to glutamate needs tetrahydrofolate.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: L-Histidine (CID 6274) (PubChem, US National Library of Medicine, 2026); Amino acids: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.