Amino acids are organic molecules that serve as the building blocks of proteins.
They play essential roles in biological processes, acting as precursors for biomolecules and providing energy under certain conditions.
Each amino acid consists of:
Amino group (-NH₂)
Carboxyl group (-COOH)
Hydrogen atom (-H)
Variable side chain (R group) that determines its properties and function.
Classification of Amino Acids
Amino acids are categorized based on the characteristics of their side chains into four main groups:
Amino acids are classified based on the chemical properties of their side chains (R groups) into four main groups:
1) Nonpolar (Hydrophobic) Amino Acids
Properties: Side chains consist mainly of carbon and hydrogen, making them water-repellent.
Function: Poorly soluble in water, participate in hydrophobic interactions in protein folding.
Examples: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan.
2) Polar (Hydrophilic) Amino Acids
Properties: Side chains contain functional groups capable of forming hydrogen bonds with water.
Function: Soluble in water, contribute to hydrophilic interactions in protein structure.
Examples: Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine.
3) Acidic (Negatively Charged) Amino Acids
Properties: Contain carboxyl (-COO⁻) groups, making them negatively charged at physiological pH.
Function: Participate in acid-base interactions and ionic bonding.
Examples: Aspartic acid, Glutamic acid.
4) Basic (Positively Charged) Amino Acids
Properties: Contain amino (-NH₃⁺) or positively charged groups, making them positively charged at physiological pH.
Function: Participate in acid-base interactions and ionic bonding.
Examples: Lysine, Arginine, Histidine.
Comparison Table: Amino Acid Classification
Classification | Properties | Function/Interaction | Examples |
| Mainly C & H, no polar groups | Hydrophobic interactions, found in protein cores | Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan |
| Contain functional groups forming hydrogen bonds | Hydrophilic interactions, often found on protein surfaces | Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine |
| Contain carboxyl (-COO⁻) groups | Acid-base interactions, ionic bonds | Aspartic acid, Glutamic acid |
| Contain amino (-NH₃⁺) groups | Acid-base interactions, ionic bonds | Lysine, Arginine, Histidin |
Properties of Amino Acids
1) Chirality
Most amino acids exist in L- and D-forms (stereoisomers), but proteins in living organisms are composed of L-amino acids.
2) Zwitterionic Nature
At physiological pH, amino acids exist as zwitterions with both positively charged amino groups and negatively charged carboxyl groups.
Functions of Amino Acids
Building Blocks of Proteins
Amino acids polymerize into proteins, which perform structural, enzymatic, and regulatory functions in cells.
Precursors for Other Biomolecules
Some amino acids serve as precursors for:
Neurotransmitters (e.g., Tryptophan → Serotonin).
Hormones (e.g., Tyrosine → Thyroid hormones).
Nucleotides (e.g., Glutamine → Purines).
Energy Source
Under starvation or metabolic stress, amino acids can be broken down to produce energy.
Examples of Amino Acids
Glycine: The simplest of all amino acids, found in collagen and involved in synthesizing other biomolecules.
Lysine: An essential amino acid important for protein synthesis and critical for tissue growth and repair.
Tryptophan: An essential amino acid that serves as a precursor for serotonin (a neurotransmitter) and melatonin (a hormone).
Chemical Nature and Biological Role
Amino acids are characterized by the presence of an amino group, a carboxyl group, a hydrogen atom, and a variable side chain attached to a central alpha carbon.
This structure forms the foundation for their diverse roles in biology, from providing structural elements to proteins to serving as precursors for various biomolecules and as potential energy sources.
Through these functions, amino acids play an indispensable role in the maintenance and regulation of life.