The role of disulfide bond in the amyloidogenic state of β2-microglobulin studied by heteronuclear NMR

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Abstract

β2-Microglobulin (β2-m) is a major component of dialysis-related amyloid fibrils. Although recombinant β2-m forms needle-like fibrils by in vitro extension reaction at pH 2.5, reduced β2-m, in which the intrachain disulfide bond is reduced, cannot form typical fibrils. Instead, thinner and flexible filaments are formed, as shown by atomic force microscopy images. To clarify the role of the disulfide bond in amyloid fibril formation, we characterized the conformations of the oxidized (intact) and reduced forms of β2-m in the acid-denatured state at pH 2.5, as well as the native state at pH 6.5, by heteronuclear NMR. {1H}-15N NOE at the regions between the two cysteine residues (Cys25–Cys80) revealed a marked difference in the pico- and nanosecond time scale dynamics between that the acid-denatured oxidized and reduced states, with the former showing reduced mobility. Intriguingly, the secondary chemical shifts, ΔCα, ΔCO, and ΔHα, and 3JHNHα coupling constants indicated that both the oxidized and reduced β2-m at pH 2.5 have marginal α-helical propensity at regions close to the C-terminal cysteine, although it is a β-sheet protein in the native state. The results suggest that the reduced mobility of the denatured state is an important factor for the amylodogenic potential of β2-m, and that the marginal helical propensity at the C-terminal regions might play a role in modifying this potential.

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