Date of Award

8-1-2022

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Chemistry

Content Description

1 online resource (xviii, 107 pages) : illustrations (some color)

Dissertation/Thesis Chair

Igor K Lednev

Committee Members

Alexander Shekhtman, Jia Sheng, Mehmet Yigit, Wilfredo Colón

Keywords

amyloid fibrils, methionine radical cation, polymorphs, spontaneous refolding, sulfur oxidation, trisulfides, Amyloid beta-protein, Polymorphism (Crystallography), Sulfur amino acids, Alzheimer's disease

Subject Categories

Analytical Chemistry | Biochemistry | Biophysics

Abstract

Protein aggregation that results in the formation of amyloid fibrils has been linked to many neurodegenerative disorders, including Alzheimer’s disease and Parkinson’s disease. The sulfur atoms in methionine (Met) and cysteine (Cys) residues of proteins can be readily oxidized, significantly affecting their properties. Oxidation of sulfur-containing amino acids has recently been shown to affect protein fibrillation. This work presents novel findings on Cys and Met redox reactions that are related to the formation of amyloid fibrils and on the polymorphism of a model fibrillogenic protein, hen egg white lysozyme (HEWL). Biophysical techniques including Raman spectroscopy, atomic force microscopy, electron paramagnetic resonance, UV–vis spectroscopy, and fluorescence spectroscopy were used to obtain information that was used in a comprehensive characterization. This dissertation presents evidence that hydrogen sulfide (H2S) causes the formation of trisulfide bridges between cysteine residues and that this modification destabilizes the structure of HEWL, preventing protein fibrillation. As a result, small spherical aggregates of unordered protein that, in contrast to HEWL fibrils, exhibit no cytotoxicity, form. The formation of trisulfide bonds and their effect on amyloid fibrillation were investigated and characterized. These findings are important because the amount of H2S, a neuromodulator and a neuroprotectant, is known to be significantly reduced in the brain tissue of patients diagnosed with Alzheimer’s disease relative to that of healthy individuals. This dissertation also reports a new phenomenon in which lysozyme fibrils spontaneously refold to a different polymorph through a disassembled intermediate when changes in environmental hydrophobicity occur. Intermediate aggregates with a PPII-like structure featuring highly solvent-exposed tryptophan residues predominate before the protein refolds to form polymorph II fibrils. Furthermore, the disulfide (SS) bonds within the protein undergo significant rearrangement when the protein refolds from one polymorphic form to another. The main SS bond conformation changes from gauche-gauche-trans in polymorph I to gauche-gauche-gauche in polymorph I. Lastly, a new type of reversible protein chromophore that results in the formation of purple fibrils was discovered and characterized. The formation of a sulfur∴π-bonded radical cation due to the interaction between methionine and phenylalanine is reported here. The purple chromophore is formed due to a specific orientation of the sulfur-centered radical cation and a phenyl ring that is stabilized by the fibril framework. Specific fibril conformation and the resulting formation of the chromophore can be reversibly controlled by varying the pH. This is the first known example of the formation of a side-chain self-assembled chromophore as a result of protein aggregation.

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