Protein Folding and Chaperones

Protein Folding Fundamentals - Nature's Origami

• Process: Polypeptide chain folds into a unique, biologically active 3D native conformation.
• Anfinsen's Dogma: Primary amino acid sequence contains all information for correct folding.
• Driving Forces:
  • Hydrophobic effect (dominant): Nonpolar residues sequestered from water.
  • Hydrogen bonds: Stabilize secondary (α-helices, β-sheets) and tertiary structures.
  • Van der Waals forces: Weak, short-range attractions.
  • Ionic bonds (salt bridges): Between charged groups.
  • Disulfide bonds (-S-S-): Covalent; stabilize structure.
• Thermodynamics: Native state is thermodynamically most stable (lowest Gibbs free energy, $ΔG < 0$).
• Levinthal's Paradox: Proteins fold rapidly via specific pathways, not random conformational search.
• Folding Funnel: Energy landscape guiding protein towards native state.

⭐ The hydrophobic effect, where nonpolar amino acid side chains cluster away from water, is the primary driving force for protein folding into its native conformation.

Molecular Chaperones & Mechanisms - Folding's Helping Hands

• Function: Proteins aiding correct folding of nascent/denatured polypeptides, prevent aggregation, assist translocation. Mostly ATP-dependent.
• Key Types & Actions:
  • Hsp70 (e.g., DnaK in E.coli):
    • Binds exposed hydrophobic patches on unfolded proteins.
    • Acts co-translationally and post-translationally.
    • Cycle: Hsp40 (DnaJ) aids substrate binding & ATP hydrolysis (tight grip); NEF (GrpE) promotes ADP/ATP exchange for release.
    • 📌 Mnemonic: 70 is when you need help (early stage, versatile).
  • Chaperonins (Hsp60 / GroEL-GroES complex in E.coli; TRiC/CCT in Eukaryotes):
    • GroEL forms barrel; GroES acts as lid.
    • Isolates unfolded proteins in cavity for undisturbed folding.
    • ATP-dependent.
    • 📌 Mnemonic: A 60-sided barrel (complex structure) for tough folding jobs.
  • Hsp90:
    • Late-stage folding/activation of specific "client" proteins (e.g., steroid receptors, signaling kinases).
    • ATP-dependent.
  • Calnexin & Calreticulin:
    • ER lumen; lectin-like, bind N-glycans on glycoproteins. Ensure quality control.

⭐ Hsp90 inhibitors (e.g., Geldanamycin, 17-AAG) are potent anti-cancer drug candidates, targeting the stability of numerous oncogenic client proteins.

• Pathology Link: Chaperone defects or overload implicated in protein misfolding diseases (e.g., Alzheimer's, Parkinson's, Cystic Fibrosis, Prion diseases).

Protein Misfolding & Diseases - Misfolded Mayhem

• Improper protein folding or misfolded protein accumulation causes cellular dysfunction and disease.
• Pathogenic Mechanisms:
  • Loss of function: Degraded or non-functional misfolded protein (e.g., CFTR in Cystic Fibrosis).
  • Gain of toxic function: Misfolded protein gains harmful activity (e.g., Huntington causing aggregation).
  • Protein Aggregation & Amyloid Formation: Misfolded proteins form insoluble amyloid aggregates.
    • Often β-sheet rich, proteolysis-resistant.
• Key Proteopathies (Misfolding Diseases):
  • Alzheimer's Disease: Extracellular Amyloid-β (Aβ) plaques; intracellular Tau neurofibrillary tangles.
  • Parkinson's Disease: α-synuclein aggregation into Lewy bodies.
  • Prion Diseases (e.g., CJD, Kuru): Pathogenic $PrP^{Sc}$ induces misfolding of normal $PrP^{C}$.
  • Cystic Fibrosis: CFTR misfolding (esp. $\Delta F508$ mutation) leads to premature degradation.
  • Amyloidosis: Systemic or localized amyloid deposition.

⭐ Prion diseases are unique as the misfolded PrPSc protein can act as a template, inducing misfolding in normal PrPC proteins, making them transmissible.

High‑Yield Points - ⚡ Biggest Takeaways

• Protein folding yields the functional native 3D structure, determined by the amino acid sequence.
• Anfinsen's experiment proved primary structure dictates tertiary structure.
• Molecular chaperones (e.g., Hsp70, Hsp60/GroEL-GroES) aid correct folding and prevent aggregation.
• Misfolded proteins are linked to diseases like Alzheimer's, Parkinson's, and prion diseases.
• Levinthal's paradox suggests folding follows defined pathways, not random search.
• PDI (Protein Disulfide Isomerase) assists disulfide bonds; PPI (Peptidyl Prolyl Isomerase) aids proline isomerization.
• The ubiquitin-proteasome pathway degrades misfolded proteins.