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 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

Properties of Amino Acids

1. Chirality:

   • Most 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, acids exist as zwitterions with both positively charged amino groups and negatively charged carboxyl groups.

Functions of Amino Acids

1. Building Blocks of Proteins:

   • Amino acids polymerize 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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