Amino Acids

Amino acids are organic acids containing at least one amino (-NH 2 ) and carboxyl (-COOH) group. They are the basic structural component of proteins. The biological properties of proteins are determined by the type of amino acids, their order and their mutual spatial relationships. Although more than 300 amino acids occur in nature, only 20 of them occur in proteins exclusively as L-α-amino acids (except for glycine, which does not have a chiral atom). These amino acids are referred to as biogenic amino acids or proteinogenic amino acids.

Reactivity
The carboxyl group and the amino group are carriers of all reactions that can be expected. They create salts, esterification, acetylation, decarboxylation (removal of the carboxyl group), deamination (removal of the amine group) takes place on them. Amino groups can be moved from one molecule to another ( transamination ). The most important reaction is the formation of a peptide bond, where the α-carboxyl group of one amino acid reacts with the α-amine group of another to split off a water molecule.

Solubility
Amino acids are very easily soluble in polar solvents (water, ethanol), they are insoluble in non-polar solvents (benzene, hexane ether).

Electrochemical properties
Amino acids contain at least two dissociable groups, -COOH (acidic character) and -NH 2 (basic character).

R-COOH R-COO− + H+
 * R-NH3+ R-NH2 + H+

At blood pH (7.4 and 7.1 respectively), carboxyl groups exist as R-COO − ions and most amino groups as R-NH 3 +. This creates an amphiont, a zwitterion that carries both a positive and a negative charge.


 * Isoelectric point: is the pH value at which the amino acid has zero free electric charge (amphiont). The isoelectric pH for amino acids that have two dissociable groups lies midway between the pK values ​​on either side of the isoionic arrangement:
 * $$pI = (\frac{pK_1 + pK_2}{2}\,)$$

Biogenic amino acids
By combining these 20 (actually 21) biogenic amino acids, all known human proteins are formed.

Amino acids with an aliphatic side chain
 * Glycine Gly (G)
 * Alanine Ala (A)
 * Valine Val (V)
 * Leucine Leu (L)
 * Isoleucine Ile (I)

With a carboxyl or amide group on the side chain (acidic groups)
 * Aspartic Acid Asp (D)
 * Asparagine Asn (N)
 * Glutamic Acid Glu (E)
 * Glutamine Gln (Q)

With an amine group on the side chain (basic groups)
 * Arginine Arg (R)
 * Lysine Lys (K)

With an aromatic nucleus or a hydroxyl group on the side chain
 * Histidine His (H)
 * Phenylalanine Phe (F)
 * Serine Ser (S)
 * Threonine Thr (T)
 * Tyrosine Tyr (Y)
 * Tryptophan Trp (W)

With sulfur in the side chain
 * Methionine Met (M)
 * Cysteine Cys (C)

Imino acids
 * Proline Pro (P)

21. amino acid
 * Selenocysteine SeCys – replaces cysteine ​​in the human enzyme glutathione peroxidase and in some bacterial enzymes.

Essential Amino acids
Essential are those that the body cannot create by itself (they contain chains that the body cannot process). They are valine, leucine, isoleucine, phenylalanine, tryptophan, lysine, methionine and threonine. For essential amino acids, we are therefore dependent on their intake in food or their supplementation (targeted use through supplements).

Non-essential Amino acids
The body can create non-essential amino acids and thus partially cover their need for the organism.

Related Articles

 * Amino acids (1. LF UK, NT)
 * Important reactions of amino acids