Histidine

Histidine
	Structure of histidine
Ball-and-stick model	Space-filling model
Names
	IUPAC name Histidine
	Systematic IUPAC name 2-Amino-3-(1H-imidazol-4-yl)propanoic acid
Identifiers
CAS Number	71-00-1 ;
3D model (JSmol)	Interactive image; Zwitterion: Interactive image; Protonated zwitterion: Interactive image;
Beilstein Reference	84088
ChEBI	CHEBI:15971 ;
ChEMBL	ChEMBL17962 ;
ChemSpider	6038 ;
DrugBank	DB00117 ;
ECHA InfoCard	100.000.678
EC Number	200-745-3;
Gmelin Reference	83042
IUPHAR/BPS	3310;
KEGG	D00032 ;
PubChem CID	6274;
UNII	4QD397987E ;
CompTox Dashboard (EPA)	DTXSID9023126 ;
	InChI InChI=1S/C6H9N3O2/c7-5(6(10)11)1-4-2-8-3-9-4/h2-3,5H,1,7H2,(H,8,9)(H,10,11)/t5-/m0/s1 Key: HNDVDQJCIGZPNO-YFKPBYRVSA-N ;
	SMILES O=C([C@H](CC1=CNC=N1)N)O; Zwitterion: O=C([C@H](CC1=CNC=N1)[NH3+])[O-]; Protonated zwitterion: O=C([C@H](CC1=CNC=[NH1+]1)[NH3+])[O-];
Properties
Chemical formula	C6H9N3O2
Molar mass	155.157 g·mol−1
Solubility in water	45.6mg/mL
Hazards
NFPA 704 (fire diamond)	1 1 0
Supplementary data page
	Histidine (data page)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH₃⁺ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO⁻ form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also. It is encoded by the codons CAU and CAC.

Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896. The name stems from its discovery in tissue, from ἱστός histós "tissue". It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.