amino acids


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Basic structure of α-amino acids
(residue R is an H atom in the case of glycine )

Amino acids ( AS ), uncommonly also aminocarboxylic acids , out of date called amido acids , are chemical compounds with an amino group containing nitrogen (N) and a carboxylic acid group containing carbon (C) and oxygen (O) . Amino acids are found in all living things . They are the building blocks of proteins (protein) and are released when proteins are broken down ( proteolysis ). Essential amino acids an organism cannot produce it itself, so they must be taken in with food.

The class of amino acids includes organic compounds that contain at least one amino group (-NH 2 or substituted -NR 2 ) and one carboxy group  (-COOH) as functional groups , that is, have structural features of amines and carboxylic acids . Chemically, they can be differentiated according to the position of their amino group to the carboxy group - if the amino group on the C α -atom is immediately adjacent to the terminal carboxy group, this is called α-position and speaks of α-amino acids .

Selected α-amino acids are the natural building blocks of proteins. They are linked together to form chains by the carboxy group of one amino acid forming a peptide bond with the amino group of the next . The amino acids linked in this way to form a polymer differ in their side chains and together determine the shape with which the polypeptide then unfolds in an aqueous environment to form the native protein . This biosynthesis of proteins takes place in all cells on the ribosomes according to genetic information that is available in the form of mRNA .

The base sequence of the mRNA encodes the amino acid sequence in triplets , each base triplet representing a codon that stands for a specific proteinogenic amino acid . The amino acids specified as building blocks for the formation of proteins in a certain order form the proteins. In humans there are 21 different proteinogenic amino acids. After translation , the side chains of some of the amino acids built into the protein can still be modified .

The spectrum of amino acids goes well beyond these twenty proteinogenic ones . So far, over 400 non-proteinogenic naturally occurring amino acids are known that have biological functions. The comparatively rare D amino acids represent a special group. The variety of synthetically produced and that of theoretically possible amino acids is considerably greater.

Some amino acids play a special role as neurotransmitters , as do various breakdown products of amino acids; Biogenic amines not only act as messenger substances in the nervous system, but also develop diverse physiological effects in the organism as hormones and tissue mediators .

The simplest amino acid, glycine , could not only be detected on earth , but also on comets , meteorites and in gas clouds in interstellar space .

history

The first amino acid was isolated from the juice of asparagus ( Asparagus officinalis ) in the Paris laboratory of Louis-Nicolas Vauquelin and his student Pierre Jean Robiquet in 1805 and was then named asparagine . As the last of the usual protein-building amino acids, threonine was discovered in fibrin in 1931 and its structure was clarified by William Rose in 1935 . Through experiments with various feeds, Rose had found that the 19 amino acids that had been discovered so far were not sufficient as an additive. He also determined the essentiality of other amino acids and determined the minimum daily dose required for optimal growth .

In the period between 1805 and 1935, many of the chemists and pharmacists known at the time were involved in isolating amino acids for the first time and clarifying their structure. Emil Fischer , to whom the Fischer projection also goes back, succeeded in finally elucidating the structure of serine (1901), lysine (1902), valine (1906) and cysteine (1908). Even Albrecht Kossel (1896 histidine from Störsperma), Richard Willstätter (1900 proline via synthesis) and Frederick Hopkins (1901 tryptophan from casein ) were Nobel laureates later. The German chemist Ernst Schulze isolated three amino acids for the first time - glutamine from beets in 1877 , phenylalanine in 1881 and arginine from lupins in 1886 - and was involved in the structural analysis of other amino acids. Heinrich Ritthausen had previously obtained crystalline glutamic acid from grain protein , gluten , in 1866 . In 1872 Wilhelm Dittmar clarified the structure of glutamine and glutamic acid, the salts of which are glutamates .

Simplified structural formulas of 20 natural α-amino acids, without configuration information

Already in 1810 William Hyde Wollaston discovered the sulfur-containing cystine as "cystic oxide" in bladder stones , but it was not until 1884 that Eugen Baumann discovered the monomeric cysteine . In 1819 Henri Braconnot separated the glycine from glue and Joseph Louis Proust separated the leucine from grain. Eugen von Gorup-Besánez isolated the valine from pancreatic juice in 1856 . Justus von Liebig was able to separate the tyrosine from casein for the first time in 1846 , the structure of which Ludwig von Barth clarified in 1869 . In the hydrolyzate of casein, Edmund Drechsel discovered lysine in 1889 and later John Howard Mueller discovered sulphurous methionine in 1922 as the 19th amino acid, the structural formula of which George Barger and Philip Coine stated in 1928. In molasses had Felix Ehrlich in 1903 as 18, the isoleucine found, a structural isomer of leucine.

