Amino acids and Peptides 2024

1. Introduction to Amino Acids

  • Amino acids are the building blocks of proteins and are produced during protein breakdown.

  • Biological Functions:

    • Serve as energy sources (e.g., alanine for muscles).

    • Aid in tissue repair (e.g., leucine for skin and wound healing).

    • Participate in enzymatic and hormonal activities.

  • There are 20 common amino acids, each playing a specific role in the body.

2. Properties of Amino Acids

  1. Structure:

    • All amino acids have a central α-carbon attached to:

      • An amino group (-NH2).

      • A carboxyl group (-COOH).

      • A side chain (R group).

      • A hydrogen atom.

    • Chirality:

      • Except for glycine, amino acids are chiral and exist as L-isomers in proteins.

  2. Chemical Behavior:

    • At physiological pH (~7.0), amino acids exist as zwitterions (carry both a positive and a negative charge).

3. Classification of Amino Acids

Amino acids are categorized based on the properties of their side chains:

  1. Non-polar Side Chains:

    • Hydrophobic, cluster inside proteins (e.g., glycine, alanine, valine).

  2. Uncharged Polar Side Chains:

    • Have a neutral charge at pH 7 but can engage in hydrogen bonding (e.g., cysteine, tyrosine).

  3. Acidic Side Chains:

    • Contain negatively charged carboxyl groups (e.g., aspartic acid, glutamic acid).

  4. Basic Side Chains:

    • Contain positively charged groups (e.g., lysine, arginine, histidine).

4. Essential Amino Acids

  • These are amino acids that the body cannot synthesize and must be obtained through diet:

    • Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.

5. Peptides and Peptide Bond Formation

  1. Peptide Bond Formation:

    • A condensation reaction occurs between the α-carboxyl group of one amino acid and the α-amino group of another.

  2. Peptide Terminology:

    • Dipeptide: Two amino acids.

    • Tripeptide: Three amino acids.

    • Oligopeptide: 4-20 amino acids.

    • Polypeptide: >20 amino acids.

    • Protein: >50 amino acids.

  3. Peptide Nomenclature:

    • Written from the N-terminal (free amino group) to the C-terminal (free carboxyl group) using either three-letter or single-letter codes.

6. Peptides of Biological Importance

  1. Aspartame (Phe-Asp):

    • A low-calorie sweetener.

  2. Glutathione (γ-Glu-Cys-Gly):

    • Acts as a reducing agent, protects red blood cell membranes, and aids in amino acid transport.

  3. Oxytocin:

    • Stimulates uterine contractions.

  4. Vasopressin:

    • Controls body fluid balance by reducing urination.

  5. Angiotensin:

    • Regulates blood pressure.