Histidine
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| Structure of L- histidine, the naturally occurring enantiomer | ||||||||||||||||||||||
| General | ||||||||||||||||||||||
| Surname | Histidine | |||||||||||||||||||||
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| Molecular formula | C 6 H 9 N 3 O 2 | |||||||||||||||||||||
| Brief description |
colorless crystals |
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| properties | ||||||||||||||||||||||
| Molar mass | 155.16 g · mol -1 | |||||||||||||||||||||
| Physical state |
firmly |
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| Melting point |
287 ° C (decomposition) |
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| pK s value |
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| solubility |
poor in water (38.2 g l −1 at 20 ° C) |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||||||||||||||
Histidine , abbreviated to His or H , is a semi-essential , proteinogenic , α- amino acid in its natural L form and was discovered by Albrecht Kossel in 1896 .
Together with the amino acids arginine and lysine, histidine is one of the basic amino acids, which are also known as hexonic bases because of their six carbon atoms . In addition to the obligatory α-amino group, basic amino acids have a further basic group. In histidine, the ring of the heterocyclic amine imidazole is the basic group, which at the same time also determines the aromaticity of the histidine. This means that histidine is one of the aromatic amino acids, as well as phenylalanine , tyrosine and tryptophan .
Isomers
Histidine has a stereocenter, so there are two chiral enantiomers . The L -form [synonym: ( S ) -histidine] occurs naturally as a protein component.
In this article, the physiology information pertains to L- histidine alone . Whenever histidine is mentioned in this text and in the scientific literature without any addition , L- histidine is always meant. Racemic DL -histidine [synonym: ( RS ) -histidine] and enantiomerically pure D -histidine [synonym: ( R ) -histidine] are synthetically accessible and are of little practical importance.
The racemization of L- amino acids can be used for amino acid dating - an age determination for fossil bone material.
| Isomers of histidine | ||
| Surname | L -histidine | D -histidine |
| other names | ( S ) -histidine | ( R ) -histidine |
| Structural formula | |
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| CAS number | 71-00-1 | 351-50-8 |
| 4998-57-6 (racemate) | ||
| EC number | 200-745-3 | 206-513-8 |
| 225-660-9 (racemate) | ||
| ECHA info card | 100,000,678 | 100.005.922 |
| 100.023.328 (racemate) | ||
| PubChem | 6274 | 71083 |
| 773 (racemate) | ||
| DrugBank | DB00117 | - |
| - (racemate) | ||
| Wikidata | Q485277 | Q27077043 |
| Q27103201 (racemate) | ||
properties
The imidazole ring of histidine is subject to tautomerism , more precisely to imine - enamine tautomerism.
This rearrangement is reversible and both tautomers are in equilibrium. Here, the hydrogen atom bound to one of the nitrogen atoms of the ring can switch to the other nitrogen atom. At the same time, the double bond between the two nitrogen atoms in the ring shifts.
An isoelectric point of 7.59 makes histidine a neutral amino acid in the physiological environment; its van der Waals volume is 118, the degree of hydrophobicity −3.2. Histidine forms an orange azo dye with a suitable diazo component and can thus be qualitatively detected using the Pauly reaction .
Occurrence
L- histidine occurs in young plant tissue ( Greek ἱστός: tissue), hence the name is derived from it. L- histidine fulfills an important role as a blood buffer in hemoglobin (see also functions ).