Friedrich Wöhler , whose syntheses opened up the field of biochemistry in the 1820s , did not discover any amino acids, but three of his students were involved, along with the aforementioned Gorup-Besánez and Schulze, Georg Städeler (1863 serine from raw silk). 18 of the 20 discovered amino acids were isolated from plant or animal material, only the two amino acids alanine (1850 Adolph Strecker ) and proline (Willstätter) were obtained through organic synthesis. While the analysis of the material composition up to the molecular formula was easy to accomplish with the methods of the time, the structural formula of many amino acids could often only be finally clarified through partial steps of the synthesis, which sometimes only succeeded years later. The structure of asparagine and of aspartic cleared Hermann Kolbe until 1862, 57 years after the first description.

Amino acids owe their generic names to two functional groups, their individual names sometimes due to a light appearance (e.g. arginine , leucine ), a sweet taste (e.g. glycine ) or the material in which they were found (e.g. asparagine , Cysteine , serine , tyrosine ), characteristics of the chemical structure (e.g. proline , valine , isoleucine ) or both (e.g. glutamine , glutamic acid ) and sometimes the starting materials of their synthesis (e.g. alanine ).

That proteins are built up as chains of amino acids, linked by peptide bonds, was first proposed in 1902 at the meeting of German naturalists and doctors in Karlsbad, simultaneously and independently from both Emil Fischer and Franz Hofmeister (Hofmeister-Fischer theory).

structure

Carbamic acid
Carbamic acid

Amino acids consist of at least two carbon atoms. The unstable carbamic acid has only one carbon atom and is therefore not an amino acid, but a carbonic acid amide. Amino acids can be divided into classes depending on the carbon atom on which the amino group is located relative to the carboxy group. If several amino groups are represented in the molecule , the carbon atom whose amino group is closest to the carboxy carbon determines which class of amino acids it is.

General structure of
amino acids (R: side chain)
Alpha-Amino Acid Formula V2.svg
α-amino acid
Beta-Amino Acid Formula V3.svg
β-amino acid
Gamma-Amino Acid Formula V2.svg
γ-amino acid
  • α-Amino acids: The amino group of the α-amino acids is on the second carbon atom, including the carboxy carbon atom. The count always starts with the carboxy carbon. The IUPAC name is therefore 2-aminocarboxylic acids. The simplest representative of the α-amino acids is the proteinogenic amino acid glycine . All proteinogenic amino acids are α-amino acids.
The term amino acids often refers to a certain group of α-amino acids, which mainly consists of L- α-amino acids: the proteinogenic amino acids . These are the building blocks of all proteins of all life on earth and, in addition to the nucleic acids, basic building blocks of life .
  • β-amino acids: The amino group of the β-amino acids is located on the third carbon atom (including the carboxy carbon atom). The IUPAC name is 3-aminocarboxylic acids. The simplest representative is β-alanine .
  • γ-amino acids: The amino group of the γ-amino acids is located on the fourth carbon atom (including the carboxy carbon atom). The IUPAC name is 4-aminocarboxylic acids. The simplest representative is γ-aminobutyric acid (GABA).

The designation of further classes of amino acids follows the same scheme.

The amino acids of a class are distinguished by their side chain R . If the side chain R is different from the other substituents located on the carbon with the amino group, then there is a stereocenter here and there are two enantiomers of the corresponding amino acid . If the side chain R itself contains further stereocenters, diastereomers also result and the number of possible stereoisomers increases correspondingly to the number of further stereocenters. There are four stereoisomers of amino acids with two differently substituted stereocenters .

Aminoacyl group

Reaction of glycine

Aminoacyl group formed from the amino acid glycine . R here denotes a radical to which the aminoacyl group is attached; for example, a transfer RNA ( tRNA ) is loaded into aminoacyl-tRNA .
Reaction of (S) -glutamine

Aminoacyl group formed from the amino acid L - glutamine . R here denotes a radical to which the aminoacyl group is attached; for example, a transfer RNA ( tRNA ) is loaded into aminoacyl-tRNA .

Aminoacyl group refers to the monovalent group that is created from an amino acid by removing the hydroxyl (-OH) from the carboxyl group (-COOH), i.e. the univalent radical. An α-aminoacyl group is thus formed from an α-amino acid; from the amino acid tyrosine, for example, the tyrosyl group is created as a special α-aminoacyl group.

