Protein folding and stability
Why sequence determines shape, the forces involved, and what happens when folding fails.
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- Explain what Anfinsen’s experiment showed
- Rank the forces that stabilize folded proteins
- Describe the role of chaperones
- Give examples of protein-misfolding diseases
Key concepts
In the 1950s and 60s, Christian Anfinsen unfolded the enzyme ribonuclease A with urea and a reducing agent that breaks disulfide bonds. When he removed both slowly, the enzyme refolded and regained its activity. The conclusion, recognized with the 1972 Nobel Prize: the amino acid sequence contains the information needed to fold.
What holds a protein together
- The hydrophobic effect, the main driving force: nonpolar side chains cluster away from water.
- Hydrogen bonds, ionic interactions (salt bridges) and van der Waals contacts.
- Covalent disulfide bonds between cysteines, especially in secreted proteins.
Folded proteins are only modestly more stable than unfolded ones, which is why heat, extreme pH and chemicals can denature them. Denaturation unfolds a protein without breaking its peptide bonds.
Help and failure
A random search of all shapes would take longer than the age of the universe (Levinthal’s paradox), so folding follows an energy “funnel” toward the native state. Chaperones such as Hsp70 and GroEL help proteins fold in the crowded cell without dictating the final shape. Misfolded proteins can aggregate into amyloid fibrils, seen with amyloid-β in Alzheimer’s disease, prions, and amylin in type 2 diabetes. Since 2021, AI systems such as AlphaFold can predict many structures from sequence alone.
Worked example
Problem: In a variation of Anfinsen’s experiment, the reducing agent was removed first while urea was still present. The enzyme recovered only about 1% of its activity. Why?
Solution: Disulfide bonds re-formed while the chain was still unfolded, pairing cysteines at random (“scrambled” ribonuclease). Adding a trace of reducing agent let the bonds shuffle until the protein found its stable, native fold, restoring activity.
Common mistakes
- Thinking chaperones encode the final structure. They prevent mistakes; the sequence determines the fold.
- Assuming denaturation breaks peptide bonds. It disrupts the 3D structure only.
- Overrating hydrogen bonds as the main driving force. The hydrophobic effect dominates.
Check yourself
What is the main driving force of protein folding?
What did Anfinsen’s experiment demonstrate?
Name a disease linked to protein aggregation.
14 cards on protein folding.
Go deeper
- Anfinsen (1973): Principles that govern the folding of protein chains
- AlphaFold Protein Structure Database
- Jumper et al. (2021): AlphaFold
VALREXA Biosciences. (2026, November 7). Protein folding and stability: study guide. VALREXA Biosciences. https://valrexa.com/study/protein-folding.html
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