L- histidine is found in foods rich in protein. The following examples give an overview of the histidine content and each relate to 100 g of the food; the percentage of histidine in the total protein is also given:
| Food | protein | Histidine | proportion of |
|---|---|---|---|
| Beef, raw | 21.26 g | 678 mg | 3.2% |
| Chicken breast fillet, raw | 21.23 g | 791 mg | 3.7% |
| Salmon, raw | 20.42 g | 549 mg | 2.7% |
| Chicken egg | 12.57 g | 309 mg | 2.4% |
| Cow's milk, 3.7% fat | 3.28 g | 89 mg | 2.7% |
| Walnuts | 15.23 g | 391 mg | 2.6% |
| Wheat germ, dried | 23.15 g | 643 mg | 2.8% |
| Wholemeal wheat flour | 13.70 g | 317 mg | 2.3% |
| Wholemeal corn flour | 6.93 g | 211 mg | 3.0% |
| Rice, unpeeled | 7.94 g | 202 mg | 2.5% |
| Soybeans, dried | 36.49 g | 1097 mg | 3.0% |
| Peas, dried | 24.55 g | 597 mg | 2.4% |
All of these foods contain almost exclusively chemically bound L- histidine as a protein component, but no free L- histidine.
It is also a component of some medicines and vitamin preparations.
synthesis
In the metabolism, L- histidine is made from phosphoribosyl pyrophosphate (PRPP) and ATP in a sequence of eleven reactions that are catalyzed by eight enzymes , via several intermediate products, etc. a. Imidazole glycerol phosphate, synthesized.
L- histidine is a precursor in the biosynthesis of histamine and carnosine .
Dismantling
For the breakdown including structural formulas, see section Web links
L- histidine can be decarboxylated to the biogenic amine histamine .
The deamination (by the enzyme histidase ) leads to urocanic acid , the further degradation after hydration by urocanase to imidazol-4-one-5-propionic acid. Imidazolone propionase catalyzes its conversion to formiminoglutamate (FIGLU), from which, under the action of the bifunctional enzyme formiminotransferase-cyclodesaminase , L - glutamate is produced, another amino acid.
Functions
The isoelectric point of histidine is in the neutral range. It is therefore the only proteinogenic amino acid that can be both proton donor and proton acceptor under physiological conditions. An example of this is its role in the “ catalytic triad ” (Asp-His-Ser) of serine proteases . In the protein component of the oxygen-transporting blood pigment hemoglobin and the oxygen-storing muscle pigment myoglobin , the “distal” and “proximal” histidine of the peptide chain are of particular importance for the binding site of iron in the prosthetic heme group. Histidine also occurs as a ligand of metal ion complexes of the electron transport chains in the mitochondria ( oxidative phosphorylation ) and in the chloroplasts ( photosynthesis ).
In aqueous solution, histidine protolyzes according to the pH and its pKa values (see figure).
use
Component of infusion solutions for parenteral nutrition, orally for rheumatoid arthritis and against renal anemia .
Web links
Individual evidence
- ↑ a b entry on histidine. In: Römpp Online . Georg Thieme Verlag, accessed on June 21, 2014.
- ↑ a b c F.A. Carey: Organic Chemistry. 5th edition, The McGraw Companies 2001, p. 1059, Link
- ↑ a b c d data sheet histidine (PDF) from Merck , accessed on December 25, 2019.
- ↑ Jan Koolman, Klaus-Heinrich Röhm: Pocket Atlas of Biochemistry . 3. Edition. Georg Thieme Verlag, 2003, ISBN 3-13-759403-0 , p. 60.
- ↑ Hans-Dieter Jakubke, Hans Jeschkeit: Amino acids, peptides, proteins. Verlag Chemie, Weinheim, 62, 1982, ISBN 3-527-25892-2 .
- ↑ PM Hardy: The Protein Amino Acids. In: GC Barrett (Ed.): Chemistry and Biochemistry of the Amino Acids. Chapman and Hall, 1985, ISBN 0-412-23410-6 , p. 9.
- ↑ Student assignment: CHEMKON 3/2018 . In: CHEMKON . tape 25 , no. 3 , June 2018, p. 121–122 , doi : 10.1002 / ckon.201880371 .
- ↑ nutrient database of the US Department of Agriculture , 22nd edition.
- ↑ S. Ebel and HJ Roth (eds.): Lexikon der Pharmazie. Georg Thieme Verlag, 1987, ISBN 3-13-672201-9 , p. 66.