Proteinogenic amino acids

Amino acids are referred to as proteinogenic which are used in living organisms as building blocks of proteins during translation according to genetic information. In the biosynthesis of proteins, which takes place on the ribosomes of a cell, selected amino acids are linked by peptide bonds in a certain order to form the polypeptide chain of a protein in the course of protein biosynthesis . The amino acid sequence of the ribosomally formed peptide is predetermined by the genetic information contained in the base sequence of a nucleic acid , with an amino acid being encoded by a base triplet according to the genetic code .

Proteinogenic amino acid: L-proline

L- proline
(proteinogenic amino acid)
Non-proteinogenic amino acid: D-proline

D- proline
(non-proteinogenic amino acid)

The proteinogenic amino acids are always α-amino acids. Except for the smallest, glycine , they are chiral and occur with a special spatial arrangement. The amino acid proline has a special feature, the amino group of which has a secondary amine and which therefore does not fit into a protein fold as flexibly as other proteinogenic amino acids - proline, for example, is regarded as a helix breaker in α-helical structures in proteins. Due to the secondary amino group, proline is also referred to as a secondary amino acid - often incorrectly or out of date also as an imino acid .

Of the mirror- inverted enantiomers , only the L amino acids are proteinogenic (for D  /  L nomenclature see Fischer projection ; in cases such as hydroxyproline there are further stereoisomers ). The molecular components of the cellular apparatus necessary to build proteins - in addition to ribosomes, tRNAs and aminoacyl-tRNA synthetases , which are loaded with amino acids - are themselves also chiral and only recognize the L variant.

Nevertheless, D- amino acids also occur occasionally in living things . However, these are synthesized independently of proteinogenic metabolic pathways and are not used for the ribosomal structure of proteins. For example, D - alanine is incorporated into peptidoglycans of the bacterial cell wall or D - valine is incorporated into bacterial cyclo- depsipeptides such as valinomycin . Different types of archaea , bacteria , fungi and nudibranchs have multi - enzyme complexes called non- ribosomal peptide synthetases with which such (non-proteinogenic) amino acids can be incorporated into a non- ribosomal peptide .

Canonical amino acids

For 20 of the proteinogenic amino acids are found codons in the (most frequently used) Standard Version of the genetic code. These are therefore called standard amino acids or canonical amino acids .

In amino acid sequences , the amino acids are usually given with a name abbreviation in a three-letter code or represented in the one -letter code by a symbol.

The 20 canonical amino acids
amino acid Acyl
group
eat-
tially?
Ø in
proteins
Surname Abbr. symbol
Alanine Ala A. Alanyl No 9.0%
Arginine Arg R. Arginyl semi 4.7%
Asparagine Asn N Asparaginyl No 4.4%
Aspartic acid Asp D. α-aspartyl No 5.5%
Cysteine Cys C. Cysteinyl   no  * 2.8%
Glutamine Gln Q Glutaminyl No 3.9%
Glutamic acid Glu E. α-glutamyl No 6.2%
Glycine Gly G Glycylic No 7.5%
Histidine His H Histidyl semi 2.1%
Isoleucine Ile I. Isoleucylic Yes 4.6%
Leucine Leu L. Leucylic Yes 7.5%
Lysine Lys K Lysyl Yes 7.0%
Methionine Mead M. Methionyl Yes 1.7%
Phenylalanine Phe F. Phenylalanyl Yes 3.5%
Proline Per P Prolyl No 4.6%
Serine Ser S. Seryl No 7.1%
Threonine Thr T Threonyl Yes 6.0%
Tryptophan Trp W. Tryptophyl Yes 1.1%
Tyrosine Tyr Y Tyrosyl   no  * 3.5%
Valine Val V Valyl Yes 6.9%
* Essential for children and pregnant women.

In addition to the codes given above, additional characters are used as placeholders if the precise amino acid cannot be inferred from protein sequencing or X-ray structure analysis.

Possible amino acids Abbr. symbol
Asparagine or aspartic acid Asx B.
Glutamine or glutamic acid Glx Z
Leucine or isoleucine Xle J
unknown amino acid Xaa (rarely Unk) X

Non-canonical amino acids

In addition to the canonical amino acids, the naturally occurring amino acids include the other amino acids called non-canonical amino acids, including proteinogenic and non-proteinogenic amino acids. Several groups can be distinguished:

