Protein Misfolding and Disease States

Protein Folding Funnel

  • Proteins fold spontaneously from high disorder (high entropy) to native state (low entropy), resulting in a negative change in free energy (ΔG\Delta G).

  • The loss of entropy is overcome by favorable enthalpic contributions from bond formation.

  • Folded proteins are only ~0.4 kJ/mol0.4 \text{ kJ/mol} per amino acid more stable than unfolded structures due to generally weak non-covalent interactions.

  • Stability is enhanced by metal ions (coordination) and increased disulfide bonds (especially in oxidative environments).

Protein Denaturation and Renaturation

  • Proteins denature (unfold) by disrupting stabilizing interactions (e.g., pH changes, detergents, chaotropic agents).

  • Christian Anfinsen's RNase A experiment showed denaturation and subsequent renaturation under physiological conditions.

  • This revealed that a protein's primary sequence contains sufficient information for proper folding, but environmental changes can increase misfolding.

Proper Disulfide Bond Formation

  • Disulfide bonds are covalent interactions important for structural integrity.

  • Anfinsen observed improper disulfide bond formation, particularly with environmental changes.

  • Protein disulfide isomerase (PDI) aids proper disulfide bond formation: PDI-reduced breaks and reforms improper bonds; PDI-oxidized forms new proper bonds.

Ensuring Proper Folded Proteins (Molecular Chaperones)

  • Physiological backup systems exist to ensure proper protein folding, collectively known as molecular chaperones.

  • Hsp proteins (heat shock proteins): Recognize misfolded/unfolded proteins, recruit other Hsp family proteins (e.g., DnaJ), and use ATP hydrolysis for refolding. Properly folded proteins are released; partially folded ones are passed to chaperonins.

  • Molecular chaperonins (e.g., GroEL and GroES): Sequester misfolded proteins in a unique folding environment using ATP hydrolysis; GroEL acts as the container, GroES the lid.

Protein Misfolding and Disease State

  • Misfolded proteins are targeted for destruction via ubiquitination and protease cleavage.

  • Disease states occur when misfolded proteins are not effectively managed, leading to:

    • Increased aggregation, preventing proper physiological function.

    • Increased degradation, leading to inability to maintain healthy function.

  • In both cases, essential proteins cannot fulfill their intended function.