Amino Acids

Amino acids are organic compounds that serve as the building blocks of proteins, which are essential macromolecules in all living organisms. They play a central role in biological processes, forming the structural framework of cells, enzymes, hormones, and neurotransmitters. Each amino acid consists of a basic amino group, an acidic carboxyl group, a hydrogen atom, and a distinctive side chain attached to a central carbon atom. The chemical diversity of these side chains gives amino acids their unique properties and biological functions.

Chemical Structure

The general chemical formula of an amino acid is NH₂–CHR–COOH, where:

• NH₂ is the amino group (basic in nature),
• COOH is the carboxyl group (acidic in nature),
• H is a hydrogen atom, and
• R represents the variable side chain or radical group that determines the identity and characteristics of each amino acid.

The central carbon atom is known as the α-carbon. In most amino acids, except glycine, the α-carbon is chiral (attached to four different groups), leading to optical isomerism. Naturally occurring amino acids in proteins are predominantly in the L-form, which is biologically active.

Classification of Amino Acids

Amino acids can be classified according to several criteria, such as nutritional requirement, polarity, structure, and metabolic fate.
1. Based on Nutritional Requirement:

• Essential Amino Acids: These cannot be synthesised by the human body and must be obtained through diet.
  • Examples: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
• Non-Essential Amino Acids: These can be synthesised by the body from other metabolic intermediates.
  • Examples: Alanine, Aspartic acid, Glutamic acid, Serine, Asparagine.
• Semi-Essential (Conditionally Essential) Amino Acids: These are required under certain physiological conditions such as growth or illness.
  • Examples: Arginine, Cysteine, Tyrosine, Glycine, Proline, Glutamine.

2. Based on the Nature of Side Chain (R Group):

• Non-Polar (Hydrophobic): Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline.
• Polar (Uncharged): Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine.
• Acidic: Aspartic acid, Glutamic acid.
• Basic: Lysine, Arginine, Histidine.

3. Based on Structure:

• Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine.
• Aromatic: Phenylalanine, Tyrosine, Tryptophan.
• Sulphur-Containing: Cysteine, Methionine.
• Hydroxylic: Serine, Threonine, Tyrosine.
• Amidic: Asparagine, Glutamine.

Peptide Bond Formation

Amino acids join together through peptide bonds to form peptides and proteins. A peptide bond is a covalent bond formed between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of another, releasing a molecule of water (H₂O) in a condensation reaction.
The resulting compound is a dipeptide, and successive additions form polypeptides, which fold into complex three-dimensional structures to form proteins.

Physical and Chemical Properties

• Amphoteric Nature: Amino acids contain both acidic (–COOH) and basic (–NH₂) groups and can act as either acids or bases, making them amphoteric.
• Zwitterion Formation: In aqueous solution, amino acids exist as zwitterions, carrying both a positive charge on the amino group and a negative charge on the carboxyl group, making them electrically neutral overall.
• Isoelectric Point (pI): The pH at which an amino acid has no net electrical charge and does not move in an electric field.
• Solubility: Most amino acids are soluble in water but insoluble in organic solvents.
• Optical Activity: Except glycine, all amino acids exhibit optical activity due to their chiral α-carbon.

Biological Functions of Amino Acids

Amino acids perform numerous critical functions in living organisms, including:

1. Protein Synthesis:
   • The primary role of amino acids is to serve as the monomeric units of proteins.
2. Enzyme and Hormone Production:
   • Many hormones and enzymes are either amino acids themselves or derived from them. For example, thyroxine (from tyrosine) and epinephrine (from phenylalanine).
3. Neurotransmitter Precursors:
   • Amino acids act as neurotransmitters or their precursors. For instance, glutamate is an excitatory neurotransmitter, and tryptophan forms serotonin.
4. Metabolic Regulation:
   • Participate in the urea cycle, gluconeogenesis, and energy metabolism.
   • Some amino acids, like alanine, help maintain glucose homeostasis.
5. Formation of Biological Compounds:
   • Serve as precursors for molecules such as porphyrins, purines, pyrimidines, and coenzymes.
6. Tissue Growth and Repair:
   • Essential for the synthesis of structural proteins such as collagen, actin, and myosin, contributing to tissue repair and muscle maintenance.

Metabolic Fate of Amino Acids

Based on their metabolic pathways, amino acids are classified as:

• Glucogenic: Yield glucose through gluconeogenesis (e.g., Alanine, Aspartic acid).
• Ketogenic: Yield ketone bodies upon degradation (e.g., Leucine, Lysine).
• Both Glucogenic and Ketogenic: Some yield both glucose and ketone bodies (e.g., Isoleucine, Phenylalanine, Tyrosine, Tryptophan).

Essential Amino Acid Sources

To ensure adequate intake of essential amino acids, a balanced diet is vital.

• Animal Sources: Eggs, milk, meat, fish, and poultry provide complete proteins containing all essential amino acids.
• Plant Sources: Pulses, lentils, soybeans, and quinoa; however, plant proteins are often incomplete and need to be combined (e.g., rice with lentils) for full amino acid profiles.

Clinical and Nutritional Importance

Deficiency or imbalance of amino acids can lead to several health conditions:

• Kwashiorkor and Marasmus: Protein-energy malnutrition disorders in children.
• Phenylketonuria (PKU): A genetic disorder where the body cannot metabolise phenylalanine properly.
• Homocystinuria: Caused by abnormal metabolism of methionine.
• Albinism: Results from defective metabolism of tyrosine.

Amino acid supplements and balanced nutrition are often prescribed for conditions involving muscle wasting, liver disorders, or stress recovery.

Industrial and Pharmaceutical Applications

Amino acids have extensive applications beyond biology:

• Food Industry: Used as flavour enhancers (e.g., monosodium glutamate), sweeteners, and nutritional supplements.
• Pharmaceuticals: Used in intravenous solutions, amino acid-based drugs, and therapeutic nutrition.
• Cosmetics: Incorporated into skin and hair care products for hydration and repair.
• Biotechnology: Used in culture media and as precursors for bioactive compounds.

Importance in Evolution and Origin of Life

Amino acids are considered fundamental to the origin of life. Experiments such as the Miller–Urey experiment (1953) demonstrated that amino acids could form spontaneously under prebiotic conditions, suggesting their primordial role in the evolution of living systems.