Amino-acid

An amino-acid is a organic Molécule having a carbonaceous skeleton and two functions: a amine (- NH₂) and a Carboxylic acid (- COOH). The amino-acids are the basic structural units of the Protéine S. They measure approximately 100 picometers (pm).

The carbon atoms of the carbonaceous chain are ordered compared to the group carboxyl and are named by a Greek letter: the carbon atom directly related to the carboxyl group is carbon α, and if the group amino is also on this carbon, it is about an amino carboxylic acid in position α, in other words an α-amino acid. For example, the Lysine is an α-amino acid carrying a second amino group in position ε.

There exists more than 100 α-amino acids present in nature, some were discovered on Météorite S, in particular the carbonaceous Chondrite S.

In the cell, the amino-acids can exist in a free state or of biopolymères (Peptide S or proteins). One can thus distinguish various categories of amino-acids:

Certains amino-acids is found in proteins, and is able to take part in vivo in the synthesis of these proteins. They are thus at the same time components and precursors of proteins.
Certains amino-acids is found in proteins only after their biosynthesis (because they are formed only after incorporation of another amino-acid in the proteinic molecule).
Certains amino-acids exists only in a free state. Although there exist many amino-acids in nature, the hydrolysis of proteins or natural peptides leads to 20 amino-acids.

These amino-acids are the " maillons" who constitute the Protéine S. Those are comparable with " colliers" composed of one hundred to several thousands of these " perles" , connected in a covalent way by means of functions Amide (peptide Connection). The sequence of the amino-acids constitutes the primary structure of proteins. However, these last draw their properties from their tertiary structure, or space folding up. If the number of amino-acids is lower than 20, one speaks about Peptide, and 20 to 100 of Polypeptide.

General structure

The natural amino-acids are primarily α-amino acids answering the general structure:

The complex opposite without R is called Radical or chains principal, it is common to all the amino-acids. R represents a side chain specific to each amino-acid. The amino-acids are in general classified according to the properties of the side chain in four groups: Acid, basic, Absorbent (polar) and Hydrophobic (Non-polar).

Isomerism

Separately the Glycine, where R = H, the amino-acids exist in the form of two stéréoisomères possible, called D and L, according to whether the group (- NH₂) is respectively on the right or on the left of representation of Fischer. They are thus optically active (they deviate the light polarized in a plan). It results the existence from it from optical isomers: each isomer deviates the light planes polarized and is dextrogyre (+) or laevogyrous (-) according to whether the rotation of the plan of polarization of the light follows a time direction, or anti-clockwise. There is no correlation between the direction of rotation of the plan of polarization (or optical activity) and the configuration of the amino-acid: thus L-alanine is dextrogyre and notes L (+) - alanine. By convention, there are correspondence between the representation of the oses and that of the amino-acids.

The amino-acids L represent the great majority of the amino-acids which are in the Protéine S. Some of these amino-acids, like the Thréonine, have one the 2nd asymmetrical carbon. In this case, the natural compound is called L, the 2 others stéréoisomères from which the relative positions of the substituents are different are called " allo". Those are not present in the Protéines. The amino-acids D meet in certain proteins produced by exotic organizations at the bottom of the oceans, as some Mollusque S. They are also abundant components of the cellular walls of the Bactérie S.

General properties

Solubility

The majority of the amino-acids easily undergo solvation by polar solvents such as water, or the alcohol (particularly Proline and hydroxyproline) in which they are soluble. In addition, the α-amino acids are soluble, but with less degree in nonpolar solvents. It is important to retain that this solubility is largely dependant on the properties of the side chain: solubility falls with the number of carbon atoms of the radical, but conversely increases if this radical R is carrying polar functions (NH2, COOH) or absorbent (OH). Ex of solubility: The Tyrosin, by its aromatic nucleus, is not very water soluble: 0,04%. In the same way, the Cystein, the Leucine.

