Classification of Amino Acids

Classification of amino acids or types of amino acids based of structure of substituent that is "R" group or side chain of amino acid is discussed herein.  

Amino acids are organic compounds which consists of amino group and carboxylic acid functional group. Amino acids are the building blocks of the proteins, DNAs & RNAs.

The classification of amino acids based on the types of the side chain is one of the methods to categorize the various amino acids. Other most important methods of classification is essential vs non-essential amino acids.

Keywords: Amino acid, Amine, Carboxylic acid, side chain.

Table of Contents

1. Introduction
2. Classification of Amino acids
3. Conclusion
4. FAQs

1. Introduction

Proteins are complex macromolecules composed of long chains of amino acids. There are 20 standard amino acids that are commonly found in proteins. These amino acids differ in their chemical properties, side chain structures, and functions. The classification of amino acids provides a framework for understanding their distinct characteristics and contributions to protein structure and function.

Typical structure of alpha-amino acid is shown below;

Fig 1 : Structure of Amino acid

2. Classification of amino acids

There are 20 amino acids found in nature which are building blocks of protein molecules. The amino acids are can be classified based on following criteria;

• Nature of "R" group
• Polarity & Charge
• Nutritional requirements
• Number of amino & carboxylic acid group

Here in this article we will learn about classification of amino acids based on the structure of the side chain ("R" group) as below;

• Aliphatic, 
• Aromatic, 
• Acidic, 
• Basic, 
• Hydroxylic, 
• Sulphur containing & 
• Amide group containing. 

2.1 Aliphatic Amino acids 

Aliphatic amino acids have hydrophobic side chains (R group) that are repelled by water and tend to cluster together in the interior of proteins

There are six aliphatic amino acids present in nature; 

1. Alanine

2. Glycine

3. Isoleucine

4. Proline

5. Leucine

6. Valine

Fig 2: Aliphatic amino acids

2.2 Aromatic Amino acids

Aromatic amino acids are a special class of amino acids that possess an aromatic ring in their side chains. These amino acids contribute to the structural stability and functionality of proteins. There are three natural amino acids which has aromatic side chain. 

1. Phenylalanine

2.Tryptophan

3. Tyrosine

They are involved in protein-protein interactions, ligand binding, and the formation of hydrophobic cores within proteins.

Fig 3: Aromatic amino acids

2.3 Acidic Amino acids

Acidic amino acids have side chains that can donate a proton, resulting in a negative charge. 

Two amino acids has acidic side chain

1. Aspartic acid

2. Glutamic acid

Fig 4: Acidic amino acids

2.4 Basic Amino acids

Basic amino acids have side chains that can accept a proton, resulting in a positive charge. 

There are three basic amino acids found in nature. 

1. Arginine

2. Histidine

3 Lysine

Fig 5: Basic amino acids

Acidic and basic amino acids play crucial roles in protein function, particularly in enzyme catalysis and protein-protein interactions.

2.5 Hydroxylic Amino acids

Hydroxylic amino acids have hydrophilic side chains (With OH group) that can interact with water molecules and other polar substances. These amino acids are vital for protein solubility and the formation of hydrogen bonds

Serine & Threonine are hydroxylic group containing amino acids.

Fig 6: Hydroxylic amino acids

2.6 Sulphur containing amino acids

Cysteine & Methionine are Sulphur containing amino acids.

Fig 7: Sulphur containing amino acids

2.7 Amide group containing amino acids

Asparagine & Glutamine are amide group containing amino acids. They are also known as polar amino acids.

Fig 8: Amide group containing amino acids

3. Conclusion

The classification of amino acids provides a systematic understanding of their properties and contributions to protein structure and function. Understanding the classification of amino acids is crucial for unravelling the intricate world of proteins and their biological functions.

To summarize this article, we have learned the classification of amino acids based on the structure of "R" group. The "R" group can be Aliphatic, Aromatic, Acidic, Basic, Hydroxylic, Sulphur containing & Amide group containing amino acids. 

4. FAQs

Q1: How many different types of amino acids are there?

There are 20 standard amino acids that are commonly found in proteins. However, there are a few additional amino acids that are found in specific proteins or organisms, making the total number slightly higher.

Q3: Are there amino acids with both acidic and basic properties?

Yes, there are amino acids with side chains that can exhibit both acidic and basic properties. For example, histidine has a side chain that can act as a weak acid or a weak base, depending on the pH of its environment.

Q4: Can amino acids with aromatic side chains participate in chemical reactions?

Yes, aromatic amino acids can participate in various chemical reactions due to the presence of their aromatic rings. For example, the aromatic side chains of tyrosine and tryptophan can undergo oxidation or form hydrogen bonds with other molecules.

Q5: Can amino acids be modified or chemically altered within proteins?

Yes, amino acids within proteins can undergo post-translational modifications, where chemical groups are added, removed, or modified. These modifications can alter the structure, function, and stability of proteins.

Friends, if you feel this article helpful then do write in the comment box. Also you can suggest me topics which you would be interested to learn. 

That's all for this topic. Thank you.