What are the Classification of Amino acids?

amino acids classification

Amino acids can be classified into 4 types:

  1. Classification based on the position of “-NH2
  2. Classification based on the composition of “-R’ side chain
  3. Classification based on the Nutritional requirement
  4. Classification based on the Metabolic Fate

a) Classification based on the position of “-NH2”:

Amino acids can be classified into THREE types:

i) α-amino acid: The amino group attached to the next carbon of the carboxyl group is called “α-amino acid” All naturally occurring amino acids are in “α-L-amino acids”.

ii) β-amino acid: the amino group attached to the third carbon (numbering from Carboxyl group) of the amino acid is called “β-amino acid”. Eg: β-alanine, it is one of the end product of Pyrimidine catabolism.

iii) γ-amino acid: the amino group attached to the Fourth carbon (numbering from Carboxyl group) of the amino acid is called “γ-amino acid”. Eg: GABA (Gamma Amino Butyric Acid)

 

amino acid types based on amino group position

b) Classification based on the composition of “-R’ side chain:

Based on the composition of ‘R’ side chain, amino acids can be categorized into 8 types: (Fairley & Kigour, 1966)

a) Neutral Amino Acids (or) Simple amino acids:

These have no functional group in the side chain. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
Glycine (glycosG=sweet) G Gly α-amino acetate Animal source are Scleroproteins, Gelatin and silk fibroin.Plant source are Glycine Max (Soya been)
AlanineIsolated from Silk Fibroin in 1888. A Ala α-amino propionate Alanine is present in Silk fibroin alon with Glycine
Valine V Val α-aminoisovalarate
LeucineIsolated from Cheese by Proust in 1819. L Leu α-aminoisocaproate Isolated from cheese, but later it was obtained in purer form fro hydrolysates of wool.
IsoleucineDiscovered by Paul Erhlish (LT 1854 to 1915) I Ile α-amino-β-methylvalarate

aliphatic side chain amino acid

b) Hydroxyl Group containing amino acids:

These contain a hydroxyl group in their side chain. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
SerineDerived from the Serum S Ser α-amino-β-hydroxyl propionate Silk protein, Sericin, and Fibroin
ThreonineDiscovered by Meyer & Rose in 1936 T Thr α-amino-β-hydroxyl butyrate Threonine is less abundant than serine in most proteins.

hydroxy side chain amino acid

c) Sulphur Containing amino acids:

These posses a sulfur atom in the side chain. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
CysteineIsolated from Urinary stones in 1843 C Cys α-amino-β-mercaptopropionate Fibrous proteins such as Keratin from hair are especially rich in cysteins (12%)
Methionine M Met α-amino-β-methylmercaptobutyrate

sulphar containing amino acid

d) Acidic amino acids:

These have a Carboxyl group in the side chain

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
Aspartic acidDiscovered by Ritthausen in 1868. D Asp α-aminosuccinate It is the parent compound of aspargine.
Glutamic acidDiscovered by Ritthausen in 1866 E Glu α-aminoglutarate It is found in Gluten. It is the parent compound of Glutamine.

acidic side chain containing amino acid

e) Basic amino acids:

These possess an amino group in the side chain. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
Arginine R Arg α- amino-δ-guanidinovalarate(Guanidonium group is present) It is abundant in highly basic proteins of the cell nucleus (histones) and in Sperm proteins.
Lysine K Lys α, ε- diaminocaproate It is present in plant proteins like Corn and Wheat.

basic group amino acid

f) Heterocyclic amino acid:

These amino acids have in their side chain a ring which possess at least one atom other than the carbon. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
TryptophanIt was discovered in the laboratory of F.G.Hopkins W Trp α-amino-β—3-indolepropionate (or) β–indolylalanine
Histidine H His α-amino-β-Imidazolepropionate Hemoglobin, Protamines and Histones

 base group amino acids

g) Aromatic amino acid:

These have a benzene ring in the side chain. Eg:

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
Phenylalanine F Phe α-amino-β-phenylpropionate
TyrosineIsolated from Cheese in 1857 Y Tyr α-amino-β-(p-hydroxy phenyl) propionate Cheese

phenylalanine tyrosine

h) Imino acid:

These are also heterocyclic compounds, which have “imino group” (-NH-) instead of amino group (-NH2).

Name of the Amino acid Single letter symbol Three letter symbol IUPAC name Source
Proline P Phe 2-pyrrolidinecarboxylate Zein from Corn and Gelatin
Hydroxy Proline Hy.Pro

c) Classification based on the Nutritional requirement:

Based on Nutritional requirement, amino acids can be divided into 3 types.

  1. Essential Amino acids (EAA)
  2. Non-Essential Amino acids (NEAA)
  3. Semi-Essential Amino acids (SEAA)

1. Essential Amino acids (EAA):

Some of the amino acids don’t synthesize in the human body. It should be supplied through diet. They are required for proper growth and maintenance of the individual.

Eg:

MATT VIL PHLy

      (or)

      PVT TIM HALL

M= Methionine A=Arginine T=Threonine T=Tryptophan V=Valine

I=Isoleucine L=Leucine P=Phenylalanine H=Histidine L=Lysine

2. Non-Essential Amino acids (NEAA):

The body can synthesize about 10 amino acids to meet the biological needs, hence they need not be consumed in the diet.

Eg: Gly, Ala, Ser, Cys, Asp, Asn, Glu, Gln, Tyr and Pro.

3. Semi-Essential Amino acid:

Histidine and Arginine are semi-essential amino acids. Growing children require them in food. But they are not essential for the adult individual.

d) Classification based on the Metabolic Fates

Amino acids can be classified based on the metabolic fate:

i) Purely ketogenic amino acids

ii) Ketogenic and Glucogenic amino acids

iii) Purely Glucogenic amino acids

i) Purely ketogenic amino acids:

Leucine is purely ketogenic because it is converted into ketone bodies.

ii) Ketogenic and Glucogenic amino acids:

During metabolism, part of the carbon skeleton of these amino acids will enter the ketogenic pathway and the other part of the glucogenic pathway.

Eg: Lys, Ile, Phe, Tyr & Trp are partially ketogenic and partially glucogenic.

iii) Purely Glucogenic amino acids:

All the remaining 14 amino acids are purely glucogenic as they enter only into the glucogenic pathway.

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