AMINO ACIDS CLASSIFICATION AND PROPERTIES

AMINO ACIDS

CLASSIFICATION

AND

PROPERTIES

 Amino acids are organic compounds containing

amine [- NH

]

carboxyl [-COOH]

side chain [R group]

 The major key elements if amino acids are carbon,

hydrogen, nitrogen, oxygen.

 About 500 amino acids are known (though only 20

appear in the genetic code) and can be classified in

many ways

WHAT ARE AMINO ACIDS.

BASIC STRUCTURE[SKELETON]

 Classification of amino acids gives the grouping

between 20 acids and a basic outline for grouping.

 It makes a clear idea to pick the amino acid type

 This is much useful for biochemists for the easy

understanding between each amino acids.

NEED FOR CLASSIFICATION

Classification:

Based on

 R group

 Polarity and R group

 Distribution in protein

 Nutritional requirements

 Number of amino and carboxylic groups

Based on R-Group

• Simple amino acids:

these have no functional group in their side

chain.

Example: glycine, valine, alanine, leucine,

isoleucine

• Hydroxy amino acids:

these have a hydroxyl group in their side

chain

Eg: serine, threonine

• Sulfur containing amino acids:

have sulfur in their side chain

Eg: cysteine, methionine

• Aromatic amino acids:

have benzene ring in their side chain

Eg: phenylalanine, tyrosine

• Heterocyclic amino acids:

having a side chain ring which possess at

least on atom other than carbon

Eg: Tryptophan, histidine, proline

• Amine group containing amino acids:

derivatives of amino acids in which

one of carboxyl group has been transformed

into an amide group

Eg: Asparagine, glutamine

• Branched chain amino acids:

A branched-chain amino acid (BCAA) is

an amino acid having aliphatic side-chains with

a branch

Eg: leucine, isoleucine, valine

• Acidic amino acids:

have carboxyl group in their side chain

Eg: Aspartic and Glutamic acid

• Basic amino acids:

contain amino group in their side chain

Eg: Lysine, Arginine

• Imino acid:

Amino acids containing a secondary amine

group

Eg: Proline

Polarity and R Group

• Amino acids with non polar R group:

these are hydrocarbons in nature,

hydrophobic, have aliphatic and aromatic groups

[aliphatic R groups]

Eg: Alanine, Valine, Leucine, Isoleucine, Proline.

[Aromatic groups]

Eg: Phenylalanine, Tryptophan,

Methionine(sulfur)

• Amino acids with polar but uncharged R

Group:

these amino acids are polar and possess

neutral pH value.

Eg: Glycine, Serine, Threonine, Cysteine,

Tyrosine, Glutamine, Asparagine.

• Negatively charged amino acids:

their side chain [R Group] contain extra

carboxyl group with a dissociable proton.

And renders electrochemical behaviour to

proteins

Eg: Aspartic acid and Glutamic acid

Aspartic acid

• Positively charged amino acid:

their side chain have extra amino group

Rendering basic nature to protein,

Eg: Lysine, Arginine, Histidine.

Lysine

Distribution in protein:

• Standard protein amino acids:

the amino acids that are used to form

proteins,

recognized by ribozyme autoaminoacylation

systems

Eg:

Histidine, isoleucine, leucine, lysine, methionine,

phenylalanine, threonine, tryptophan,

and valine

• Non standard protein amino acids:

these amino acids are not required to build

proteins.

have a vital role as metabolic intermediates.

Eg. Hydroxyproline, Hydroxylysine,

Carboxyglutamate, Diaminopimelate.

• Non standard non protein amino acid:

These are the derivative of amino acids and

have role in metabolism.

Eg: Alpha amino butyrate, Citruline, Ornithine,

beta-alanine.

Based on nutritional requirements:

• Essential amino acids:

Essential amino acids cannot be made by the

body.

As a result, they must come from food.

The essential amino acids are: Arginine,

histidine, isoleucine, leucine, lysine, methionine,

phenylalanine, threonine, tryptophan, and

valine.

• Non essential amino acids:

An amino acid that can be made by humans and

so is essential to the human diet.

The nonessential amino acids: Alanine,

asparagine, aspartic acid, cysteine,

glutamic acid, glutamine, glycine, proline,

serine, and tyrosine.

On basis of number of amino and carboxylic

groups:

Monoamino- monocarboxylic amino

acids

• glycine, alanine

• proline

• phenylalanine

• methionine

• serine, threonine

• Monoamino-dicarboxyli amino acid:

Aspartic and glutamic acid

• Diamino-monocarboxylic amino acids:

Lysine, arginine, histidine.

Properties : Physical

• Colourless

• Crystalline in nature

• Tasteless[tyrosine], sweet[glycine, alanine]

• Melting point above 200•C

• Soluble in polar solvent and Insoluble in non

polar solvent

• Have absorbance at 280nm

• Mol wt: 100 – 50,000Dt

• All amino acids possess optical isomers due to

the presence of asymmetric α-carbon atoms.

• Some are structurally stable and sterically

hindered [Glycine]

• Amino acids [proteins]posses enzymatic

activities

• Amino acids exhibit colloidal nature and

denaturing property

Chemical properties

• Decarboxylation:

The amino acids will undergo decarboxylation

to form the corresponding “amines”. Thus

amines are produced

• Histidine → Histamine + CO

• Tyrosine →Tyramine + CO

• Lysine →Cadaverine + CO

• Reaction with Ninhydrin:

• Reaction with Alkalies (Salt formation):

• The carboxyl group of amino acids can release a

ion with the formation of Carboxylate (COO

–

) ions.

• Reaction with Alcohols (Esterification) :

• the amino acids is reacted with alcohol to form,

“Ester”. The esters are volatile in contrast to the form

amino acids.

• Reaction with DANSYl Chloride:

DANSYl chloride means “Dimethyl Amino Naptha

Sulphonyl Chloride”.

When the amino acid reacts with DANSYl chloride

reagent, it gives a “Flourescent DANSYl derivative

• Reaction with acylating agents (Acylation):

When the amino acids react with “Acid chloride” and

acid anhydride in alkaline medium it gives “pthaloyl

amino acid

• Reaction with Sanger’s reagent:

• “1-flouro-2,4-dinitrobenzene” is called

Sanger’s reagent (FDNB).sanger’s reagent

reacts with α-amino acid to produce Yellow

coloured derivative, DNB-amino acid.

• Reaction with Edmann’s reagent:

Edmann’s reagent is “phenylisothiocyanate”. When amino

acids react with Edmann’s reagent

it gives “phenyl thiohydantoic acid” finally it turns into

cyclized form “Phenyl thiohydantoin” (Edmann’s derivative).

• Reference :

Dr. J.L. Jain – Fundamentals of Biochemistry by

Chand publications, New Delhi.