Copper(II)-Induced Secondary Structure Changes and Reduced Folding Stability of the Prion Protein

The cellular isoform of the prion protein PrPC is a Cu2+-binding cell surface glycoprotein that, when misfolded, is responsible for a range of transmissible spongiform encephalopathies. As changes in PrPC conformation are intimately linked with disease pathogenesis, the effect of Cu2+ ions on the structure and stability of the protein has been investigated. Urea unfolding studies indicate that Cu2+ ions destabilise the native fold of PrPC. The midpoint of the unfolding transition is reduced by 0.73 ± 0.07 M urea in the presence of 1 mol equiv of Cu2+. This equates to an appreciable difference in free energy of unfolding (2.02 ± 0.05 kJ mol− 1 at the midpoint of unfolding). We relate Cu2+-induced changes in secondary structure for full-length PrP(23–231) to smaller Cu2+ binding fragments. In particular, Cu2+-induced structural changes can directly be attributed to Cu2+ binding to the octarepeat region of PrPC. Furthermore, a β-sheet-like transition that is observed when Cu ions are bound to the amyloidogenic fragment of PrP (residues 90–126) is due only to local Cu2+ coordination to the individual binding sites centred at His95 and His110. Cu2+ binding does not directly generate a β-sheet conformation within PrPC; however, Cu2+ ions do destabilise the native fold of PrPC and may make the transition to a misfolded state more favourable.