histidine

 

(hĭs'tĭdēn) , organic compound, one of the 22 α-amino acids commonly found in animal proteins. Only the L-stereoisomer appears in mammalian protein. Histidine is the direct precursor of histamine; it is also an important source of carbon atoms in the synthesis of purines. The imidazole group on the side chain of histidine can act as both an acid and a base, i.e., it can both donate and accept protons under some conditions. This turns out to be an important property when histidine is incorporated into proteins, particularly when it becomes a part of the primary structure of some enzymes. It is thought that the side chain of this amino acid acts as a general acid and base as it participates in the catalytic functions of chymotrypsin, as well as those of a number of enzymes dealing with the metabolism of carbohydrates, proteins, and nucleic acids. It has even been implicated in the workings of cocoonase, the enzyme that allows adult silk moths to escape from their cocoons. Histidine is considered to be an essential amino acid for infants (it must be supplied in the diet); experiments with adults indicate that they can go for at least short periods without dietary intake of this amino acid. It was isolated from protein in 1896; its structure was confirmed by chemical synthesis in 1911.

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
Molar mass	155.16 g/mol
SMILES	N[C@@H](Cc1[nH]cnc1)C(O)=O
Complete data

Histidine (abbreviated as His or H)^[1] 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. Its codons are CAU and CAC.

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

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 coordinating 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.

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.

Additional images

Histidine

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.

References

External links

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

Major families of biochemicals
Peptides | Amino acids | Nucleic acids | Carbohydrates | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides
Analogues of nucleic acids:	The 20 Common Amino Acids	Analogues of nucleic acids:
Alanine (dp) | Arginine (dp) | Asparagine (dp) | Aspartic acid (dp) | Cysteine (dp) | Glutamic acid (dp) | Glutamine (dp) | Glycine (dp) | Histidine (dp) | Isoleucine (dp) | Leucine (dp) | Lysine (dp) | Methionine (dp) | Phenylalanine (dp) | Proline (dp) | Serine (dp) | Threonine (dp) | Tryptophan (dp) | Tyrosine (dp) | Valine (dp)

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