Amino Acids and Their Classification
Amino Acids
Amino acids are organic acids having an amino group attached to a chain containing an acid group. It is the building blocks of proteins and play a crucial role in various biological processes. Although the amino group can be anywhere on the chain, amino acid derived from proteins have the amino group on the alpha carbon, that is, the carbon atom next to the carboxyl group. The general formula of alpha amino acids is shown in left side figure. Amino acids are generally colorless crystalline solids having melting points or decomposition points is above 200°C. They are soluble in water but insoluble in non-polar organic solvents.
More than 200 amino acids have been isolated and identified but only 25 are obtained upon hydrolysis of typical proteins. Out of 25, only 20 amino acids are of general occurrence because they are usually found in all proteins. Again, 10 out of 25 amino acids are essential amino acids. Also, 23 out of 25 amino acids are α-amino acids whereas remaining 2, i.e., proline and hydroxyproline, are imino acids.
Twenty amino acids are specified by the genetic code and are utilized in ribosomes. They are: Glycine, Serine, Asparagine, Alanine, Threonine. Glutamic acid, Valine, Cysteine, Glutamine, Leucine, Methionine, Arginine, Isoleucine, Aspartic acid, Lysine, Histidine, Phenylalanine, Tyrosine, Tryptophan and Proline
Classification of Amino Acids
Amino acids may be classifies in a number of ways. The following classification which is based on the properties of the side chain of the amino acids is considered to be the most convenient one.
Neutral Amino Acids
These amino acids contains same number of amino and carboxyl groups.
Acidic Amino Acids
These amino acids contains more carboxyl groups than amino groups.
Basic Amino Acids
These amino acids contains more amino groups than carboxyl groups.
Sulphur Containing Amino Acids
These amino acids contains sulphur atom(s).
Aromatic Amino Acids
These amino acids contains aromatic ring.
Heterocyclic Amino Acids
These amino acids contains heterocyclic ring.
Essential and Non-essential Amino Acids
The body can synthesize some but not all of the amino acids that it needs for maintaining good health. Those amino acids that cannot be synthesized by the body and must be supplied in the diet are called Indispensable or Essential Amino Acids. These amino acids must be supplied in the diet either as free amino acids or as constituents of dietery proteins to maintain the normal growth and the nitrogen balance in the body. The deficiency of essential amino acids may cause various diseases like nervous breakdown, inhibition of full mental growth and their continuous deficiency may even cause death in young animals. The nine essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
| Daily Requirements of Essential Amino Acids for Adults | |
|---|---|
| Essential Amino Acids | Requirements in gms |
| Isoleucine | 1.4 |
| Leucine | 2.2 |
| Lysine | 1.6 |
| Methionine | 2.2 |
| Phenylalanine | 2.2 |
| Threonine | 1.0 |
| Tryptophan | 0.5 |
| Valine | 1.6 |
Since the formation of cystine and tyrosine in the body is dependent on the presence of adequate amounts methionine and phenylalanine respectively, they are considered as partly essential amino acids. Arginine and glycine are also considered to be partly essential amino acids because of their limited rate of synthesis in the body.
The amino acids that can be synthesized from other compounds by the tissues of the body are called Dispensable or Non-essential Amino Acids. Alanine, Asparagine, Aspartic Acid, Glutamic Acid and Serine are the non-essential amino acids.
Conditional Amino Acids
Conditional amino acids are usually not essential except during illness, stress or pregnancy. During these times, it's necessary to get them from food or supplements. There are seven conditional animo acids. They are Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine and Ornithine.
| Classification of Amino Acids | ||
|---|---|---|
| Non-essential | Conditionally essential | Essential |
| Alanine | Arginine | Histidine |
| Asparagine | Cysteine | Isoleucine |
| Aspartate | Glutamine | Leucine |
| Glutamate | Glycine | Lysine |
| Serine | Proline | Methionine |
| Tyrosine | Phenylalanine | |
| Threonine | ||
| Tryptophan | ||
| Valine | ||
