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Histidine
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{{short description|Chemical compound}} {{chembox | Name = Histidine | ImageFile1_Ref = {{chemboximage|correct|??}} | ImageFile1 = Amino Acid Histidine.png | ImageSize1 = 175 | ImageCaption1 = Structure of histidine | ImageFileL2 = Histidine-from-xtal-3D-bs-17.png | ImageSizeL2 = 125 | ImageCaptionL2 = [[Ball-and-stick model]] | ImageFileR2 = Histidine-from-xtal-3D-sf.png | ImageSizeR2 = 125 | ImageCaptionR2 = [[Space-filling model]] | IUPACName = Histidine | OtherNames = | SystematicName = 2-Amino-3-(1''H''-imidazol-4-yl)propanoic acid | Section1 = {{Chembox Identifiers | CASNo = 71-00-1 | CASNo_Ref = {{cascite|correct|CAS}} | Beilstein = 84088 | ChEMBL_Ref = {{ebicite|correct|EBI}} | ChEMBL = 17962 | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 15971 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 6038 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 4QD397987E | EINECS = 200-745-3 | Gmelin = 83042 | IUPHAR_ligand = 3310 | KEGG_Ref = {{keggcite|correct|kegg}} | KEGG = D00032 | SMILES = O=C([C@H](CC1=CNC=N1)N)O | SMILES1 = O=C([C@H](CC1=CNC=N1)[NH3+])[O-] | SMILES1_Comment = [[Zwitterion]] | SMILES2 = O=C([C@H](CC1=CNC=[NH1+]1)[NH3+])[O-] | SMILES2_Comment = Protonated zwitterion | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 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 | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = HNDVDQJCIGZPNO-YFKPBYRVSA-N | PubChem = 6274 | DrugBank_Ref = {{drugbankcite|correct|drugbank}} | DrugBank = DB00117 }} | Section2 = {{Chembox Properties | C=6 | H=9 | N=3 | O=2 | Appearance = | Density = | MeltingPt = | BoilingPt = | Solubility = 45.6mg/mL<ref>{{cite web |title=Histidine (HMDB0000177) |url=https://www.hmdb.ca/metabolites/HMDB0000177 |website=Human Metabolome Database |access-date=22 March 2025 |language=en}}</ref> }} | Section3 = {{Chembox Hazards | NFPA-H = 1 | NFPA-F = 1 | NFPA-R = 0 | MainHazards = | FlashPt = | AutoignitionPt = }} }} [[File:Histidine-spin.gif|thumb|Histidine ball and stick model spinning]] '''Histidine''' (symbol '''His''' or '''H''')<ref name=":7">{{cite web | url = http://www.chem.qmul.ac.uk/iupac/AminoAcid/AA1n2.html | title = Nomenclature and Symbolism for Amino Acids and Peptides | publisher = IUPAC-IUB Joint Commission on Biochemical Nomenclature | year = 1983 | access-date = 5 March 2018 | archive-url = https://web.archive.org/web/20081009023202/http://www.chem.qmul.ac.uk/iupac/AminoAcid/AA1n2.html | archive-date = 9 October 2008 | url-status = dead }}</ref> is an [[essential amino acid]] that is used in the biosynthesis of [[protein]]s. It contains an [[Amine|α-amino group]] (which is in the [[protonated]] –NH<sub>3</sub><sup>+</sup> form under [[Physiological condition|biological conditions]]), a [[carboxylic acid]] group (which is in the deprotonated –COO<sup>−</sup> 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 amino acid|essential]] only for infants, it has now been shown in longer-term studies to be essential for adults also.<ref>{{cite journal |doi=10.1172/JCI108016 |title=Evidence that histidine is an essential amino acid in normal and chronically uremic man |year=1975 |last1=Kopple |first1=Joel D |last2=Swendseid |first2=Marian E |journal=Journal of Clinical Investigation |volume=55 |issue=5 |pages=881–91 |pmid=1123426 |pmc=301830}}</ref> It is [[Genetic code|encoded]] by the [[Genetic code|codons]] CAU and CAC. Histidine was first isolated by [[Albrecht Kossel]] and [[Sven Gustaf Hedin]] in 1896.<ref>{{Cite journal|last1=Vickery|first1=Hubert Bradford|last2=Leavenworth|first2=Charles S.|title=On the Separation of Histidine and Arginine|date=1928-08-01|url=http://www.jbc.org/content/78/3/627.full.pdf|journal=Journal of Biological Chemistry|language=en|volume=78|issue=3|pages=627–635|doi=10.1016/S0021-9258(18)83967-9|issn=0021-9258|doi-access=free}}</ref> The name stems from its discovery in tissue, from {{wikt-lang|grc|ἱστός}} ''histós'' "tissue".<ref name=":7" /> It is also a [[Precursor (chemistry)|precursor]] to [[histamine]], a vital inflammatory agent in immune responses. The acyl [[radical (chemistry)|radical]] is '''histidyl'''.
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