L-selenocysteine
L-pyrrolysine
  • The first group includes those proteinogenic amino acids that are incorporated into proteins through recoding of the genetic material. The 21st and 22nd proteinogenic amino acids belong to this: selenocysteine (in eukaryotes and some bacteria and archaea ) and pyrrolysine (in some bacteria and archaea). Specific tRNAs - tRNA Sec and tRNA Pyl - were found for both amino acids, which enable incorporation into the ribosome during translation. Their anticodon pairs, depending on structural elements in the context of the mRNA (see Secis ), with the codon UGA or UAG ; in the standard code these represent a stop codon . However, not all organisms use the non-canonical proteinogenic amino acids of this group.
amino acid Abbr. symbol
Pyrrolysine Pyl O
Selenocysteine Sec U
L - N -formyl methionine
The usual start codon AUG codes for the amino acid methionine . In addition to the tRNA Met, bacteria have a special tRNA fMet , which is also loaded with methionine and serves as an initiator tRNA . However , the amino acid bound to tRNA i fMet is formylated in bacteria at the N terminus to N -formylmethionine (fMet) before it can become the first amino acid of a peptide chain during initiation on the ribosome. This amino acid derivative formylmethionine is therefore occasionally also counted as (23rd) proteinogenic amino acid. Also, mitochondria and chloroplasts use fMet initial. In contrast, it is not used in translation in the cytosol of eukaryotic cells and in archaea .
  • The third group, strictly speaking, is non-proteinogenic amino acids, which the organism cannot distinguish from the canonical amino acids and which it incorporates into proteins non-specifically instead. This includes selenomethionine , which can be incorporated in place of methionine , or canavanine , which the organism cannot distinguish from arginine , or azetidine-2-carboxylic acid , which acts as a poisonous proline analogue. Many of the amino acids in this group are toxic as they often lead to misfolding of the protein, which can impair the shape and thus the functionality of the protein. Azetidine-2-carboxylic acid is a toxic component of the lily of the valley , whereby the lily of the valley protects itself against the uncontrolled incorporation of this amino acid into its proteins with a highly specific prolyl tRNA synthetase .

In addition to the canonical 20, humans also use selenocysteine as a proteinogenic amino acid. Of the 20 canonical amino acids, 12 are synthesized by the human organism or by microorganisms living in the human digestive tract. The remaining 8 amino acids are essential for humans, which means that they have to be ingested through food.

The incorporation of artificial, almost arbitrarily constructed amino acids in the course of a protein design is possible, among other things, by replacing the ligand in the corresponding aminoacyl-tRNA synthetase . These methods are partially advanced so far that this target specific proteins, a marker can be obtained, for example, the protein by treatment with specific reagents for fluorescence stimulate (for example, incorporation of norbornene-amino acid via pyrrolysyl-tRNA synthetase / codon CUA). This enables the protein to be precisely localized even without production and reaction with antibodies .

Biochemical significance

Amino acids as building blocks of proteins

The 20 naturally occurring proteinogenic standard amino acids, grouped according to their physicochemical properties

L- amino acids are of great importance in biochemistry because they are the building blocks of peptides and proteins . So far, more than twenty so-called proteinogenic amino acids are known. These are first of all those 20 L -α-amino acids which are coded as standard amino acids by codons of three nucleobases each in the DNA according to the standard code . In addition to these canonically named amino acids, two more have been added, selenocysteine and pyrrolysine . Both non-canonical are also α-amino acids, based on the terminal carboxy group , the amino group is bonded to the immediately adjacent carbon atom (C α ). In addition, there are other amino acids that occur as components of proteins or peptides, but are not coded.

Amino acid chains with a chain length of less than 100 amino acids are usually referred to as peptides, the larger ribosomally formed ones are called proteins. The individual amino acids are linked within the chain via peptide bonds (acid amide). An automated method for the synthesis of peptides provides the Merrifield synthesis .

Proteins ingested in the form of food are broken down into L- amino acids during digestion . They are then used in the liver . They are either used for protein synthesis or broken down ( see also: amino acid index ). The most important mechanisms of amino acid breakdown are:

Essential amino acids

Amino acids that an organism needs but cannot produce itself are called essential amino acids and must be taken in with food. All of these essential amino acids are L -α amino acids. For humans, valine , methionine , leucine , isoleucine , phenylalanine , tryptophan , threonine and lysine are essential amino acids. The WHO has also classified the amino acid histidine as an essential amino acid since 1985. There are therefore nine essential amino acids. Conditionally essential or semi-essential amino acids only need to be taken in with food in certain situations, for example during growth or after severe injuries. The remaining amino acids are either synthesized directly or obtained from other amino acids by modification. Cysteine can be synthesized from the essential amino acid methionine. As long as the ability to produce the amino acid tyrosine from phenylalanine is not yet fully developed, this is one of the other essential amino acids in childhood. For a similar reason, tyrosine must also be taken in the case of phenylketonuria . There are also other diseases that impair the amino acid metabolism and may require the intake of an actually non-essential amino acid. All the essential or semi-essential amino acids that the human body needs are contained in chicken eggs.

