Amino acids can be classified into three group:
1 On the Basis of the Carbon Chain: amino acids are of three types:
i) Aliphatic Amino Acids: These are further classified into four:
a) Neutral (Monoamino-Monocarboxylic Acids):
example:- glycine, alanine, serine, threonine, valine, leucine, and isoleucine.
b) Acidic (Monoamino-Dicarboxylic Acids):
example:-glutamine aspartic acid, asparagine, glutamic acid
c) Basic:
example:-
arginine,
lysine
hydroxylysine.
d) Sulphur Containing Amino Acids:
example:- cysteine, cystine (dicysteine), and methionine.
ii) Aromatic Amino Acids:
example:- phenylalanine, tyrosine, and thyroxine.
iii) Heterocyclic Amino Acids :
example:-
proline
histidine
hydroxyproline
tryptophan
2) On the Basis of Nutritional Requirement :
Amino acids are of two types.
i ) Essential Amino Acids(EAA):
Also termed as indispensable amino acids.
They are not synthesised in the body and are obtained from dietary sources
Examples valine , isoleucine , tryptophan , methionine , leucine , phenylalanine , threonine , and lysine .
Exceptionally , histidine and arginine are indicated as semi - essential amino acids as a little amount of them are synthesised in the body . Deficiency of these amino acids may give adverse effects like retarded growth , weak immunity , early ageing , etc.
ii ) Non - Essential Amino Acids : These amino acids are also termed as dispensable amino acids , and are synthesised in the body
Examples glycine , serine , threonine , glutamate , glutamine , aspartate asparagine , cysteine , and proline ,
Table 1.1 : Essential and Non - Essential Amino Acids
Essential │ Non - Essential
Valine │ Glycine
Isoleucine │ Tyrosine
Tryptophan │ Proline
Methionine │ Cysteine
Arginine │ Aspartic acid
Leucine │ Alanine
Phenylalanine │ Serine
Threonine │ Hydroxyproline
Lysine │ Cystine
Histidine │ Glutamic acid
3 ) On the Basis of Polarity : On the basis of polarity, amino acids are of four types :
i ) Non-Polar Amino Acids :
example:- alanine , valine , leucine , isoleucine , phenylalanine , glycine , tryptophan , methionine , and proline ) are neutral molecules having an equal number of amino and carboxyl groups . The nature of these amino acids is hydrophobic and the ' R ' side chain does not have any charges
ii) Polar Amino Acids with No Charge : These amino acids do not bear any charge on the side chain ' R ' .
example:-
serine
threonine
tyrosine
cysteine
glutamine
asparagine
They are involved in hydrogen bonding of protein structure .
iii) Polar Amino Acids with Positive Charge :
example:- lysine , arginine ,and histidine) carry more number of amino groups . thus , are basic in nature , The ' R side chain of these amino acids bears a positive charge .
iv ) Polar Amino Acids with Negative Charge :
example:- (aspartic acid and glutamic acid ) are acidic in nature because they carry more number of carboxyl groups . The ' R ' group of these amino acids bears negative charge. These amino acids are also termed as dicarboxylic mono-amino acids
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