Ionic properties

The amino-acids contain a grouping carboxyl - acid COOH and a grouping amino - basic NH₂. In solution, these groupings exist in two forms, one charged, the other neutral: _{R-COOH; R-COO^- + H⁺

R-NH ₃⁺; R-NH₂ + H⁺

The amino-acids are called for this diionic structure amphoteric.
Ionization varies with the pH: the amino-acids exist, in aqueous solution, in 3 possible forms:

a) In acid medium:
The function amine ionizes by collecting a proton and the dissociation of carboxyl is inhibited. The amino-acid is in the form of Cation.

b) In basic medium
The acid function ionizes by releasing a proton, the base of the medium blocks the ionization of the grouping amino. The amino-acid is in the form of Anion.

c) The pH for which 2 dissociations are carried out is called not isoelectric: or phi. With this pH, there is a dipolar ion or zwitterion of null clear load, therefore not migrating in an electric field.

On both sides of the phi, one defines pH which corresponds to a half dissociation of COOH and NH₃⁺, they are the pKs.
There thus exist 2 km No:
the km No of COOH: approximately 2 - 3
the km No of NH₃⁺: approximately 10

The point (or pH) isoelectric or isoionic is equal to the half summons pKs. The radical R, when it contains a ionizable group, takes part in the value of the isoelectric point.
An additional km No appears then. For example for the Histidine:
pK₁ acid
pK₂ half dissociation of the group imidazol
pK₃ amine

Absorption of the light

The amino acid solutions are colorless.
The aromatic amino-acids absorb in the U.V between 260 and 280 Nm.
With the top of 260 Nm, most of the ultraviolet absorption of proteins comes from their content of Tryptophane and sometimes of Tyrosine.

Properties of the group carboxyl

1. Amidation

Carboxyl can form amides with the amines.

$R - COOH + R' NH_2 \ Leftrightarrow R - CO - NHR' + H_2O$

Asparagine and glutamine are two examples of physiological derivatives formed according to this reaction.
The amidation can be obtained in vitro by using carbodiimides (R1-N=C=N-R2). The Groupe carboxyl in a first stage is activated by carbodiimide, then the derivative activated thus formed reacts with the amine.

2. Decarboxylation

Chemical or enzymatic by a décarboxylase. Decarboxylation in the form of CO2. The décarboxylases are specific of each amino-acid. Decarboxylation is important in biochemistry because it leads to the " amines biologiques" corresponding very active:

Examples:
Histidine décarboxylée out of histamine (shock, allergy);
5OH tryptophan décarboxylée out of serotonin (hypertension)

Properties of the grouping amino

They are general properties of primary amines. Two types of aminos groups can be distinguished: amines in alpha and the amine in epsilon of the side chain of the lysin whose km No is slightly basic (>8). The difference of the values of km No can be used for selective modifications, by controlling the pH of the reactional medium.

Acetylation

The Acetylation of the groupings aminos of the amino-acids by acetic anhydride reduced their positive loads and changes their interactions with the components of the environment.

Reaction with the Aldehyde S

With the Aldehyde S Aliphatic S: it is formed the derivative diméthylol amino-acid. With the Aldehyde S Aromatic S, one obtains bases of Schiff.

A reaction of the same type can in vivo occur between Amino-acid and Oligosaccharide S (reaction of Glycation of the Protéines with the Amino-acid residues of having a free function amine). In the sequences saccharidic, the Sucre reducing terminal exists in a prevalent way in cyclic form, with only of the traces in open form. A base of Schiff can be formed with this minority form, thus moving balance between the two forms towards the open form.

In vitro, this reaction with saccharides is generally carried out in the presence of sodium cyanoborohydrure (NaCNBH3). The base of Schiff formed is thus quickly reduced by the anions cyanoborohydrides in more stable amine secondary.

Arylation

The substitution of a H of function NH2 by a grouping Aryl (Aromatic) led to a function secondary amine. For example with dinitro-fluoro-benzene (reactive of Sanger) it is formed dinitrophényl-acid amino coloured, therefore measurable.
This reaction can occur when the amino-acid is built-in a Protéine. If a protein is hydrolized one releases from the amino-acids and the amino-acid DNP corresponding to the amino-acids whose groups NH2 are free in the protein (terminals).

Cette reaction made it possible Frederick Sanger (in 1953) to establish the first primary structure of a protein (insulin).

Carbamylation

It takes place with the Isocyanate S, in particular the phénylisothiocyanate (PITC).

The PITC is particularly used to determine the sequence of the Amino-acid in the peptide chains.
Phénylthiocarbamyl-amino acid (PTC-AA) resulting is a compound characteristic of each amino-acid (natural of the grouping R). It is very stable and detectable in the Ultraviolet (245 Nm).