Plants and microorganisms can synthesize all the amino acids they need by themselves. Therefore, there are no essential amino acids for them .

Set diagram - representation of the properties of the side chains of proteinogenic standard amino acids

Chemical-physical properties

The proteinogenic amino acids can be divided into groups according to their residues (see table overview of properties ). An amino acid can appear in different groups at the same time. The overlapping of the groups can be shown graphically in a quantity diagram .

The authors have different views on the properties of the side chain of cysteine: Löffler considers it polar, while Alberts considers it non-polar. Correctly, sulfur is a heteroatom , so the following applies: The side chain of cysteine ​​has weakly polar properties.

Acid and base behavior

Titration curves of the proteinogenic amino acids

Due to the basic amino group and the acidic carboxylic acid group, amino acids are both bases and acids . As solids and in neutral aqueous solutions, amino acids are present as zwitterions , that is, the amino group is protonated and the carboxy group is deprotonated. The zwitterion can be generalized as follows:

Amino acid as a zwitterion

As a zwitterion, the protonated amino group can react as an acid ( proton donor ) and the carboxylate group can react as a base ( proton acceptor ). In acidic solutions, amino acids are present as cations and in basic solutions as anions :

Structure of amino acids at different pH values

The charge of an amino acid molecule depends on the pH of the solution. In the case of a zwitterion with an acidic and a basic group, the total charge of the molecule is zero at neutral pH. In addition, the side chains of the amino acids have partially acidic or basic charged groups. The pH with a net charge of zero is the isoelectric point (pH I , pI) of an amino acid. The water solubility of an amino acid is lowest at the isoelectric point.

pK S values of certain amino acid side chains (as free amino acids residues and protein)
amino acid property free in protein
Asp angry 03.68 03.7-4.0
Glu angry 04.25 04.2-4.5
His basic 06.00 06.7-7.1
Cys semi-sour 08.33 08.8-9.1
Tyr semi-sour 10.07 09.7-10.1
Lys basic 10.53 09.3-9.5
Arg basic 12.48 -

For the acid-base behavior of proteinogenic amino acids, the behavior of their side chain (hereinafter referred to as R ) is of particular interest. In proteins, the NH 2 and COOH groups cannot be protonated at a physiological pH value (around pH 7) because of the peptide bond, and therefore cannot be titrated . Exceptions are the amino and carboxy terminus of the protein. The side chain residue R is therefore decisive for the acid-base behavior of proteins and peptides .

The behavior of the side chain R depends on its constitution, that is, whether the side chain itself can act as a proton acceptor or as a proton donor . The proteinogenic amino acids are divided according to the functional groups into those with non-polar or polar amino acid side chains and further subdivided into subgroups sorted by polarity : aliphatic , aromatic , amidated , sulfur-containing , hydroxylated , basic and acidic amino acids.

Although the side chains of tyrosine and cysteine ​​are relatively acidic compared to the other non-polar side chains, they only tend to deprotonate at unphysiologically high pH values. Proline is a secondary amino acid because the N-terminus forms a five-atom ring with the side chain. Within a protein, the carboxy terminus of a preceding amino acid binds to the nitrogen of the proline, which cannot be protonated due to the peptide bond already mentioned. Histidine, tyrosine and methionine each come in two subgroups.

Electrical properties of amino acids
amino acid pK 2
COOH
pK 1
COOH
Isoelectric
point
pK 1
NH 2
pK 2
NH 2
Alanine - 02.30 06.10 09.90 -
Arginine - 02.81 10.76 09.09 12.50
Asparagine - 02.02 05.41 08.80 -
Aspartic acid 03.65   01.88 02.85 09.60 -
Cysteine 08.33  * 01.71 05.05 10.78 -
Glutamine - 02.17 05.65 09.13 -
Glutamic acid 04.25   02.19 03.22 09.67 -
Glycine - 02.21 05.97 09.15 -
Histidine - 01.78 07.47 08.97 05.97
Isoleucine - 02.32 05.94 09.76 -
Leucine - 02.40 05.98 09.60 -
Lysine - 02.20 09.59 08.90 10.28
Methionine - 02.28 05.74 09.21 -
Phenylalanine - 02.58 05.84 09.24 -
Proline - 01.99 06.30 10.60 -
Serine - 02.21 05.68 09.15 -
Threonine - 02.10 05.60 09.12 -
Tryptophan - 02.15 05.64 09.12 -
Tyrosine 10.07  ** 02.20 05.66 09.11 -
Valine - 02.30 05.96 09.60 -
* Thiol group
** phenolic hydroxyl group
Aliphatic amino acid side chains
Aromatic amino acid side chains
Amidated amino acid side chains
Sulfur-containing amino acid side chains
Hydroxylated amino acid side chains
Basic amino acid side chains
Acid Amino Acid Side Chains

The pK value is the pH value at which the titratable groups are protonated and deprotonated in equal parts; the titratable group is then present in equal parts in its basic as in its acidic form (see also: Henderson-Hasselbalch equation ).

