Chemistry of amino acids

Amino acids are of great importance in chemical processes of living organisms (biochemistry), as they are the building blocks of proteins (peptides and proteins). Twenty-two amino acids are encoded in the genetic material (genome), from which vital proteins are produced. These twenty-two amino acids are known as proteinogenic amino acids.

Amino acids are strung together in chains and depending on the length of an amino acid chain, they are called either peptides (up to 100 amino acids) or proteins (more than 100 amino acids). The proteinogenic amino acids are divided into different groups depending on which reactive side chains they have. This also results in the different chemical-physical properties of amino acids.

For example, if an amino acid has only one long non-polar side chain, this influences, among other things, the solubility properties of the amino acid. In addition, the pH value (measure of the acidic or basic character of an aqueous solution) plays an important role for the properties of the side chain, since the side chain behaves differently when it is charged or uncharged. For example, in polar solvents, charged side chains make an amino acid more soluble, while uncharged side chains make the amino acid more insoluble.

In proteins, many differently charged amino acids are attached to each other, making certain sections more hydrophilic (water-attracting) or hydrophobic (water-repelling). For this reason, the folding and activity of enzymes (catalysts of biochemical reactions, fulfil important functions in metabolism) depends on the pH value. Likewise, the charges and the dissolution behaviour of the side chains explain why proteins can be denatured by strongly acidic or basic solutions.

Amino acids are also known as so-called zwitterions because they can carry different charges depending on the environment (positive or negative charges). This phenomenon is due to the two functional groups of an amino acid, i.e. the amino and the carboxyl group. Simplified, one can remember that an amino acid dissolved in an acidic solution carries a positive charge and an amino acid in an alkaline solution carries a negative charge.

In a neutral aqueous solution amino acids are equally present in positive and negative charge form. Contact with heat, acids and alkalis can destroy the proteins or amino acid chains and make them unusable. The classification of proteinogenic amino acids into polar or non-polar amino acids is also based on the functional groups.

However, the classification according to the chemical-physical properties of the individual amino acids is not only based on polarity, but also on character, molar mass, hydrophobicity (water-repellent property), acidity or basicity (acidic, basic or neutral amino acids) and the electrical properties of the amino acids. In addition to the proteinogenic amino acids, there is also a large number (more than 400) of amino acids that do not occur in proteins, the so-called non-proteinogenic amino acids. Examples of these are L-thyroxine (thyroid hormone), GABA (inhibitory neurotransmitter), ornithine (metabolic intermediate in the urea cycle), and many others.

Most non-proteinogenic amino acids are derived from the proteinogenic amino acids. Each of the 20 proteinogenic amino acids has at least two carbon atoms (C atoms). This carbon atom is essential for the classification of the respective amino acid.

This means that the carbon atom to which the amino group is attached determines which class of amino acid it is. However, there are also amino acids in which several amino groups are represented. In such cases the carbon atom whose amino group is closest to the carboxy carbon determines which class of amino acid it is.

In general, a distinction is made between alpha-amino acids, beta-amino acids and gamma-amino acids: Within the individual classes the amino acids have a similar structure, but differ in the structure of their side chain. It is the individual components of the side chains that are responsible for the behaviour of the amino acid in acidic or basic environments. In nature, there are about twenty amino acids, whereas man himself can only build up some amino acids independently.

Amino acids which the body itself is not capable of forming are called essential amino acids. Humans must take up these amino acids through food. Essential amino acids in adult humans are: The amino acid cysteine is not essential in the true sense, but it is indispensable as a source of sulphur for the human body.

In infants, histidine and arginine are also essential. Amino acids can form chain-like combinations with each other. One speaks then of protein molecules (proteins).

The combinations of amino acids determine how a protein functions and what its main function is. The combination of amino acids is not arbitrary. It is given (coded) in the respective gene.

Always three base pairs, which are arranged in a certain way, correspond to a so-called code word (= codon). This codon represents the construction manual for the respective amino acid. – Leucine

Isoleucine
Methyonine
Threonine
Valine
Lysine
Phenylalanine
And tryptophan.
Alpha-amino acids: The amino group of this amino acid class can be found on the second carbon atom. Another name for these amino acids is 2-aminocarboxylic acids (IUPAC name). The most important representative of this class is the amino acid glycine, which has a rather simple structure.

All amino acids that are important for the human organism are classified according to their structure as alpha-amino acids. In this case one speaks of so-called proteinogenic amino acids. They are the building blocks from which all proteins are built.

Beta-amino acids: The class of beta-amino acids is characterized by the fact that their amino group is located on the third carbon atom. The IUPAC term “3-aminocarboxylic acids” is also used synonymously for this class. – Gamma-amino acids: The amino group of all amino acids from the gamma group is bound to the fourth carbon atom.

The structure of the amino acids of this class therefore differs significantly from the structure of the proteinogenic amino acids. The IUPAC designation of this group is 4-aminocarboxylic acids. Although gamma-amino acids are not used in the human organism for the synthesis of proteins, some representatives of this class can be found in humans. The simplest representative of this group, gamma-aminobutyric acid (GABA for short), serves as an inhibitory neurotransmitter (messenger) in the nervous system.