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4.2.1. Classification of amino acids

Classification of amino acids

The amino acids according to the characteristics of the radical group, are divided by ionization, polarity and reactivity, in:

  • Neutral amino acids. Its side chain does not have carboxyl or amino groups and, therefore, at neutral pH its net electric charge is 0. They can be:
    • Polar neutrals. Its side chain has hydrophilic groups with which it can form hydrogen bonds with polar molecules, making it soluble in water.
    • Nonpolar neutrals. Its long side hydrocarbon chain is hydrophobic, and it is less soluble in water.
  • Acidic amino acids: the R group carries an acid group (carboxyl), so that at neutral pH, they have a negative charge, since that group releases H+.
  • Basic amino acids: when the R group carries a basic group (amino), in such a way that at neutral pH, they have a positive electrical charge (take H+).

Essential amino acids

Heterotrophic organisms are not able to synthesize all the amino acids they need from simpler compounds. The so-called essential amino acids must be obtained from the diet, and they vary for each species. For example, in humans they are: valine (Val), leucine (Leu), isoleucine (Ile), tryptophan (Trp), phenylalanine (Phe), methionine (Met), threonine (Thr) and lysine (Lys).

The foods that serve as a source of these amino acids are meat, fish, eggs, cheese, etc.

Autotrophic organisms can indeed synthesize all the amino acids necessary for their proteins.