It is usually customary to speak of the pK instead of the pK S , i.e. the pK of the acid . In this sense, however, the pK of the lysine would have to be referred to as pK B and the pK of the base . For the sake of simplicity, this notation is generally omitted, since the context also shows whether the group acts as a base or an acid.

The pK is not a constant, but depends on the temperature, the activity , the ionic strength and the immediate environment of the titratable group and can therefore fluctuate widely.

If the pH is higher than the pK of a titratable group, the titratable group is in its basic (deprotonated) form. If the pH is lower than the pK of the titratable group, the titratable group is in its acidic (protonated) form:

  • For Asp (pK =  03.86) at pH 7: the side chain is almost completely deprotonated.
  • For Lys (pK = 10.53) at pH 7: the side chain is almost completely protonated.

The side chains of basic amino acids are simply positively charged in their protonated (acidic) form and uncharged in their deprotonated (basic) form. The side chains of the acidic amino acids (including cysteine ​​and tyrosine) are uncharged in their protonated (acidic) form and simply negatively charged in their deprotonated (basic) form. Since the behavior of the side chain is completely different when it is charged or uncharged, the pH value plays such an important role in the properties of the side chain.

The titratable side chains influence, for example, the solubility behavior of the corresponding amino acid. In polar solvents, the following applies: Charged side chains make the amino acid more soluble, uncharged side chains make the amino acid more insoluble.

In proteins this can lead to certain sections becoming more hydrophilic or hydrophobic , which means that the folding and thus also the activity of enzymes depends on the pH value. Proteins can therefore be denatured by strongly acidic or basic solutions .

Table overview of the properties

Properties of the 20 canonical amino acids ( R : side chain) according to Taylor
amino acid Side chain R
Surname Abbr. symbol Structural formula Constitutional formula relative
molecular weight
van der
Waals
volume
pola-
rity
Hydro
phobi-
capacity
Acidity
or
basicity
Acid
constant

(pK S )
Alanine Ala A.
L-alanine
-CH 3 015th 067 non-polar +1.8 neutral -
Arginine Arg R.
L-arginine
-CH 2 CH 2 CH 2 NH-C (NH) NH 2 100 148 polar −4.5 basic
(strong)
12.48
Asparagine Asn N
L-asparagine
-CH 2 CONH 2 058 096 polar −3.5 neutral -
Aspartic
acid
Asp D.
L-aspartic acid
-CH 2 COOH 059 091 polar −3.5 angry 3.90
Cysteine Cys C.
L-cysteine
-CH 2 SH 047 086 polar +2.5 neutral 8.18
Glutamine Gln Q
L-glutamine
-CH 2 CH 2 CONH 2 072 114 polar −3.5 neutral -
Glutamic
acid
Glu E.
L-glutamic acid
-CH 2 CH 2 COOH 073 109 polar −3.5 angry 4.07
Glycine Gly G
L-glycine
-H 001 048 non-polar −0.4 neutral -
Histidine His H
L-histidine
-CH 2 ( C 3 H 3 N 2 ) 081 118 polar −3.2 basic
(weak)
6.04
Isoleucine Ile I.
L-isoleucine
-CH (CH 3 ) -CH 2 CH 3 057 124 non-polar +4.5 neutral -
Leucine Leu L.
L-leucine
-CH 2 CH (CH 3 ) 2 057 124 non-polar +3.8 neutral -
Lysine Lys K
L-lysine
-CH 2 CH 2 CH 2 -CH 2 NH 2 072 135 polar −3.9 basic 10.54
Methionine Mead M.
L-methionine
-CH 2 CH 2 SCH 3 075 124 non-polar +1.9 neutral -
Phenylalanine Phe F.
L-phenylalanine
-CH 2 ( C 6 H 5 ) 091 135 non-polar +2.8 neutral -
Proline Per P
L-proline
One H is missing from the NH 2 042 090 non-polar −1.6 neutral -
Serine Ser S.
L-serine
-CH 2 OH 031 073 polar −0.8 neutral -
Threonine Thr T
L-threonine
-CH (OH) CH 3 045 093 polar −0.7 neutral -
Tryptophan Trp W.
L-tryptophan
-CH 2 ( C 8 H 6 N ) 130 163 non-polar −0.9 neutral -
Tyrosine Tyr Y
L-tyrosine
-CH 2 ( C 6 H 4 ) OH 107 141 polar −1.3 neutral 10.46
Valine Val V
L-valine
-CH (CH 3 ) 2 043 105 non-polar +4.2 neutral -

