amino acids

Learning Objectives

In this lesson, we will learn

1. What is an amino acid? 

2. What are the essential and non-essential amino acids? 

3. Different types of amino acids based on their structure and the structure of their side chains 

4. Some key properties of amino acids

5. A basic overview of peptides

What is an amino acid?

An amino acid is an organic molecule that contains an amino group, a carboxyl group, and a side chain (R). They are the building blocks of proteins. About 500 amino acids are known, but there are 20 standard amino acids out of which almost all proteins are made.

9 out of the 20 standard amino acids are “essential” amino acids for humans. They cannot be built from other compounds by the human body, and so must be taken in as food.

The amino and carboxyl groups of the basic 20 amino acids are both covalently bonded to a central carbon atom. That carbon atom is also bonded to a hydrogen atom and an R group. It is this R group, referred to as a side chain, that varies from one amino acid to another. The nature of the side chains accounts for the variability in the physical and chemical properties of the different amino acids.

Classification of amino acids

The amino acid can be classified based on their structure and the structure of their side chains i.e. the R chains. Now, two basic subcategories are

Non-polar amino acids – These are also known as hydrophobic. The R group can be either of alkyl groups with an alkyl chain or aromatic groups. The acids falling in this group are as below. The first seven are alkyl and the last two are aromatic.

1. Glycine
2. Alanine
3. Valine
4. Methionine
5. Leucine
6. Isoleucine
7. Proline
8. Phenylalanine
9. Tryptophan

Polar amino acids – If the side chains of amino acid contain different polar groups like amines, alcohols, or acids they are polar in nature. These are also known as hydrophilic acids. These are further divided into three categories:

1. Acidic – If the side chain contains an extra element of carboxylic acid component these are acid-polar amino acids. They tend to donate their hydrogen atom. These are – Aspartic Acid and Glutamic Acid.
2. Basic – These have an extra nitrogen group that tends to attract a hydrogen atom. The three basic polar amino acids are – Histidine, Lysine, and Arginine.
3. Neutral – These are neither acidic nor basic. They have an equal number of amino and carboxyl groups. Also, they have at least one hydrogen component connected to electronegative atoms. Some of these neutral acids are – Serine, Threonine, Asparagine, Cysteine, Tyrosine.

Amino acids can also be classified on the basis of their need for the human body and their availability in the human body.

1. Essential amino acids – These are the acids that cannot be synthesized in our bodies. We must rely on food sources to obtain these amino acids. They are - Leucine, Isoleucine, Lysine, Threonine, Methionine, Phenylalanine, Valine, Tryptophan and Histidine
2. Non-essential – These acids are synthesized in our bodies itself and we need not rely on outside sources for them. They are either produced in our bodies or obtained from protein breakdown.

Properties of amino acids

1. Each amino acid has both acidic and basic groups. This is the reason they behave like salts.
2. Any amino acid in the dry state is in crystalline form. They exist as a dipolar ion. The COOH group exists as an anion and the NH2 group exists as a cation. This dipolar ion has a special name “Zwitter ion”.
3. In aqueous solution, alpha-amino acids exit in equilibrium between a cationic form, an anionic form, and dipolar ion.
4. The isoelectric point is the pH point at which the concentration of zwitterions is the highest and the concentration of cationic and anionic form is equal. This point is definite for every α-amino acid.
5. They are generally water-soluble and also have high melting points.

Peptides

Amino acids can undergo a condensation reaction in which the carbon atom of the carboxyl group of one amino acid bonds to the amino nitrogen atom of another amino acid. The result is called a peptide bond.

Polypeptides and proteins are long strings of amino acids. In general, a polypeptide is a sequence of 10 or more amino acids, while a protein is a polypeptide with a molecular weight of more than about 10,000 g/mol.

Proteins are very prevalent in living organisms. Hair, skin, nails, muscles, and the hemoglobin in red blood cells are some of the important parts of your body that are made of different proteins. The wide array of chemical, physiological, and structural properties exhibited by different proteins is a function of their amino acid sequences.