Classification of Amino Acid

CLASSIFICATION OF AMINO ACID

INTRODUCTION

• Amino acids are basic unit of protein
• Contain an amino group and carboxyl group (except proline).
• Unique side chain bonded to alpha carbon distinctly differentiates amino acid.
• Amino acids having an asymmetric carbon atom exhibit optical activity.
• Glycine is the simplest amino acid and has no asymmetric carbon atom and shows no optical activity.

Amino acid can be classified in 4 ways:

• Based on structure
• Based on polarity
• Based on ability of body to synthesize
• Based on metabolism

CLASSIFICATION BASED ON STUCTURE 

• Aliphatic Side Chains : Glycine, Alanine, Valine, Leucine, Isoleucine
• Side Chains Containing Hydroxyl (OH) Groups  :Serine, Threonine, Tyrosine
• Side Chains Containing Sulphur Atoms  :Cysteine, Cystine, Methionine
• Side Chains Containing Acidic Groups or their amides  : Aspartic acid, Asparagine, Glutamic acid, Glutamine
• Side Chains Containing Basic Groups  : Arginine, Lysine, Histidine
• Containing Aromatic Rings  : Phenylalanine, Tyrosine, Tryptophan(photochromosity seen in this group.)
• Imino acids or heterocyclic amino acids : – One of the 20 amino acids, proline is an imino (-NH) acid not an amino (-NH₂) acid, as are other 19. 

CLASSIFICATION BASED ON POLARITY

•  Cell membrane has its hydrophobic ends in the centre of membrane while its hydrophilic ends lie on the outer surface of membrane.
• Hydrophobhic or Non polar aminoacid are present in transmembrane region.
•  Hydrophillic or polar amino acids are present on outer surface of membrane.

Polar Amino Acids (Hydrphilic)
a. Charged

• Acidic Amino acids-Aspartic Acid (Aspartate), Glutamic Acid (Glutamate)
• Basic Amino Acids-Histidine, Arginine,Lysine

b. Uncharged

• Aliphatic amino acid with hydroxyl group as side chain: Serine, Threonine
• Aliphatic amino acids with amide group: Asparagine, Glutamine
• Simple Amino acid: Glycine alone
• Sulfur containing Aminoacid: Cysteine alone. 

Nonpolar Amino Acids (Hydrophobic)                                                

• Simple amino acid: Alanine alone
• Sulfur containing amino acid: Methionine alone
• Aromatic Amino acids except Histidine .
• All branched chain amino acids: Leucine(most non polar),Isoleucine and Valine
• Imino acid: Proline 

CLASSIFICATION BASED ON ABILITY OF BODY TO SYNTHESIZE

Essential

• Eight of these amino acids are essential (or indispensable) and cannot be produced by the body.
•  Leucine, Isoleucine, Lysine, Threonine, Methionine, Phenylalanine, Valine, Tryptophan

Nonessential

• Asparagine, Alanine, Arginine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Proline ,Glycine, Tyrosine, Serine

Semi-essential

• Histidine and Arginine

CLASSIFICATION BASED ON METABOLISM

• Ketogenic :Leucine, Lysine
• Glucogenic:Valine, Cysteine, Serine,Alanine, Histidine,Threonine, Arginine, Glycine, Glutamate, Proline/Hydroxy proline
• Both Glucogenic & Ketogenic :Isoleucine, Tyrosine ,Tryptophan , Phenylalanine 

AMINO ACID	CLASSIFICATION	SPECIAL Products
Isoleucine	Ketogenic+Glucogenic/Essntl/Aliphatic
Valine	Glucogenic/Essntl/Aliphatic(neutral)
Leucine	Ketogenic/ Essntl/Aliphatic(neutral)
Lysine	Ketogenic/ Essntl/Basic	Carnitine
Methionine	Glucogenic/Essntl/Sulphur(neutral)	Creatine, epinephrine, poiyamines
Phenyl alanine	Ketogenic+Glucogenic/Essntl/aromatic(neutral)
Threonine	Glucogenic/Essntl/Aliphatic(neutral)
Tryptophan	Ketogenic+Glucogenic/ Essntl/aromatic(neutral)	NAP, NADP, serotonin, melatonin
Alanine	Glucogenic/Non-Essntl/Aliphatic(neutral)	Coenzyme-A
Glycine	Glucogenic/ Non-Essntl/ Aliphatic(neutral)	Creatine, glutathione, heme, purines, conjugated bite acids
Serine	Glucogenic/Non-Essntl/ Aliphatic(neutral)	Phosphatidyt serine, sphingomyelins, choline
Cystiene	Glucogenic/Non-Essntl/Sulphur(neutral)	Glutathione, taurine, co enzyme-A, active sulfate
Tyrosine	Ketogenic+Glucogenic/ Non-Essntl/Aromatic(neutral)	Thyroxine, epinephrine, nor epinephrine, dopamine, melanin
Valine	Glucogenic/Non-Essntl/Aliphatic(neutral)
Proline	Glucogenic/Non-Essntl/Heterocyclic
Glutamate	Glucogenic/Non-Essntl/Acidic	GABA, glutathione, gamma- carboxy glutamate
Asparagine	Glucogenic/Non-Essntl/acidic
Glutamine	Glucogenic/Non-Essntl/acidic	Purines, pyrimidines, amino sugars
Arginine	Glucogenic/Semi-essntl/basic	Creatine , nitric oxide
Histidine	Glucogenic/Semi-essntl/Imidazole ring	Histamine

Limiting amino acid

• Insufficient in food.
• Wheat-lysine (the most deficient) and threonine (second limiting amino acid)
•  Rice-lysine
• Legumes-tryptophan or methionine (or cysteine)
• Maize-lysine and tryptophan

Isoelectric pH,

• In isoelectric ph an amino acid is in zwitterion form (dipolar molecule), i.e. has both positive and negative charge on same acid.
•  Overall molecule is electrically neutral

Important chemical reactions of amino acids

Reaction used to determine amino acid sequence in polypeptide chain.

• Sanger’s reaction : – Uses Sanger’s reagent ( 1 -fluoro-2,4-dinitrobenzene) to tag amino terminal.
• Edman’s reaction : – Uses Edman’s reagent (phenylisothiocyanate) to tag amino terminal.

Reaction used to identification of individual or group of amino acids

• Ninhydrin test: – All amino acids.
• Xanthophoretic reaction : – Aromatic amino acids^e (Tyrosine, tryptophan, phenylalanine).
• Millan’s test^e (Millon-Nasse reaction) : – Tyrosine° (phenol group of tyrosine). Therefore millon’s test is positive in tyrosinosis°.
• Aldehyde test : – Tryptophan (indole ring)
• Hopkins-tole reaction : – Tryptophan (indole ring)
• Sakaguch’s reaction : – Arginine (guanidinium group of arginine).
• Sulphur test : – Cysteine (sulphydryl group)
• Nitroprusside test : – Cysteine (sulphydryl group)
• Pauly’s test : – Histidine (imidazole group)
• Biuret reaction : – Peptide bond
• Diazo reaction ^Q (Pauli’s) : – Histidine or tyrosine.

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