Stereochemistry

According to the Cahn-Ingold-Prelog convention, 18 of the 20 proteinogenic amino acids have the ( S ) configuration on the α-carbon atom , only cysteine has the ( R ) configuration, since the carbon with the thiol group has a higher priority than the Has carboxylic acid group. Glycine is achiral , so no absolute configuration can be determined.

In addition to the stereocenter on the α-C atom, isoleucine and threonine each have a further stereogenic center in their R group . Proteinogenic isoleucine [ R  = -C * H (CH 3 ) CH 2 CH 3 ] is ( S ) -configured there, threonine [ R  = -C * H (OH) CH 3 ] ( R ) -configured.

Non-proteinogenic amino acids

The amino acid L- DOPA ( L -3,4-dihydroxyphenylalanine) is a precursor in the biosynthesis of adrenaline , noradrenaline , dopamine and melanins

To date, over 400 non-proteinogenic (i.e., not incorporated into proteins during translation ) amino acids found in organisms are known. Among them is the L - thyroxine , a hormone produced by the thyroid , L -DOPA , L - ornithine or in almost all types of cyanobacteria proven neurotoxin β-Methylaminoalanin (BMAA).

Most of the non-proteinogenic amino acids are derived from the proteinogenic, which are L -α-amino acids. Nevertheless, β-amino acids ( β-alanine ) or γ-amino acids ( GABA ) can also be formed.

Among the non-proteinogenic amino acids are all D - enantiomers of the proteinogenic L -amino acids. D -serine, in the brain by the serine - racemase from L generated -serine (its enantiomer). It serves both as a neurotransmitter and as a glio transmitter by activating the NMDA receptor , which together with glutamate allows the channel to open. To open the ion channel, glutamate and either glycine or D- serine must bind. D -Serine is a stronger agonist than glycine itself at the glycine binding site of the glutamate receptor of the NMDA type , but was still unknown at the time the glycine binding site was first described. After D -aspartate, D -serine is the second D -amino acid that has been found in humans.

The synthetic amino acid (all- S ) - endo - cis -2-azabicyclo- [3.3.0] -octane-3-carboxylic acid, a structural element of the drug ramipril .

The synthetic amino acids include 2-amino-5-phosphonovaleric acid (APV), an antagonist of the NMDA receptor and the economically important D- phenylglycine [synonym: ( R ) -phenylglycine], which is found in the side chain of many semisynthetic β-lactam antibiotics as Partial structure is included. ( S ) - and ( R ) - tert -leucine [synonym: ( S ) - and ( R ) -β-methylvaline] are synthetic structural isomers of the proteinogenic amino acid ( S ) -leucine and are used as starting material in stereoselective syntheses.

There are also aminosulfonic acids [example: 2-aminoethanesulfonic acid (synonym: taurine )], α-aminophosphonic acids and α-aminophosphinic acids. These are also α-amino acids, but no α-amino carboxylic acids. Instead of a carboxy group (–COOH), these α-amino acids contain a sulfonic acid, phosphonic acid or phosphinic acid group.

Some non-proteinogenic amino acids
amino acid Biological importance
Thyroxine Thyroid - hormone
GABA inhibitory neurotransmitter
L - homoser Metabolic intermediate in arginine synthesis
Ornithine Metabolic intermediate in the urea cycle
Citrulline Metabolic intermediate in the urea cycle
Argininosuccinate Metabolic intermediate in the urea cycle
L -DOPA Metabolic intermediate in the synthesis of catecholamines
5-hydroxytryptophan Metabolic intermediate in serotonin synthesis
β-alanine Building block of coenzyme A
β-methylamino-alanine Neurotoxin of cyanobacteria
Ibotenic acid Mushroom poison
D - valine Part of the antibiotic valinomycin
D - alanine Part of bacterial cell walls
D - glutamate Part of bacterial cell walls
2,6-diaminopimelic acid Part of bacterial cell walls

proof

Quantitative photometric detection of amino acids can be carried out, among other things, by the Kaiser test with ninhydrin or with the Folin reagent , whereby primary amines can be detected. For secondary amines, the Isatin test or the Chloranil test are used. Separation and detection of amino acids can also be carried out by capillary electrophoresis or by HPLC , sometimes as liquid chromatography with mass spectrometry coupling . While most amino acids do not absorb UV light with wavelengths above 220 nm, the amino acids phenylalanine, tyrosine, histidine and tryptophan are aromatic and absorb UV light with a maximum between 260 nm and 280 nm. The amino acid composition of a protein can be determined by hydrolysis of the protein to be examined. The slowly occurring racemization of the amino acids in the proteins originally composed exclusively of L-amino acids is investigated during amino acid dating .

