1 Definition 
Amino acids are carboxylic acids, in which a hydrogen atom is replaced by an amino group (NH2). Strictly speaking, it denotes the 20 different amino acids that are made up of human proteins. They are also designated as "proteinogenic" amino acids. There are several hundred other amino acids in nature that however play no role in the protein synthesis. All proteinogenic amino acids have an amino group attached to the alpha C atom (carbon atom next to the carboxyl group) and are present in an L-shape.
2 Systematics 
The essential and non-essential amino acids are differentiated; the non-essential amino acids can be synthesized by the organism itself, whereas the essential amino acids have to be supplied from external sources, primarily through ingestion.
- Essential amino acids
- Non-essential amino acids
- Amino acids that are used for protein synthesis in other species:
The semi-essential amino acids are also distinguished. They are amino acids that need be absorbed with the food only in certain situations, for instance in case of increased protein requirement (growth, convalescence). L-arginine and L-histidine belong to this group.
3 Chemical properties 
The chemical properties of amino acids vary according to the occurrence of other functional groups. The classifications of amino acids are based on these functional groups, on the distinction between essential and non-essential amino acids, on the water solubility (hydrophilic and hydrophobic amino acids), on their ability to metabolize carbohydrates (glucoplastic and ketoplastic amino acids), or on the pH value in solution (acidic and alkaline amino acids).
4 Chirality 
- L-amino acids ("levorotatory" amino acids) or as
- D-amino acids ("dextrorotatory" amino acids).
Both these forms are also termed enantiomers. They behave chemically identically, but differ in their optical properties. L-amino acids turn the linear polarized light toward left (Lat. laevus - left), and D-amino acids turn it toward right (Lat. dexter - right).
L-amino acids are exclusively used for the protein synthesis in biological organisms. This is attributed to the fact that the enzymes taking part in the synthesis can process only this form.
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