Amino Acids

In simplest terms, proteins are polymers. A polymer is any compound made up of multiple smaller units or compounds known as monomers.

Every protein molecule is a ‘polypeptide’ (poly + peptide). It means that it is a polymer of smaller units called ‘peptides’ or ‘amino acids’. There are a total of 20 amino acids. All the proteins on Earth, yes, millions of biomolecules identified to date, are made up of varying numbers and compositions of these 20 amino acids*. These amino acids are also depicted using 3-letter and 1-letter codes as shown below:

These 3-letter and 1-letter codes are especially helpful in bioinformatics. For instance, suppose a protein has the following amino acid sequence:

MGHIVTA

This sequence can be translated as follows:

methionine-glycine-histidine-isoleucine-valine-threonine-alanine

Of course, the actual proteins are much longer, with the number of amino acids ranging from <100 to >25000 (including the proteins identified to date).

In mammals, such as humans, some of these amino acids are not synthesized within the body and are hence required in the diet. These amino acids are known as essential amino acids. On the other hand, the remaining amino acids are produced within our bodies and are thus not required to be ingested via food.

The essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

The non-essential amino acids include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

Furthermore, each amino acid comprises an amino group, a carboxylic acid group, and a side chain. The side chains present in the amino acids are primarily responsible for the distinct characteristics of these amino acids. Based on the characteristics of these side chains, amino acids are classified as follows:

Acidic: Aspartic acid, glutamic acid

Aliphatic: Alanine, glycine, isoleucine, leucine, proline, valine

Amidic (Containing amide group): Asparagine, glutamine

Aromatic: Phenylalanine, tryptophan, tyrosine

Basic: Arginine, histidine, lysine

Hydroxylic: Serine, threonine

Sulphur-containing: Cysteine, methionine

OR

Non-polar (Hydrophobic): Alanine, glycine, isoleucine, leucine, methionine, phenylalanine, proline, tryptophan, valine

Polar: Asparagine, cysteine, glutamine, serine, threonine, tyrosine

Positively-charged: Arginine, histidine, lysine

Negatively-charged: Aspartate, Glutamate