Extraction and production

Amino acids are obtained either from natural substances by separating a hydrolyzed protein or by synthetic means. Originally, the development of a synthesis for the various amino acids mainly served to clarify the structure. In the meantime, these structural questions have been resolved and the desired amino acids are specifically represented with the various syntheses, as far as they are still relevant. The syntheses initially give rise to racemic mixtures which can be separated. One method for this is selective enzymatic hydrolysis, which is used to resolve racemates .

Adolph Strecker (around 1869)

The following is an overview of various syntheses that were developed by chemists as early as the mid-19th century. Some of these older syntheses are only of historical interest because of low yields or other problems. However, some of these old methods have been further developed and some are still relevant today for the representation of amino acids. Further details on these syntheses, including the equations for the syntheses, are given under the links to the syntheses and the amino acids given .

Amino acids are industrially produced today using the following processes:

use

Amino acids are of fundamental importance for human nutrition , especially because the essential amino acids cannot be produced by themselves. As a rule, in the course of a balanced diet, the requirement for essential amino acids is completely covered by animal proteins or a suitable combination of different vegetable proteins (e.g. from cereals and legumes). Vegetable proteins usually have a lower biological value . Animal feeds are also enriched with amino acids, e.g. B. DL - methionine and L - lysine , but also branched amino acids (leucine, isoleucine, valine), which increases their nutritional value. Various amino acids are sold as dietary supplements .

Amino acids and their derivatives are used as additives for food . The human tongue has a glutamate receptor , the activation of which is generally associated with increased taste. Sodium glutamate is therefore used as a flavor enhancer . The sweetener aspartame contains an amino acid. Amino acids are precursors for certain flavoring substances that are created by the Maillard reaction during dry cooking of food .

Amino acids are used in cell biology and microbiology as components of cell culture media. In biochemistry , derivatives of amino acids such as photo-leucine or photo-methionine are used to determine the structure of proteins and others for marking molecules. In addition, amino acids are also used as auxiliaries, e.g. B. as a salt former , buffer . In pharmacy and medicine , L- amino acids are used as infusion solutions for parenteral nutrition and as stabilizers for certain liver diseases . L- Dopa is used for diseases with a deficiency of neurotransmitters . Amino acids are necessary starting materials for synthetic peptide hormones and for the biosynthesis of antibiotics . Magnesium and potassium aspartates play a role in the treatment of heart and circulatory diseases.

Cysteine , or the derivatives acetylcysteine and carbocysteine , are also used for infectious bronchial diseases with increased bronchial secretions . In addition, L cysteine as a reducing agent in the perm used. Amino acids are added to skin care products and shampoos in cosmetics .

metabolism

Breakdown of proteinogenic amino acids

Amino acids can be divided into ketogenic , glucogenic and mixed keto and glucogenic amino acids according to their breakdown pathways . Ketogenic amino acids are added to the citrate cycle during breakdown , glucogenic amino acids to gluconeogenesis . Furthermore, various degradation products with biological activity (e.g. neurotransmitters ) are formed from amino acids in the metabolism . Tryptophan is the precursor to serotonin . Tyrosine and its precursor phenylalanine are the precursors of the catecholamines dopamine , epinephrine (synonymous with adrenaline) and norepinephrine (synonymous with noradrenaline). Phenylalanine is the precursor to phenethylamine in humans. In plants, phenylalanine is the precursor to the phenylpropanoids . Glycine is the starting material for porphyrin synthesis ( heme ). The secondary messenger substance nitrogen monoxide is formed from arginine . Ornithine and S-adenosylmethionine are precursors to the polyamines . Aspartate, glycine and glutamine are the starting materials for the biosynthesis of nucleotides.

In various human infections with pathogens , competition with the host for the amino acids asparagine, arginine and tryptophan has been described.

literature

Books

Magazine articles

Web links

Wiktionary: Amino acid  - explanations of meanings, word origins, synonyms, translations

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