Amino acids : Classification and Properties

Amino acids are building blocks of proteins and each amino acid bind to its neighboring residue through a peptide bond.

There are 20 main amino acids present in nature. Out of these, the asparagine was discovered first in the year 1806. The amino acid composed of an amino group, a carboxyl group, and R group attached to the α – carbon atom.

The proline amino acid is an exception as it has a cyclic structure and instead of an amino group, it has imino group.

Classification of amino acids

Classification of the amino acid based on the chemical properties of the alkyl group.

For this type of classification, the glycine is an exception(R=H), the amino acids have at least one asymmetrical carbon atom (the α – carbon).

Classification of the amino acid based on solubility characteristics of the side chains

The ionization and polarity are referred to as solubility characteristics of the side chains.

On the basis of this, the R-groups fall into four classes:

1. Nonpolar (hydrophobic – 9)- Glycine, Alanine, Valine, Leucine, and Isoleucine, Phenylalanine, Tryptophan, Proline, methionine
2. Polar, negatively charged (acidic-hydrophilic – 2),- Aspartate, Glutamate
3. Polar, positively charged (basic-hydrophilic – 3) – Histidine, Lysine, arginine
4. Polar, neutral (un-ionized-Hydrophobic – 6) – Serine, Threonine, Asparagine, Glutamine, Tyrosine, and cysteine.

Out of these 2o amino acids, eight amino acids are essential as they are not synthesized by humans and they must be supplied through diet. Rest of the amino acids are synthesized by the body through various pathways.

Amino acid Single letter code Abbrevation Property
Alanine A Ala Non-essential
Arginine R Arg Conditionally essential
Asparagine N Asn Non-essential
Aspartic acid D Asp Non-essential
Cysteine C Cys Non-essential
Glutamic Acid E Glu Non-essential
Glutamine Q Gln Conditionally essential
Glycine G Gly Non-essential
Histidine H His Conditionally essential
Isoleucine I Ile Essential
Leucine L Leu Essential
Lysine K Lys Essential
Methionine M Met Essential
Phenylalanine F Phe Essential
Proline P Pro Non-essential
Serine S Ser Non-essential
Threonine T Thr Essential
Tryptophan W Trp Essential
Tyrosine Y Tyr Essential
Valine V Val Non-essential

Conditionally essential amino acids are synthesized by the body but may require supplement during certain physiological condition such as pregnancy.