Criteria for Classification of Amino Acids:
Each of the twenty common amino acids of protein has a unique side chain but some amino acids could share identical features. Amino acids have been classified on the basis of the following criteria:
i. Functional group of the side chain.
iii. Nutritional requirements
Classification Based on Structure:
On the basis of the type of functional groups present in their side chains, amino acids can be placed into nine classes. Note that you are expected to study the structure of each amino acid taking into consideration their uniqueness.
1. Amino acid having hydrogen atom as R group-Gly
2. Amino acids possessing unsubstituted aliphatic chain as their R group-Ala, Val, Leu, Ile
3. Amino acids whose R groups possess aliphatic chain bearing a hydroxyl group-Ser, Thr
4. Amino acids whose R group is an aliphatic chain terminating in an acidic carboxyl Group-Asp, Glu
5. Amino acids whose side groups are aliphatic chain terminating in basic amino group- Arg, Lys
6. Amino acids whose side chains are aliphatic chain terminating in amide group-Asn, Gln
7. Amino acids with sulphur-containing aliphatic R groups- Cys, Met
8. Amino acids whose side chains terminate in an aromatic ring- Phe, Tyr
9. Amino acids whose side chains terminate in a heterocyclic ring- Trp, Pro, His
Categorization Based on Polarity:
Amino acids categorized according to polarities of their side chains. Polarity is maybe the most useful criterion of amino acid classification. This is due to protein folding is administered by tendency of amino acid residues of protein to interact with water. In protein conformation, hydrophobic side chains are detached from contact with water while hydrophilic (water-loving) ones interact liberally with aqueous environment. These interactions are in harmony with second law of Thermodynamics.
This categorization scheme places every amino acid in either of the two major classes given below:
i) Those with nonpolar R groups
ii) Those with polar R groups
i) Amino Acids with Nonpolar Side Chains:
Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine and Tryptophan have nonpolar R group. The table provides description of nature of individual side chains that account for their nonpolarity.
Nonpolar Amino acid
Nature of R group
Gycine, Alanine, Valine,
Contain aliphatic side chain
Contains a thio ether end
Contains a cyclic pyrrolidine side chain
Contains a phenyl moiety in its side
Contains an aromatic indole group
ii) Amino Acids with Polar Side Chains:
This class is more divided in two sub-classes:
a) Those having uncharged polar R groups:
Serine and Threonine - are polar due to their hydroxyl groups.
Asparagine and Glutamine -polarity is because of the presence of amide-bearing R groups.
Tyrosine - has an OH functional group attached to benzene ring. This OH is hydrophilic.
Cysteine - its thiol or (SH) group is responsible for its polarity.
b) Those with charged polar R groups:
At physiological pH values, Lysine, Arginine and Histidine are positively charged because of their terminal ammonium, guanidinium, and imidazolium groups respectively. These 3 amino acids are basic amino acids as their side chains have net positive charges at neutral pH.
Aspartic acid and Glutamic acid are negatively charged above pH 3. Such amino acids are also referred to as acidic amino acids as their side chain have acidic carboxyl group.
Categorization Based on Nutritional Requirements:
Few, but not all, of 20 common amino acids can be synthesized by body and as such, they may not be comprised in diet. On the basis of needs in our diet, amino acids have been grouped in:
i) Essential amino acids, and
ii) Nonessential amino acids
Those amino acids which can be synthesized in body are known as nonessential amino acids as they may not be incorporated in diet. In contrast, those amino acids which body can't synthesize de novo (from existing molecules) should be supplied in human/animal diet. Such amino acids are referred to as essential amino acids. If only one of these essential amino acids is missing from diet, synthesis of new protein by body will be affected. The majority bacteria and plants can synthesize all common amino acids.
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