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Nearly Reversible Conformational Change of Amyloid Fibrils as Revealed by pH-Jump Experiments
pH-jump
induced conformational transitions between substates of
preformed amyloid fibrils made by a fragmented peptide of helix 2
(H2 peptide) of MoPrP were detected, and their kinetics were analyzed
using a novel pH-jump apparatus specially designed for observing amyloids.
Previously, we reported that H2 peptide formed ordered fibrils with
a minimum at 207 nm on CD spectra at pH 2.9 (named pH 2.9 fibrils),
but formed aggregate-like fibrils with a minimum at 220 nm at pH 7.5
(named pH 7.5 fibrils). When pH-jump from 2.9 to 7.5 was performed,
the CD spectrum changed instantly, but the finally observed ellipticities
were clearly distinct from those of pH 7.5 fibrils. Thus, the finally
observed state is termed ‘pH 7.5-like fibrils’. However,
pH 7.5-like fibrils reverted to the conformation very similar to that
of the pH 2.9 fibrils when the pH of the solution was restored to
2.9. Then, we examined the kinetics of the nearly reversible conformational
changes between pH 2.9 fibrils and pH 7.5-like fibrils using ANS fluorescence
stopped-flow, and we observed relatively fast phases (0.7–18
s<sup>–1</sup>). In contrast, the conversion between pH 7.5-like
fibrils and pH 7.5 fibrils never occurred (<0.2 day<sup>–1</sup>). Thus, H2 fibrils can be switched readily between distinct conformations
separated by a low energy barrier, while a large energy barrier clearly
separated the different conformations. These conformational varieties
of amyloid fibrils may explain the physical basis of the diversity
in prion