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'''Histidine''' (symbol '''His''' or '''H''')<ref>{{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 [[Amino acid|α-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=J D |last2=Swendseid |first2=M 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 German physician [[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> It is also a [[Precursor (chemistry)|precursor]] to [[histamine]], a vital inflammatory agent in immune responses. The acyl [[radical (chemistry)|radical]] is '''histidyl'''.
==Properties of the imidazole side chain==
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=== Conversion to other biologically active amines ===
* The histidine amino acid is a precursor for [[histamine]], an amine produced in the body necessary for inflammation.<ref>{{Cite journal|last1=Andersen|first1=Hjalte H.|last2=Elberling|first2=Jesper|author-link3=Lars Arendt-Nielsen|last3=Arendt-Nielsen|first3=Lars|date=2015-09-01|title=Human surrogate models of histaminergic and non-histaminergic itch|journal=Acta Dermato-Venereologica|volume=95|issue=7|pages=771–777|doi=10.2340/00015555-2146|issn=1651-2057|pmid=26015312|url=http://vbn.aau.dk/files/219083590/4442_9.pdf|doi-access=free}}</ref>
* The enzyme [[histidine ammonia-lyase]] converts histidine into [[ammonia]] and [[urocanic acid]]. A deficiency in this enzyme is present in the rare metabolic disorder [[histidinemia]], producing [[urocanic aciduria]] as a key diagnostic finding.
* Histidine can be converted to [[3-methylhistidine]], which serves as a [[biomarker (medicine)|biomarker]] for skeletal muscle damage, by certain [[methyltransferase]] enzymes.<ref name="HMDB 3-Methylhistidine">{{cite web | title=3-Methylhistidine | url=http://www.hmdb.ca/metabolites/HMDB00479 | work=HMDB Version 4.0 | publisher=Human Metabolome Database | access-date=25 December 2017 | date=20 December 2017 }}</ref>
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