Under certain conditions, normally soluble proteins misfold and self-assemble into insoluble amyloid fibrils -- highly ordered structures characterized by a repeating cross-beta sheet architecture. This process underlies some of the most devastating human diseases, including Alzheimer's, Parkinson's, and Huntington's disease.
Nucleation-Dependent Polymerization
Amyloid formation follows a sigmoidal kinetics curve with three phases. During the lag phase, monomers undergo rare primary nucleation events to form small, unstable oligomers. Once a critical nucleus forms, the growth phase begins with rapid fibril elongation as monomers add to fibril ends. Secondary nucleation -- new nuclei forming on existing fibril surfaces -- dramatically accelerates the process, creating an explosive positive feedback loop.
Disease-Associated Aggregates
- Alzheimer's Disease: Amyloid-beta (A-beta) peptides form extracellular plaques, while tau protein forms intracellular neurofibrillary tangles.
- Parkinson's Disease: Alpha-synuclein aggregates into Lewy bodies within dopaminergic neurons.
- Huntington's Disease: Expanded polyglutamine (polyQ) tracts in huntingtin protein drive aggregation in striatal neurons.
- Prion-Like Propagation: Misfolded proteins can template the misfolding of native copies, enabling cell-to-cell spread of pathology through the brain.
Why It Matters
Soluble oligomeric intermediates -- not the mature fibrils -- are now considered the most toxic species. With aging, the capacity of chaperones and the proteasome to clear misfolded proteins declines, tipping the balance toward aggregation. The ThT (Thioflavin T) fluorescence assay used in the chart below is the standard experimental method for monitoring amyloid formation in real time.
Category: Biochemistry & Molecular Biology — Protein misfolding and neurodegenerative disease