Reactions with Ester S of N-hydroxysuccinimide and para-nitrophényl

These reactions allow the grafting of a grouping R on - NH2 of an amino-acid, with elimination of the reactive grouping.

These reactions are used for the synthesis of derived from amino-acids or Protéine S " marquées" on their functions free Amines (fluorescent derivatives, biotinylation by the Biotine - N-hydroxysuccinimide,…); for the synthesis of chromatographic supports by grafting of amino-acids or proteins,…

Properties due to the simultaneous presence of - COOH and of - the NH2

Formation of metal complexes (Chelation)

These stable chelates are used to carry out chemical reactions on the level of R, in synthesis.

oxydative Decarboxylation and desamination. Reaction with the Ninhydrine

Some Oxydant S attack the amino-acid and carry out a desamination associated with a Décarboxylation. During the reaction there is production of CO2, NH3 and a Aldéhyde having a carbon atom of less than the amino-acid from which it comes.

$\ rm {
\begin{array}{rl}
R- &CH- \ color {Red} COOH \ \
&|\ \
& \ color {Red} NH_2
\end{array}
\ xrightarrow {oxidant} R-CHO + NH_3 + CO_2
}$

The oxidants are varied: hydrogen peroxide, hypochlorite etc to make this reaction quantitative, one can proportion CO2 by alkalimetry or NH3 by colorimetry. The oxidant more used is the Ninhydrine (see the corresponding page).

When an amino-acid in solution is heated in the presence of Ninhydrine in excess, it leads to chromophoric with a maximum of absorption with 570 Nm (blue-purple). The intensity of coloring is at the base of a quantitative method to proportion the amino-acids. The reaction is carried out in 3 stages. 1st corresponds to the action of a first molecule of Ninhydrine on the amino-acid leading to a iminoacide and a reduced molecule of Ninhydrine. 2nd to the action of one 2nd molecule of Ninhydrine corresponds on the iminoacide to give a Aldéhyde. This 2nd molecule condenses finally with the reduced molecule of Ninhydrine to form the chromophoric one.

Coloring is not specific amino-acids. It occurs with others made up having free groupings aminos: glucosamine, Peptide S and Protein S.
This colorimetric method is a good technique for the proportioning of a pure amino-acid, but it is less valid for a total proportioning because the amino-acids react by giving colorings of variable intensity.
The iminoacides give with the ninhydrine, a yellow coloring.

Metabolism

From the metabolic point of view, one distinguishes the essential amino-acids for the man from the others: in fact amino-acids cannot be synthesized in the human cells and which must thus be brought by the food.

See also: Amino-acid essential

When the proteins break up in the intestine, the amino-acids are " libérés" " collier". Thus, they can penetrate the intestinal wall. They mix thereafter with other amino-acids (in particular those coming from degraded body proteins) to form the " pool of the acids aminés". Of this " pool" the amino-acids are selected which the organization to synthesize the proteins which it needs miss. Once chosen, they are dependant in the ribosome of the cells which, them, determine the order of different the " perles" starting from the information held in the DNA.
Other amino-acids of the " pool" are also used to produce Glucose and Fatty-acids. The process by which the organization synthesizes glucose starting from the amino-acids calls the " néoglucogénèse". It consists first of all of the suppression of the group amino (the NH₂ part) thanks to a reaction implying of the Pyridoxine (B₆ vitamin). The amino group, which is now in the form of Ammoniac (NH₃) is immediately transformed into urea by the liver because this substance is toxic. The liver transforms then the remainder of the group (called carbonaceous chain) into glucose or fatty-acids (which are the elements of bases of the Lipides), or any. That depends if the carbonaceous chain is glucogenic (transformable in glucose) or cetogenic (transformable in fatty-acids). This capacity is important in the cases of a too weak Glycémie.

List principal amino-acids

List of the 20 amino-acids represented in the genetic Code

It should be noted that the Sélénocystéine (code with a letter: U , with three letters: Sec ), which is an amino-acid rare but essential to the function of the Sélénoprotéine S, is sometimes regarded as the twenty-and-unième amino-acid .

Structure of the 20 amino-acids represented in the genetic Code

See too

Classification set of themes of the neurosciences
Genetic disease of the metabolism of the amino-acids
essential Nutrient

External bonds

shown animated acids

Simple: Amino acid

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