classification of amino acids

Classification of amino acids

Biochemistry for medicswww.namrata.co

05/02/2023 1Namrata Chhabra

NAMRATA CHHABRA,M.D.

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Amino acids

Provide the monomer units from which the long polypeptide chains of proteins are synthesized


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L-amino acids and their derivatives participate in cellular functions as diverse as:

Nerve transmission and the Biosynthesis of porphyrins, Purines, pyrimidines, and urea.

Amino acids

05/02/2023Namrata Chhabra

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Short polymers of amino acids called peptides perform prominent roles in the neuroendocrine system as :

hormones, hormone-releasing factors, neuromodulators, or neurotransmitters.

Amino acids


General structure of an amino acid

Each amino acid (except proline) has a carboxyl group, an amino group and a distinctive side chain bonded to the alpha carbon atom.

At physiological pH the carboxyl group is dissociated forming the negatively charged carboxylate ion(-COO-), and the amino group is protonated(-NH3

+)


Classification of amino acids

Amino acids can be classified in 4 ways:1.Based on structure2.Based on the side chain characters3.Based on nutritional requirements4.Based on metabolic fate


1) Aliphatic amino acidsThey are classified in three broad categories: Mono amino mono carboxylic acids They are further

subdivided in 5 groups:a. Simple amino acids-example: glycine, alanineb. Branched chain amino acids-e.g.: valine,leucine,isoleucinec. Hydroxyl group containing amino acids-e.g.: serine, threonined. Sulphur containing amino acids-e.g.: cysteine, cystine,

methioninee. Amide group containing amino acids-e.g.- asparagine,glutamine

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1) Classification based on structure

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Mono amino dicarboxylic acidExample :aspartic acid, glutamic acid Di /poly amino mono carboxylic acidExample : lysine, arginine2) Aromatic amino acids3) Heterocyclic amino acids4) Imino acids5) Derived amino acids

1) Classification based on structure(contd.)


1) Aliphatic amino acids:a) Mono-amino mono-carboxylic acids:

Simple amino acids: Glycine , Alanine


a) Mono-amino mono-carboxylic acids(contd.)

Branched chain amino acids: Valine, Leucine and Isoleucine


a) Mono-amino mono-carboxylic acids (contd.)

-OH group-containing amino acids: Serine and Threonine


a) Mono-amino mono-carboxylic acids(contd.)

Sulfur-containing amino acids: Cysteine, Cystine(Formed by linking of two cysteine residues) and Methionine.


a) Mono-amino mono-carboxylic acids (contd.)

Amide group-containing amino acids:Glutamine and Asparagine


a) Mono-amino di-carboxylic acidsAspartic acid and Glutamic acid


a) Di- basic mono-carboxylic acids:

Arginine and Lysine


ii ) Aromatic amino acids

Phenyl alanine and tyrosine


iii) Heterocyclic Amino Acids:Tryptophan and Histidine


iv) Imino acid

Proline


v) Derived Amino Acids:

Non-α-amino acidse.g.: β-alanine, γ-amino butyric acid (GABA), δ-amino Levulinic acid

Derived and Incorporated in tissue proteins:e.g.: Hydroxy-proline, hydroxy-lysine

Derived but not incorporated in tissue proteins:e.g.: Ornithine, Citrulline, Homocysteine, Arginosuccinic acid


2) Classification based on side chain characters

A. Amino acids with a non-polar side-chain:e.g.: Alanine, Valine, Leucine, Isoleucine,

Phenylalanine, Tryptophan, Proline

Each of these amino acids has a side chain that does not bind or give off protons or participates in hydrogen or ionic bonds.

Side chains of these amino acids can be thought of as “Oily” or lipid like, a property that promotes hydrophobic interactions.



A) Amino acids with a non-polar side-chain:

B) Amino acids with a polar but uncharged side-chain:

e.g. Serine, Threonine, Tyrosine, Cysteine, Asparagine and Glutamine. These amino acids are uncharged at neutral pH, although the side

chains of cysteine and Tyrosine can lose a proton at an alkaline pH. Serine , Threonine and Tyrosine each contains a polar hydroxyl group

that can participate in hydrogen bond formation. Side chains of Asparagine and Glutamine contain a carbonyl group and

amide group, they can also participate in hydrogen bond formation.



Amino acids with a polar but uncharged side-chain

C) Amino acids with a charged side-chain

a) Amino acids with a positively charged side-chain:The basic amino acids- Lysine, Arginine and Histidine

b) Amino acids with a negatively charged side-chain:The acidic amino acids- Glutamic acid and Aspartic acid

They are hydrophilic in nature.



Amino acids with a charged side-chain

3)- Classification based on nutritional requirements

I. Essential amino acids:These amino acids cannot be synthesized in the body and have to be present essentially in the diet. Examples-

Valine, Isoleucine, Leucine, Lysine, Methionine, Threonine, Tryptophan, and Phenylalanine.


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II. Semi-essential amino acids: These amino acids can be synthesized in the body but

the rate of synthesis is lesser than the requirement(e.g. during growth, repair or pregnancy)

Examples-Arginine and Histidine.



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III. Non-essential amino acids: These amino acids are synthesized in the body, thus

their absence in the diet does not adversely affect the growth.

Examples- Glycine, Alanine, and the other remaining amino acids.



4)-Classification based on metabolic fate

The carbon skeleton of amino acids can be used either for glucose production or for the production of ketone bodies, Based on that I.Both glucogenic and ketogenic amino acids: Isoleucine, Tyrosine, Phenylalanine and Tryptophan


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II. Purely Ketogenic amino acids:

Leucine and Lysine



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II.Purely Glucogenic amino acids:

The remaining 14 amino acids are glucogenic- Alanine, Valine Serine, Threonine, Glycine, Methionine, Cysteine, Cystine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Histidine and Arginine.



Non standard amino acids

Of the over 300 naturally occurring amino acids, 20 constitute the monomer units of proteins. These 20 amino acids are called the Primary or Standard amino acids.Selenocysteine is the 21st Amino AcidThe other are Pyroglutamate and Pyrolysine.


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classification of amino acids

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