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Parallel β-sheet fibril and antiparallel β-sheet oligomer: new insights into amyloid formation of hen egg white lysozyme under heat and acidic condition from FTIR spectroscopy.

Hen egg white lysozyme (HEWL) is widely used as a model protein for amyloid research. In this study, we aim to use Fourier transform infrared (FTIR) spectroscopy to gain new structural insights into amyloid formation of HEWL under heat and acidic condition. We reveal that the fibril-forming solution of HEWL has the capability to form fibril and oligomer with distinct β-sheet configurations under different temperatures. Amyloid fibril with parallel β-sheet configuration is formed at elevated temperature, while oligomer with antiparallel β-sheet configuration is formed at room temperature. The interplay between fibrillation and oligomerization suggests that the two β-sheet aggregates consume the same amyloidogenic materials such as peptide fragments and nicked HEWL due to lysozyme hydrolysis under heat and acidic condition. Temperature-dependent FTIR reveals that the oligomer is unstable at elevated temperature, demonstrating its off-pathway nature. The temperature-dependent formation of parallel and antiparallel β-sheet configurations discovered in lysozyme system is compared with that of amyloid-β and α-synuclein systems and the implication is discussed.