Histidine

Histidine
Systematic (IUPAC) name
2-amino-3-(3H-imidazol-4-yl)propanoic acid
Identifiers
CAS number	71-00-1
PubChem	773
Chemical data
Formula	C6H9N3O2
Mol. weight	155.16
SMILES	N[C@@H](Cc1[nH]cnc1)C(O)=O
Complete data

The histidine bound heme group of succinate dehydrogenase, an electron carrier in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. From PDB 1YQ3.

Histidine (His, H) is one of the 20 most common natural amino acids present in proteins. In the nutritional sense, in humans, histidine is considered an essential amino acid, but mostly only in children.

Chemical properties

The imidazole side chains and the relatively neutral pKa of histidine (ca 6.0) mean that relatively small shifts in cellular pH will change its charge. For this reason, this amino acid side chain finds its way into considerable use as a co-ordinating ligand in metalloproteins, and also as a catalytic site in certain enzymes. The imidazole side chain has two nitrogens with different properties: One is bound to hydrogen and donates its lone pair to the aromatic ring and as such is slighty acidic, whereas the other one donates only one electron pair to the ring so it has a free lone pair and is basic. These properties are exploited in different ways in proteins. In catalytic triads, the basic nitrogen of histidine is used to abstract a proton from serine, threonine or cysteine to activate it as a nucleophile. In a histidine proton shuttle, histidine is used to quickly shuttle protons, it can do this by abstracting a proton with its basic nitrogen to make a positively-charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen. In carbonic anhydrases, a histidine proton shuttle is utilized to rapidly shuttle protons away from a zinc-bound water molecule to quickly regenerate the active form of the enzyme.

Metabolism

The amino acid is a precursor for histamine and carnosine biosynthesis.

Conversion of histidine to histamine by histidine decarboxylase

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.

Sources

Histidine is found in fruits such as bananas and grapes, meat and poultry, and milk and milk products. It is also found in root vegetables and all green vegetables, though in lesser quantities.

Forms

There are two enantiomers: D-histidine and L-histidine.

History

Histidine was first isolated in 1896 by German physician Albrecht Kossel.

Additional images

• 

  Histidine

External links

• Histidine biosynthesis (early stages)
• Histidine biosynthesis (later stages)
• Histidine catabolism
• Computational Chemistry Wiki

Template:ChemicalSources

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Histidine  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Histidine"

Note: Some restrictions may apply to use of individual images which are separately licensed.