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[Biochemistry](https://durgical.com/en/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](https://pubchem.ncbi.nlm.nih.gov/compound/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

- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)

## Related

- [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Oxygen binding: haemoglobin versus myoglobin](https://durgical.com/en/biochemistry/topics/oxygen-binding-haemoglobin-versus-myoglobin/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [Folate (vitamin B9)](https://durgical.com/en/biochemistry/molecules/folate/)

## 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.

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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)](https://pubchem.ncbi.nlm.nih.gov/compound/6274) (PubChem, US National Library of Medicine, 2026); [Amino acids: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002222.htm) (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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