In this article, we are adding Amino acids Classification details.
Amino acids are the building blocks of the molecular structure of the important and complex class of a compound known as Proteins.
The proteins on hydrolysis yield mixtures of the component amino acids.
Therefore, to understand the structure and chemistry of proteins, we have to first undertake the study of amino acids.
Table of Contents
Amino acids Classification
It can classify amino acids into 4 types:
- Classification based on the position of “-NH2”
- Classification based on the composition of “-R’ side chain
- Classification based on the Nutritional requirement
- Classification based on the Metabolic Fate
1) Classification based on the position of “-NH2”
They can classify amino acids 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 the Carboxyl group) of the amino acid is called “β-amino acid”. Eg: β-alanine, is one of the end products of Pyrimidine catabolism.
iii) γ-amino acid: the amino group attached to the Fourth carbon (numbering from the Carboxyl group) of the amino acid is called “γ-amino acid”. Eg: GABA (Gamma Amino Butyric Acid)
2) Classification based on the composition of “-R’ side chain
Based on the composition of the ‘R’ side chain, amino acids can be categorized into 8 types: (Fairly & Kigour, 1966)
i) 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 letters symbol | IUPAC name | Source |
| Glycine (glycosG=sweet) | G | Gly | α-amino acetate | Animal sources are Scleroproteins, Gelatin, and silk fibroin. Plant sources are Glycine Max (Soya been) |
| Alanine (Isolated from Silk Fibroin in 1888.) | A | Ala | α-amino propionate | Alanine is present in Silk fibroin along with Glycine |
| Valine | V | Val | α-amino isovalerate | |
| Leucine (Isolated from Cheese by Proust in 1819.) | L | Leu | α-aminoisocaproate | Isolated from cheese, but later it was obtained in purer form from hydrolysates of wool. |
| Isoleucine (Discovered by Paul Erhlish (LT 1854 to 1915)) | I | Ile | α-amino-β-methylvalarate |
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 |
| Serine (Derived from the Serum) | S | Ser | α-amino-β-hydroxyl propionate | Silk protein, Sericin, and Fibroin |
| Threonine (Discovered by Meyer & Rose in 1936) | T | Thr | α-amino-β-hydroxyl butyrate | Threonine is less abundant than serine in most proteins. |
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 |
| Cysteine (Isolated from Urinary stones in 1843) | C | Cys | α-amino-β-mercaptopropionate | Fibrous proteins such as Keratin from hair are especially rich in cysteine (12%) |
| Methionine | M | Met | α-amino-β-methylmercaptobutyrate |
d) Acidic amino acids
These have a Carboxyl group in the side chain
| Name of the Amino acid | Single letter symbol | Three letters symbol | IUPAC name | Source |
| Aspartic acid (Discovered by Ritthausen in 1868) | D | Asp | α-aminosuccinate | It is the parent compound of aspargine. |
| Glutamic acid (Discovered by Ritthausen in 1866) | E | Glu | α-aminoglutarate | It is found in Gluten. It is the parent compound of Glutamine. |
e) Basic amino acids
These possess an amino group in the side chain. Eg:
| Name of the Amino acid | Single letter symbol | Three letters 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. |
f) Heterocyclic amino acid
These amino acids have in their side chain a ring that possesses at least one atom other than carbon. Eg:
| Name of the Amino acid | Single letter symbol | Three letter symbol | IUPAC name | Source |
| Tryptophan (It 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 |
g) Aromatic amino acid
These have a benzene ring in the side chain. Eg:
| Name of the Amino acid | Single letter symbol | Three letters symbol | IUPAC name | Source |
| Phenylalanine | F | Phe | α-amino-β-phenylpropionate | |
| Tyrosine (Isolated from Cheese in 1857) | Y | Tyr | α-amino-β-(p-hydroxy phenyl) propionate | Cheese |
h) Imino acid
These are also heterocyclic compounds, which have “imino group” (-NH-) instead of an amino group (-NH2).
| Name of the Amino acid | Single letter symbol | Three letters symbol | IUPAC name | Source |
| Proline | P | Phe | 2-pyrrolidinecarboxylate | Zein from Corn and Gelatin |
| Hydroxy Proline | – | Hy.Pro |
3) Classification based on the Nutritional requirement
Based on Nutritional requirements, amino acids can be divided into 3 types.
- Essential Amino acids (EAA)
- Non-Essential Amino acids (NEAA)
- Semi-Essential Amino acids (SEAA)
1. Essential Amino acids (EAA)
Some amino acids don’t synthesize in the human body. It should be supplied through diet. They are required for the 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
In this Amino acids classification, it can be classified based on the basis of metabolic fate:
- Purely ketogenic amino acids
- Ketogenic and Glucogenic amino acids
- 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.
Reference: Structural Amino acids
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