protein and peptides

1. Peptides and Proteins: Basic Concepts

Amino Acids as Building Blocks

• Structure:

  • Central α-carbon bonded to:

    • Amino group (-NH₂).

    • Carboxyl group (-COOH).

    • Hydrogen atom.

    • Variable side chain (R-group).

• Peptide Bond Formation:

  • Condensation Reaction: The amino group of one amino acid reacts with the carboxyl group of another, releasing water and forming a covalent amide (peptide bond).

  • Planarity: Peptide bonds are planar due to resonance between the carbonyl oxygen and amide nitrogen, imparting a partial double-bond character.

Classification

• Peptides:

  • Short chains of amino acids linked by peptide bonds.

  • Length Categories:

    • Oligopeptides: 2–7 amino acids.

    • Polypeptides: 8–49 amino acids.

• Proteins:

  • Polypeptides with >50 amino acids.

  • Fold into specific 3D structures for biological activity.

  • Example: Insulin (51 amino acids), the smallest protein, regulates glucose metabolism.

2. Primary Structure: Linear Sequence

• Definition:

  • The sequence of amino acids in a polypeptide chain.

  • Written from the N-terminus (free amine group) to the C-terminus (free carboxyl group).

• Key Features:

  • Determines the protein’s higher-order structures.

  • Includes disulfide bonds (covalent bonds between cysteine residues).

• Disulfide Bridges:

  • Formed by oxidation of thiol groups in cysteine.

  • Stabilize protein structures.

  • Example: Insulin has two inter-chain and one intra-chain disulfide bridge.

• Synthesis:

  • Ribosomal synthesis is guided by mRNA, directly reflecting DNA sequences.

3. Secondary Structure: Local Folding

• Definition:

  • Regular, repeating structures formed by hydrogen bonds between the peptide backbone.

• Types:

  1. α-Helix:

     • Right-handed helical structure stabilized by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen four residues ahead.

     • Side chains project outward.

     • Example: Found in keratin (hair and nails).

     • Proline disrupts helices due to its rigid cyclic structure.

  2. β-Sheets:

     • Zigzag strands linked by hydrogen bonds, forming a sheet.

     • Two forms:

       • Parallel: Strands run in the same direction (weaker H-bonds).

       • Antiparallel: Strands run in opposite directions (stronger H-bonds).

     • Example: Found in fibroin (silk).

  3. Random Coil:

     • Irregular, flexible regions connecting structured segments.

• Interactions:

  • Stabilized by hydrogen bonds, ionic interactions, and van der Waals forces.

4. Tertiary Structure: 3D Folding

• Definition:

  • Overall 3D arrangement of a single polypeptide chain.

• Stabilizing Interactions:

  1. Hydrophobic Interactions:

     • Nonpolar side chains are buried in the protein core, away from water.

  2. Hydrogen Bonds:

     • Between polar side chains or backbone atoms.

  3. Ionic Bonds:

     • Between oppositely charged side chains (e.g., lysine and glutamate).

  4. Disulfide Bridges:

     • Covalent bonds between cysteine residues.

• Examples:

  • Myoglobin: A globular protein storing oxygen in muscles, stabilized by heme binding.

5. Quaternary Structure: Multi-Subunit Assembly

• Definition:

  • Association of multiple polypeptide chains (subunits) into a functional protein.

• Stabilizing Interactions:

  • Same as tertiary structure.

• Examples:

  • Hemoglobin:

    • Composed of two α and two β subunits.

    • Exhibits allosteric effects: Oxygen binding to one subunit enhances the affinity of others.

6. Small Peptides with Biological Functions

1. Glutathione:

   • Structure: γ-Glu-Cys-Gly.

   • Function:

     • Antioxidant protecting cells from oxidative stress.

     • Neutralizes reactive oxygen species (e.g., H₂O₂).

     • Forms a disulfide dimer (GSSG) when oxidized.

   • Clinical Importance:

     • Detoxifies harmful molecules, increasing solubility for excretion.

2. Aspartame:

   • Structure: Dipeptide of aspartic acid and methylated phenylalanine.

   • Function: Artificial sweetener, 200x sweeter than sucrose.

3. Enkephalins:

   • Structure: Pentapeptides (e.g., Met-enkephalin: Tyr-Gly-Gly-Phe-Met).

   • Function: Endogenous opioids modulating pain perception.

4. Nonapeptide Hormones:

   • Bradykinin: Vasodilator reducing blood pressure.

   • Vasopressin: Antidiuretic hormone regulating water retention.

   • Oxytocin: Stimulates uterine contractions during labor.

7. Protein Functions

1. Structural Proteins:

   • Collagen: Triple helix providing tensile strength to connective tissues.

   • Keratin: Forms hair and nails.

2. Enzymes:

   • Catalysts accelerating biochemical reactions.

   • Example: Pepsin, a protease in the stomach.

3. Transport Proteins:

   • Hemoglobin: Transports oxygen in blood.

4. Hormonal Proteins:

   • Insulin: Regulates blood glucose.

   • Glucagon: Mobilizes glucose from glycogen.

5. Membrane Proteins:

   • Ion channels (e.g., potassium channels) facilitate ion transport.

8. Denaturation and Refolding

• Denaturation:

  • Loss of secondary, tertiary, or quaternary structure, disrupting function.

  • Causes:

    • Heat, pH changes, reducing agents, detergents.

  • Example: Heat denatures egg white proteins (albumin).

• Refolding:

  • Some proteins can refold under suitable conditions, demonstrating the importance of the amino acid sequence in determining structure.

9. Experimental Methods for Protein Structure

1. X-ray Crystallography:

   • Provides atomic-level resolution.

   • Requires protein crystallization.

2. NMR Spectroscopy:

   • Analyzes protein structure in solution.

   • Suitable for smaller proteins (<40 kDa).

3. Cryo-Electron Microscopy:

   • Captures large protein complexes without requiring crystals.

4. Mass Spectrometry:

   • Identifies post-translational modifications and structural changes.

5. Computational Tools:

   • AlphaFold predicts protein structures using AI.

10. Importance of Peptides and Proteins in Drug Development

• Peptides and proteins are pivotal in drug discovery and biotechnology.

• Examples:

  • Insulin for diabetes treatment.

  • Monoclonal antibodies for cancer therapy.