Amyloid fibrils are large aggregates of misfolded proteins, which are often associated with various neurodegenerative diseases such as Alzheimer’s, Parkinson’s, Huntington’s, and vascular dementia. The amount of hydrogen sulfide (H2S) is known to be significantly reduced in the brain tissue of people diagnosed with Alzheimer’s disease relative to that of healthy individuals. These findings prompted us to investigate the effects of H2S on the formation of amyloids in vitro using a model fibrillogenic protein hen egg white lysozyme (HEWL). HEWL forms typical β-sheet rich fibrils during the course of 70 min at low pH and high temperatures. The addition of H2S completely inhibits the formation of β-sheet and amyloid fibrils, as revealed by deep UV resonance Raman (DUVRR) spectroscopy and ThT fluorescence. Nonresonance Raman spectroscopy shows that disulfide bonds undergo significant rearrangements in the presence of H2S. Raman bands corresponding to disulfide (RSSR) vibrational modes in the 550–500 cm–1 spectral range decrease in intensity and are accompanied by the appearance of a new 490 cm–1 band assigned to the trisulfide group (RSSSR) based on the comparison with model compounds. The formation of RSSSR was proven further using a reaction with TCEP reduction agent and LC-MS analysis of the products. Intrinsic tryptophan fluorescence study shows a strong denaturation of HEWL containing trisulfide bonds. The presented evidence indicates that H2S causes the formation of trisulfide bridges, which destabilizes HEWL structure, preventing protein fibrillation. As a result, small spherical aggregates of unordered protein form, which exhibit no cytotoxicity by contrast with HEWL fibrils.

A purple color is formed during the fibrillation of lysozyme, a well-studied protein lacking a prosthetic group. The application of Raman spectroscopy, electron paramagnetic resonance and UV-vis absorption spectroscopy indicates the formation of a sulfur∴π-bonded radical cation due to the methionine-phenylalanine interaction, which is consistent with a small molecule model reported in the literature. A purple chromophore with characteristic 550 nm absorption is formed due to a specific orientation of the sulfur-centered radical cation and a phenyl ring stabilized by the fibril framework. A specific fibril conformation and the resulting formation of the chromophore are controlled reversibly by varying the pH. This is the first known example of a side chain self-assembled chromophore formed due to protein aggregation.

Here, we report a new phenomenon in which lysozyme fibrils formed in a solution of acetic acid spontaneously refold to a different polymorph through a disassembled intermediate upon the removal of acetic acid. The structural changes were revealed and characterized by deep-UV resonance Raman spectroscopy, nonresonance Raman spectroscopy, intrinsic tryptophan fluorescence spectroscopy, and atomic force microscopy. A PPII-like structure with highly solvent-exposed tryptophan residues predominates the intermediate aggregates before refolding to polymorph II fibrils. Furthermore, the disulfide (SS) bonds undergo significant rearrangements upon the removal of acetic acid from the lysozyme fibril environment. The main SS bond conformation changes from gauche–gauche–trans in polymorph I to gauche–gauche–gauche in polymorph II. Changing the hydrophobicity of the fibril environment was concluded to be the decisive factor causing the spontaneous refolding of lysozyme fibrils from one polymorph to another upon the removal of acetic acid. Potential biological implications of the discovered phenomenon are discussed.

Amyloid fibrils are misfolded proteins, which are often associated with various neurodegenerative diseases such as Alzheimer's. The amount of hydrogen sulfide (H 2 S) is known to be reduced in the brain tissue of people diagnosed with Alzheimer's disease relative to that of healthy individuals. Hen Egg-White Lysozyme (HEWL) forms typical β-sheet-rich fibrils during 70 minutes at low pH and high temperatures. These results are consistent with the ThT findings that β-sheets structure is also present in myoglobin (Mb), and hemoglobin (Hb) in the presence of 45% TFE. The addition of H 2 S in the process completely inhibits the formation of amyloid fibrils in HEWL, Mb, and Hb as revealed by several spectroscopic techniques. Non-resonance Raman bands corresponding to disulfide (RSSR) vibrational modes in the 550-500 cm-1 spectral range decreases in intensity and is accompanied by the appearance of a new 490 cm-1 band assigned to the trisulfide group (RSSSR). Intrinsic tryptophan fluorescence shows a partial denaturation of HEWL containing trisulfide bonds. Overall, the Mb and Hb result ties excellent with the HEWL data showing that the presence of H 2 S during these proteins fibrillation processes protects the α-helical protein structures, preventing the formation of amyloids in these different proteins moieties.

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