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 acidSingle letter codeAbbrevationProperty
AlanineAAlaNon-essential
ArginineRArgConditionally essential
AsparagineNAsnNon-essential
Aspartic acidDAspNon-essential
CysteineCCysNon-essential
Glutamic AcidEGluNon-essential
GlutamineQGlnConditionally essential
GlycineGGlyNon-essential
HistidineHHisConditionally essential
IsoleucineIIleEssential
LeucineLLeuEssential
LysineKLysEssential
MethionineMMetEssential
PhenylalanineFPheEssential
ProlinePProNon-essential
SerineSSerNon-essential
ThreonineTThrEssential
TryptophanWTrpEssential
TyrosineYTyrEssential
ValineVValNon-essential

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