Guide: Amino acids – Classification and Characteristics (Properties)

Amino acids are the building blocks of proteins. They are carboxylic acids with a carboxyl group, primary amine group ( except Proline) and a variable side chain (R group) attached to the α Carbon atom.

The basic 20 amino acids are genetically coded and about 300 amino acids exist due to slight modification of these amino acids. The properties of these amino acids are attributed to the R group which is distinct to each amino acid.

Characteristics of Amino Acids

• The Amino acids that occur in nature are L-α – amino acids which means the primary amino group is left sided to the α C atom of amino acid. D amino acids do exist, but are not biologically useful. In the body, D form is converted to L form by liver

• At physiologic pH, the carboxyl group (-COOH) is dissociated to negatively charged carboxylate ion(-COO^-_) and amino group is protonated (-NH₃⁺)

• Amino acids may have positive, negative or Zero net charge. Molecules with equal number of ionisable group of opposite charges (like -COO^– and -NH₃⁺) are called Zwitterions. At its isoelectric pH, amino acids exist as Zwitterions.

• The charged functional groups of amino acids makes them soluble in polar solvents like water and ethanol but insoluble in non-polar solvents like ether and benzene.

• Amino acids are colourless as they do not absorb visible light. But some amino acids like Phenyl alanine and Tryptophan absorbs high UV light.

• The chemical properties of amino acids and the chemical reaction of the proteins are determined by the functional R group.

Classifications of Amino Acids

Amino acids can be classified based on

•  The C atom to which functional group is attached
  • α amino acids
  • Non α amino acids
• The  distinctive side chain – R group
  • Aliphatic amino acids
    • Amino acids with non – polar side chain
    • Amino acids with uncharged polar side chain
    • Amino acids with acidic side chain
    • Amino acids with basic side chain
  • Aromatic amino acids
• The Nutrition process
  • Essential amino acids
  • Semiessential amino acids
  • Non Essential amino acids
• The metabolic fate
  • Ketogenic
  • Glucogenic 
  • Both Ketogenic and Glucogenic

Based on the C atom to which functional group is attached

α Amino Acids

In α amino acids, the amino (-NH₃) group and carboxyl group (- COOH) are attached to the α C atom. Most of the biologically important amino acids are α amino acids. Eg:  α – alanine

Non α Amino Acids

The amino acids in which either the amino (-NH₃) group or carboxyl group (-COOH) is not attached to the α C atom. Eg: β alanine and γ amino butyrate 

Based on variable R-group

Aliphatic Amino Acids

• Simple amino acids: These are simple straight chain amino acids. Eg: Glycine, Alanine.

• Branched chain amino acids: The R group of these amino acids are aliphatic groups with branches. Eg: Valine, Leucine and Isoleucine

• Sulphur containing amino acids: Cysteine and Methionine

• Hydroxyl group containing amino acids: Serine and Threonine

• Amide group containing amino acids: Asparagine and Glutamine

Aromatic Amino acids

Phenylalanine, Tyrosine, Tryptophan and Histidine

Based on polarity and charge of the side chain:

Amino acids with non polar side chain 

• They are hydrophobic amino acids and does not dissolve in water
• All aromatic amino acids except histidine and all branched amino acids are non- polar
• Alanine, Methionine and Proline are non – polar

Amino acids with polar side chain

They are hydrophilic in nature and dissolve in water and ethanol.

• Charged polar amino acids
  1. Acidic amino acids like Aspartate and Glutamate and basic amino acids like Lysine, Arginine and Histidine are charged polar amino acids.

• Uncharged polar amino acids
  1. Hydroxyl and amide group containing aliphatic side chain amino acids are uncharged polar amino acids – Serine, Threonine, Asparagine, Glutamine.
  2. Glycine and Cysteine are uncharged polar amino acids.

• Acidic Amino acids
  1. Amino acids in which R-group is acidic or negatively charged
  2. Glutamic acid (Glutamate) and Aspartic acid (Aspartate)

Basic Amino acids

• Amino acids in which R-group is basic or positively charged
• Lysine, Arginine, Histidine

Based on Nutritional requirement

Essential Amino acids

The amino acids which are essential for biochemical processes and are not synthesised by the human body are called Essential Amino acids. They have to be supplemented through diet and dietary deficiency results in disorders.

Essential amino acids are Methionine, Threonine

Semi essential Amino acids

    Amino acids which are required for infants and not for adults are called Semi Essential Amino acids. Eg: Arginine

Non Essential Amino acids

All other amino acids are inherently synthesised by our body and are not required in diet. These are called as Non Essential Amino acids.

They are Glycine, Alanine, Serine, Cysteine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Tyrosine, Proline.

Based on Metabolic Fate

Ketogenic amino acids

Amino acid which results in the formation of Acetyl Co A and enter the ketogenic pathway to produce ketone bodies to be used as source of energy are called Ketogenic amino acid. Eg: Leucine

Glucogenic Amino acids

Amino acids which enter the glucogenic pathway to generate glucose are called Glucogenic amino acids. They are Glycine, Methionine, Aspartic acid, Alanine

Both Glucogenic and Ketogenic

Phenylalanine, Isoleucine, Tyrosine and Tryptophan are both glucogenic and ketogenic depending upon the metabolic process which is favoured to maintain homeostasis

Frequently Asked Questions

References

1. Harper’s Illustrated Biochemistry – 31^st edition
2. Lippincott Illustrated Reviews: Biochemistry
3. Leningher Principles of Biochemistry – 8^th edition
4. Self-Assessment and Review of Biochemistry – Rebecca James Perumcheril 
5. Mark’s Basic Medical Biochemistry – 4^th edition