Amino acids

• 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 acid is 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
Nonpolar (Hydrophobic)	Mainly C & H, no polar groups	Hydrophobic interactions, found in protein cores	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline, Phenylalanine, Tryptophan
Polar (Hydrophilic)	Contain functional groups forming hydrogen bonds	Hydrophilic interactions, often found on protein surfaces	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Acidic (Negatively Charged)	Contain carboxyl (-COO⁻) groups	Acid-base interactions, ionic bonds	Aspartic acid, Glutamic acid
Basic (Positively Charged)	Contain amino (-NH₃⁺) groups	Acid-base interactions, ionic bonds	Lysine, Arginine, Histidine

 

Properties of Amino Acids

1. Chirality:

   • Most acid exists in L- and D-forms (stereoisomers), but proteins in living organisms are composed of L-amino acid.

2. Zwitterionic Nature:

   • At physiological pH, acid exists as zwitterions with both positively charged amino groups and negatively charged carboxyl groups.

Functions of Amino Acids

1. Building Blocks of Proteins:

   • Amino acid polymerizes into proteins, which perform structural, enzymatic, and regulatory functions in cells.

2. 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).

3. Energy Source:

   • Under starvation or metabolic stress, acids can be broken down to produce energy.

Examples of Amino Acids

1. Glycine: The simplest of all amino acids, found in collagen and involved in synthesizing other biomolecules.
2. Lysine: An essential acid important for protein synthesis and critical for tissue growth and repair.
3. Tryptophan: An essential amino that serves as a precursor for serotonin (a neurotransmitter) and melatonin (a hormone).

Chemical Nature and Biological Role

• They 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 play an indispensable role in the maintenance and regulation of life.

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