Histidine
- (2S) - (-) -histidine
- L-histidine
- (R) - ( ) -histidine
- D -histidine
- (S )-2- amino-3- (1H -imidazol- 4-yl ) propanoic acid
- Imidazolalanin
- Abbreviations: His ( Three letter code)
- H ( single letter code )
- 71-00-1 (L- enantiomer)
- 351-50-8 (D - enantiomer)
- 4998-57-6 (DL- histidine)
V06DE00
Colorless crystals
Fixed
287 ° C ( decomposition> )
- PKCOOH: 1.82
- PKNH2: 9.17
- PKSeitenkette: 6.00
Poorly in water ( 38.2 g · l-1 at 20 ° C)
5110 mg · kg -1 ( LD50, rat, oral)
Template: Infobox chemical / molecular formula search available
Histidine, abbreviated His or H is a semi-essential proteinogenic α - amino acid in its natural L-form.
Together with arginine and lysine it is one of the basic amino acids or hexon bases, together with phenylalanine, tyrosine and tryptophan to the aromatic amino acids. Basic amino acids in addition to the obligatory α -amino group have a more basic group. In histidine is the imidazole ring, the same conditions the aromaticity of histidine.
In this article, the information concerning the physiology alone, L- histidine [ Synonyms: ( S)- histidine ]. If it is mentioned in this text and in the scientific literature without any addition " histidine " is always meant L- histidine. Racemic DL -histidine [ Synonym: ( RS)- histidine ] and enantiomerically pure D -histidine [ Synonym: (R ) -histidine ] are synthetically accessible and only have little practical significance. The racemization of L- amino acids can be used for amino acid dating - are used - an age determination for fossil bone material.
Properties
The imidazole ring undergoes tautomerism, more precisely an imine -enamine tautomerism. The hydrogen atom attached to one of the nitrogen atoms of the ring can move to another nitrogen atom. At the same time the double bond between two nitrogen atoms in the ring moves. This reaction is reversible and both tautomers are in equilibrium. In the structural formula to the right, only one tautomer is depicted.
- Isoelectric point: 7.59
- Van der Waals volume: 118
- Hydrophobicity: -3.2
Occurrence
L- histidine comes in young plant tissues ( gr ἱστός: tissue ), therefore also the name derives from. L- histidine has an important role as a blood buffer in hemoglobin ( see also functions).
L- histidine is present in protein-rich foods. The following examples provide an overview of Histidingehalte and are each based on 100 g of food, in addition, the percentage of histidine is indicated at the total protein:
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 supplements.
Synthesis
In the metabolism of L-histidine is from phosphoribosyl pyrophosphate ( PRPP ) and ATP in a series of eleven reactions that are catalyzed by enzymes, eight, via multiple intermediates, including Imidazolglycerinphosphat synthesized.
L-Histidine is a precursor in the biosynthesis of histamine and carnosine.
Reduction
For the reduction, including structural formulas, see section External links
L- histidine can be either decarboxylated to biogenic amine histamine or completely degraded to L- glutamate. The enzyme urocanase is involved.
Functions
The isoelectric point of histidine is in the neutral range. Therefore, it is the only proteinogenic amino acid, which may be under physiological conditions, both proton and proton acceptor. As an example, there is his role in the " catalytic triad " (Asp -His- Ser) of serine proteases. From functional importance and the " distal " and are " proximal " histidine (parts of the iron - binding square ) in the blood pigment hemoglobin and the muscle pigment myoglobin. It also has an important meaning as the ligand of metal ion complexes of the electron transport chain in mitochondria (oxidative phosphorylation ) and the chloroplasts (photosynthesis ).
In aqueous solution, histidine protolyzed as shown in the figure according to the pH and its pKa values .
Use
Element of infusion solutions for parenteral nutrition, perorally with rheumatoid arthritis and renal anemia.