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Perturbation of the Stability of Amyloid Fibrils through Alteration of
Electrostatic Interactions
Shammas SL, Knowles TPJ, Baldwin AJ, MacPhee CE, Welland ME, Dobson CM, and Devlin GL
Biophysical Journal 100, 2783-2791 (2011)
Na Young KimCHEM 645
Amyloid Fibrils
2
Info: Shammas SL et al. Biophys. J. 100, 2783-2791 (2011)Nelson R et al., Nature 435, 773-778 (2005)Picture: Ross CA, Poirier MA. Nature Medicine 10, S10-S17 (2004)
Structure of Amyloid Fibrils
3
Nelson R et al., Nature 435, 773-778 (2005)
Structure of Amyloid Fibrils
4
Nelson R et al., Nature 435, 773-778 (2005)
Insulin• Readily forms amyloid fibrils at acidic pH• These dissociate under alkaline conditions, but by what
mechanism?– Dissociation due to charge change at higher pH– Effects of partial distruption of beta-sheet on dissociation via
denaturant addition– Kinetics of dissociation
Bovine Insulin (Hexameric form) PDB: 2ZP6
Dissociation of Amyloid Fibrils at High pH
• Amyloid fibrils formed at pH 2.0 by seeding• Incubated amyloid fibrils at room temperature and various pH for
48h
Driving Force Behind Dissociation?
• These simple electrostatic considerations accurately described the dissociation of these charged fibrils
• •
€
Uch =Q2
4πε0εr0e−r0 rD
ΔGtot = ΔG0 +Uch Q( )
€
ΔG0 = −39.4 ±1.3kJ /mol
€
r0 =1.48nm
Is Nothing Happening Below pH 8?
Structural changes apparent above pH 4, but no dissociation of insoluble fibrils until above pH 8 (from TEM)
Structural Changes Below pH 8
Fourier transform infrared spectroscopy (FTIR) amide I’ region (1600-1700 cm-1) used to determine secondary structure content (pH* 1.6-3.6-5.6-7.6)
Beta-sheet Content Correlates Well to Fibril Stability
pH deltaG (kJ/mol)
2 -56.3 +/- 1.2
4 -55.4 +/- 1.6
6 -43.2 +/- 0.9
8 -33.3 +/- 0.6
Kinetics of Dissociation
€
%T =I
I0⋅100
OD = −log%T
100
⎛
⎝ ⎜
⎞
⎠ ⎟= −log
I
I0
⎛
⎝ ⎜
⎞
⎠ ⎟
Picture: http://en.wikipedia.org/wiki/AbsorbanceEquations: http://www.chroma.com/knowledge/introduction-fluorescence/percent-transmission-and-optical-density
Kinetics of Dissociation Continued
12
Kinetics of Dissociation Continued
13
Conclusions
Significance
• Significance of beta-sheet content on thermodynamic stability of amyloid fibrils
• “Fibrillar state can be thermodynamically more stable than the native state”
15
Questions?
16
17
Outline
• “Fibril stability is susceptible to electrostatic repulsion between constituent polypeptide chains” (pH 2 to 12)
• “Thermodynamic stability of fibrils is reduced by partial disruption of beta-sheet structure” (pH 4 to 8)
• “The kinetics of insulin fibril dissociation is dependent on the ionization state of a single side chain” (pH>10)
Explanation of Eq 2
Assumptions:• “Each monomer in the fibril experiences a repulsive elctrostatic
interaction with its nearest neighbors”• Dynamic equilibrium exists between the fibrillar and soluble states
of insulin– This can be described by classical linear polymerization theory
19
FTIR of Proteins
20
Gallagher, W. “FTIR Analysis of Protein Structure” www.chem.uwec.edu/Chem455_S05/Pages/.../FTIR_of_proteins.pdf