Classification of Amino Acids

Amino acids are organic molecules that, when combined with other amino acids. Amino acids are essential to life, since the proteins they produce are involved in almost all cell activities. Some proteins function as enzymes, while others serve as antibodies or support the structure. Although there are hundreds of amino acids in nature, proteins are made up of a set of 20. From a structural standpoint, amino acids typically contain a carbon atom, a hydrogen atom, a carboxyl group, an amino group and a variable group.

Classification of amino acids

• Non-essential amino acid

While all amino acids are required for life, not all of them can be synthesised in the body. The body can naturally produce 11 out of the 20 amino acids known as non-essential amino acids.

For example

• Alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine and tyrosine are the 11 non-essential amino acids naturally produced by the body.

Note

A.Non-essential amino acids

• Non-essential amino acids, except for tyrosine, are derived from products or intermediates of important metabolic pathways. 

• Alanine and aspartate, for example, are formed from compounds produced during cellular respiration.
• Pyruvate, a byproduct of glycolysis, is used to make alanine. 
• Oxaloacetate, a citric acid cycle intermediate, is used to make aspartate. 
• Six of the non-essential amino acids (arginine, cysteine, glutamine, glycine, proline and tyrosine) are considered conditionally essential, meaning that dietary supplementation may be necessary during illness or in children.

• Essential amino acid

The amino acids that the body cannot produce or synthesise independently, on its own, are known as essential amino acids.

For example

• The nine essential amino acids are: lysine, tryptophan, isoleucine, phenylalanine, leucine, histidine, methionine, threonine and valine. 
• Dietary sources of essential amino acids are required.
• Eggs, soy protein, and whitefish are all good suppliers of these amino acids.
• Plants, unlike humans, are able to synthesise all 20 amino acids.

• Conditional amino acid

Conditional amino acids are amino acids that are usually not essential but become so at times of disease or stress. Prematurity in babies, for example, may necessitate the use of these amino acids. Cysteine, arginine, tyrosine, glutamine, ornithine, glycine, serine and proline are among the six conditionally essential amino acids.

Classification of amino acids based on R group

Based on the properties of the “R” group in each amino acid, the amino acids can be classified  into five  groups:

1. Non-polar amino acids
2. Polar amino acids
3. Positively charged amino acids
4. Negatively charged amino acids
5. Aromatic amino acids

• Polar amino acids

Polar amino acids have hydrophilic “R” groups, which means they prefer to interact with watery solutions. 

• Non-polar amino acids

Non-polar amino acids (hydrophobic) are the polar amino acids that avoid contact with liquid. These interactions are crucial for protein folding and give proteins their three-dimensional structure.  

Note

The non-polar amino acids are hydrophobic, whereas the rest of the amino acids are hydrophilic.

• Non-polar amino acids

• Ala: Alanine  
• Gly: Glycine       
• Ile: Isoleucine           
• Leu: Leucine
• Met: Methionine
• Trp: Tryptophan   
• Phe: Phenylalanine  
• Pro: Proline
• Val: Valine

• Polar amino acids

• Cys: Cysteine   
• Ser: Serine        
• Thr: Threonine
• Tyr: Tyrosine 
• Asn: Asparagine 
• Gln: Glutamine

• Polar basic amino acids (positively charged)

• His: Histidine    
• Lys: Lysine         
• Arg: Arginine

• Polar acidic amino acids (negatively charged)

• Asp: Aspartate  
• Glu: Glutamate

• Aromatic amino acids

Aromatic amino acids, such as phenylalanine, tyrosine, and tryptophan, are nonpolar because of their aromatic side chains (hydrophobic). 

Classification system for amino acids based on their metabolic fate:

• Glucogenic amino acids: These are precursors of gluconeogenesis, which results in glucose production. Alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, methionine, proline, serine and valine are some of the amino acids found in the human body.

• Ketogenic amino acids: Ketone bodies are formed when these amino acids are broken down. Leucine and Lysine are two amino acids.

• Both glucogenic and ketogenic amino acids: Amino acids that are both glucogenic and ketogenic break down to form precursors for both ketone bodies and glucose. Isoleucine, tryptophan, phenylalanine and tyrosine are all essential amino acids.

Conclusion

Amino acids are a class of neutral substances that are separated chemically, primarily due to their ampholytic characteristics, and biochemically, primarily due to their role as protein components, from other natural compounds. An amino acid is a carboxylic acid with a specific stereochemistry and an aliphatic primary amino group in the location of the carboxyl group. Proteins are biosynthesised from 20 amino acids in a genetically controlled system. As a result, amino acids are the fundamental building blocks of proteins.