1. Based on Structure
1-A. Aliphatic amino acids
a. Mono amino mono carboxylic acids:
• Simple amino acids: Glycine, Alanine (Fig. 1)
Fig1. Simple amino acids
• Branched chain amino acids: Valine, Leucine, Isoleucine (Fig. 2)
Fig2. Branched chain amino acids
• Hydroxy amino acids: Serine, Threonine (Fig. 3)
Fig3. Hydroxy amino acids
• Sulphur containing amino acids: Cysteine, Methionine (Fig. 4)
Fig4. Sulphur containing amino acids
• Amino acids with amide group: Asparagine, Glutamine (Fig. 5)
Fig5. Amino acids with amide groups
b. Mono amino dicarboxylic acids: Aspartic acid, Glutamic acid (Fig. 6)
Fig6. Dicarboxylic amino acids
c. Di basic mono carboxylic acids: Lysine, Arginine (Fig. 7)
Fig7. Dibasic amino acids
1-B. Aromatic amino acids: Phenylalanine, Tyrosine (Fig. 8)
Fig8. Aromatic amino acids
1-C. Heterocyclic amino acids: Tryptophan (Fig. 9), Histidine (Fig. 10)
Fig9. Tryptophan (Trp) (W) with indole group
Fig10. Histidine and proline
1-D. Imino acid: Proline (Fig. 10)
1-E. Derived amino acids: 1-E-i.Derived amino acids found in proteins: After the synthesis of proteins, some of the amino acids are modified, e.g. hydroxy proline (Fig. 11) and hydroxy lysine are important components of collagen. Gamma carboxylation of glutamic acid residues of proteins is important for clotting process (Fig. 11). In ribosomal proteins and in histones, amino acids are extensively methylated and acetylated. 1-E-ii. Derived amino acids not seen in proteins (Non-protein amino acids): Some derived amino acids are seen free in cells, e.g. Ornithine (Fig. 11), Citrulline, Homocysteine. These are produced during the metabolism of amino acids. Thyroxine may be considered as derived from tyrosine.
Fig11. Some derived amino acids
1-E-iii. Non-alpha amino acids: Gamma amino butyric acid (GABA) is derived from glutamic acid. Beta alanine, where amino group is in beta position, is a constituent of pantothenic acid (vitamin) and co-enzyme A.
Each amino acid will have three-letter and one letter abbreviations which are shown in Table 1 as well as in Figs 1 to 10. Sometimes asparagine/aspartic acid may not be separately identified, for which 3-letter abbreviation is Asx and 1-letter abbreviation is B.
Similarly Glx or Z stands for glutamine/glutamic acid.
Table1. Common amino acids
Special Groups in Amino Acids
In the figures, special groups are shaded. Arginine contains guanidinium group; Phenyl alanine (benzene); Tyrosine (phenol); Tryptophan (Indole); Histidine (imidazole); and Proline (pyrrolidine) (Table 1). Proline has a secondary amino group, and hence it is an imino acid.
2. CLASSIFICATION BASED ON SIDE CHAIN
2-A. Amino acids having nonpolar side chains: These include Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine and Tryptophan.
These groups are hydrophobic (water repellant) and lipophilic. Therefore, the parts of proteins made up of these amino acids will be hydrophobic in nature.
2-B. Amino acids having uncharged or nonionic polar side chains: Glycine, Serine, Threonine, Cysteine, Tyrosine, Glutamine and Asparagine belong to this group. These amino acids are hydrophilic in nature. (Tyrosine and Cysteine may show hydrophobic character when present in the interior of the protein).
2-C. Amino acids having charged or ionic polar side chains (hydrophilic):
C-a. Acidic amino acids: They have a negative charge on the R group: Aspartic acid and Glutamic acid (Tyrosine is mildly acidic).
C-b. Basic amino acids: They have a positive charge on the R group: Lysine, Arginine and Histidine.
3. CLASSIFICATION BASED ON METABOLISM
3-A. Purely Ketogenic Leucine is purely ketogenic because it is converted to ketone bodies .
3-B. Ketogenic and Glucogenic Lysine, Isoleucine, Phenylalanine, Tyrosine and Tryptophan are partially ketogenic and partially glucogenic. How ever in humans lysine is predominantly ketogenic. During metabolism, part of the carbon skeleton of these amino acids will enter the ketogenic pathway and the other part to glucogenic pathway.
3-C. Purely Glucogenic All the remaining 14 amino acids are purely glucogenic as they enter only into the glucogenic pathway.
4. CLASSIFICATION BASED ON NUTRITIONAL REQUIREMENTS
4-A. Essential or Indispensable The amino acids may further be classified according to their essentiality for growth. Thus Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan, and Valine are essential amino acids. Their carbon skeleton cannot be synthesized by human beings and so preformed amino acids are to be taken in food for normal growth. See memory aid in Box 1.
Box1. Memory Aid for Essential Amino Acids
4-B. Partially essential or Semi-essential Histidine and arginine are semi-indispensable amino acids. Growing children require them in food. But they are not essential for the adult individual.
4-C. Non-essential or Dispensable The remaining 10 amino acids are non-essential, because their carbon skeleton can be synthesized by the body. However, they are also required for normal protein synthesis. All body proteins do contain all the non-essential amino acids.
Naming (Numbering) of Carbon Atoms
Carbon atoms in amino acids in sequence are named with letters of Greek alphabets, starting from the carbon atom to which carboxyl group is attached. As examples, naming of glutamic acid is shown in figure 6 and that of lysine is shown in Figure 7.
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