Characteristics of Amino Acids 

Amino acids are organic acids having an amine group. They are the building blocks of protein that will also regulate its structure and function.

The characteristics of amino acids are such that they fulfill their biological and structural duties. Structurally, amino acids exist as L-amino acids and D-amino acids. Only the L-amino acids form proteins. There are a total of 20 known amino acids that are seen in proteins.

Characteristics of Amino Acids

Their common characteristics are, 

• They form polymers via peptide bonds to make polypeptides. 
• They have both acidic and base properties due to the presence of carboxyl and amino groups.
• Their structure and chemical functions differ with the side chains attached to them. The side chains are generally denoted as the R group.  
• All amino acids except glycine show chirality- the α-carbon is attached to four different molecules.
• The carbon atom next to the carboxyl group is termed the α-carbon. 
• Amino acids have a high melting point.

General Structure of Amino Acids

As the amino acids have chiral carbon, they are non-superimposable except glycine. However, they show stereoisomerism in L and D amino acids. These two are determined by the position of the -OH group. 

If the -OH group is on the left-hand side, the molecule is L-amino acid and if it is on the right, it is D amino acid. L stands for the Latin word Laevus and D is Dexter. They can rotate polarized light towards the left and right respectively. 

Classification of Amino Acids

The 20 amino acids are classified into various groups according to various factors. For convenience, they are generally classified based on 

1. R group
2. Polarity
3. Nutritional requirement
4. Metabolic product
5. Distribution in protein
6. Number of amino and carboxyl groups

A. Based on the R group

Amino acids can be classified into seven distinct groups based on the structure of their R groups.

1. Aliphatic side chains: Glycine, Alanine, Valine, Leucine, and Isoleucine
2. Hydroxyl group: Serine, Threonine, and Tyrosine
3. Sulfur-containing group: Cysteine and Methionine
4. Acidic amino acids: Asparagine and Glutamine
5. Basic amino acids: Lysine, Arginine, and Histidine
6. Aromatic amino acids: Phenylalanine, Tyrosine, and Tryptophan.
7. Imino acids: Proline

B. Based on polarity

Polarity is one of the main characteristics of amino acids that are directly linked to its function in the overall protein structure. Based on its polarity, amino acids are of four types. 

1. Non-polar amino acids: They are hydrophobic amino acids due to a hydrophobic uncharged R group. Amino acids in this group are alanine, glycine, leucine, isoleucine, valine, methionine, phenylalanine, tryptophan and proline.
2. Polar amino acids with neutral R group: These are those with a no-charge polar side group such as hydroxyl, sulfhydryl or amide groups. These groups form hydrogen bonds with each other to form the protein structure. Members of this group are asparagine, cysteine, glycine, glutamine, serine, threonine, and tyrosine.
3. Polar amino acids with positive ‘R’ group: Amino acids in this group include lysine, arginine, and histidine.
4. Polar amino acids with negative ‘R’ group: This group has the dicarboxylic mono amino acids such as aspartic acid and glutamic acid.

C. Based on nutritional requirement

Amino acids are categorized into essential and nonessential amino acids based on their nutritional requirement. 

Essential Amino Acids or Indispensable Amino Acids

These are important amino acids that are needed for the proper growth and functioning of the body But the body cannot produce these amino acids so they must be taken in the diet. Among these, the adult body can produce arginine and histidine but children cannot produce them. These two are known as semi-essential amino acids. The ten essential amino acids are,  

1. Arginine
2. Histidine
3. Isoleucine
4. Leucine
5. Lysine
6. Methionine
7. Phenylalanine
8. Threonine
9. Tryptophan
10. Valine

Non-Essential or Dispensable Amino Acids

Non-essential amino acids are the ones that the human body can produce. There are 10 such nonessential amino acids present.  

1. Alanine
2. Asparagine
3. Aspartic acid
4. Cysteine
5. Glutamic acid
6. Glutamine
7. Glycine
8. Proline
9. Serine
10. Tyrosine

D. Based on Metabolic Products 

Several amino acids function as a precursor for other important compounds such as glucose and fats. Depending upon what they end up being, amino acids are classified into three categories- 

• Glucogenic amino acids end up getting converted to glucose or glycogen for energy production. Such amino acids are alanine, aspartate, glycine, and methionine 
• Ketogenic amino acids such as leucine And lysine are converted into fats. 
• Glucogenic and ketogenic amino acids can be converted into either glucose or fat. Examples of such amino acids are isoleucine, phenylalanine, Tryptophan, and tyrosine. 

E. Based on distribution in protein

1. Standard protein amino acids are recognized by ribozyme auto aminoacylation systems and are thus used for the formation of proteins. Eg, 

2. Non-standard protein amino acids that are not required to form proteins. However, they are essential for other metabolic reactions and serve as metabolite intermediates. Eg. 

• Carboxyglutamate
• Diaminopimelate
• Hydroxylysine
• Hydroxyproline

3. Non-standard non-protein amino acids are the derivatives of standard amino acids and are used in metabolic reactions. Eg, 

• Alpha amino butyrate
• Beta-alanine
• Citrulline
• Ornithine

F. Based on the Number of Amino and Carboxyl Groups

• Monoamino- monocarboxylic amino acids: Glycine, alanine, proline, phenylalanine, methionine, serine, and threonine
• Monoamino-dicarboxylic amino acid: Aspartic and glutamic acid
• Diamino-monocarboxylic amino acids: Lysine, arginine, histidine.

Additional Reading

• Fat Metabolism: Synthesis and Oxidation
• Types, Properties & Biological Functions of Lipids