Amino acids Classification: How to Classify Amino acids?

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.

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 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)

2) 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)

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 letter symbol	IUPAC name	Source
Glycine (glycos^G=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

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.

Amino Acids Classification and its Basics - Biochemistry Den

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

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 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.

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 which possesses 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

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
TyrosineIsolated 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

Additional readings:

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Amino Acids Classification and its Basics