Classification of Amino Acids Based on Structure and Physicochemical Properties
Introduction to Amino Acids
Amino acids are the building blocks of proteins. Each amino acid has a central carbon (called the alpha carbon) bonded to four groups: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom (H), and a side chain (R group). The R group varies for each amino acid and determines its chemical nature.
Amino acids are classified based on the structure and nature of their side chains. These properties affect how amino acids interact with one another and their environment, influencing the structure and function of proteins.
Classification of Amino Acids
1. Based on Polarity and Charge
a) Nonpolar (Hydrophobic) Amino Acids
These amino acids have nonpolar side chains and tend to avoid water. They are usually found in the interior of proteins.
- Examples: Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline
b) Polar Uncharged Amino Acids
These amino acids have polar side chains that can form hydrogen bonds but do not carry a charge.
- Examples: Serine, Threonine, Asparagine, Glutamine, Cysteine, Tyrosine
c) Polar Charged Amino Acids
These have side chains that carry a charge at physiological pH.
- Acidic (Negatively Charged): Aspartic acid, Glutamic acid
- Basic (Positively Charged): Lysine, Arginine, Histidine
2. Based on Side Chain Structure
a) Aliphatic Amino Acids
These amino acids have straight or branched carbon chains in their side groups.
- Examples: Glycine, Alanine, Valine, Leucine, Isoleucine
b) Aromatic Amino Acids
These have aromatic rings in their side chains.
- Examples: Phenylalanine, Tyrosine, Tryptophan
c) Sulfur-Containing Amino Acids
These amino acids contain sulfur atoms in their side chains.
- Examples: Cysteine, Methionine
d) Hydroxyl-Containing Amino Acids
These amino acids have –OH groups that can form hydrogen bonds.
- Examples: Serine, Threonine, Tyrosine
e) Acidic and Basic Amino Acids
These amino acids carry either negative or positive charges, as discussed earlier.
3. Based on Nutritional Requirement
a) Essential Amino Acids
These cannot be synthesized by the human body and must be obtained from the diet.
- Examples: Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine, Histidine
b) Non-Essential Amino Acids
These can be synthesized by the body.
- Examples: Alanine, Asparagine, Aspartic acid, Glutamic acid
Conclusion
The classification of amino acids based on structure and properties helps us understand how they function in proteins. Their side chains influence protein folding, stability, solubility, and interactions with other molecules. This knowledge is fundamental in biochemistry and molecular biology.