| Summary: | The prion protein (PrPC) is a cell surface glycoprotein that binds Cu2+ ions. The misfolding and oligomerisation of PrPC is responsible for a range of transmissible spongiform encephalopathies (TSEs) in mammals. As changes in PrPC conformation are intimately linked with disease pathogenesis, the effect of Cu2+ ions on the structure and stability of PrPC has been investigated. In chapter 3, urea unfolding studies indicate that Cu2+ ions destabilise the native fold of PrPC. The mid-point of the unfolding transition is reduced by 0.73 ± 0.05 M urea in the presence of Cu2+ ions equating to an appreciable difference in free energy of unfolding (∆∆GU[D]50%), 2.02 ± 0.05 kJmol-1. This suggests that in Prion diseases, Cu2+ ions could destabilise the native fold of PrPC and make the transition to a misfolded state more favourable. Furthermore, Cu2+ induced changes in secondary structure observed for small fragments of the protein are related to the full-length prion protein. An increase in -sheet like character is observed when Cu2+ ions are present, this is due to local Cu2+ ion coordination to the individual binding sites of the amyloidgenic region. Cu2+ induced changes in the secondary structure of the N-terminal domain, PrP(23-126) and full-length PrP(23-231), are attributed to Cu2+ ions binding within the octarepeat region of PrPC. Oxidative stress is also a well-recognised feature of Prion diseases. In chapter 4, the effects of Cu2+ catalysed oxidation of PrP on the structure and stability are discussed. 2D 1H-15N HSQC NMR studies of PrPC indicate that specific key residues are perturbed upon methionine (Met) oxidation by H2O2. These residues are involved in the hydrophobic packing of the structured core of the protein, stabilising its ternary structure. Urea unfolding studies indicate that the oxidation of PrPC by H2O2 and to a greater extent Cu2+ ions with peroxide significantly reduce the thermodynamic stability of PrPC. Cu2+ catalysed oxidation of PrP causes much more significant alteration of the structure. 2D 1H-15N HSQC NMR spectroscopy indicates that the structured C-terminal portion of PrP becomes a large molten-globule, made of monomeric species. FT-IR and far-UV-CD spectroscopy indicate that this molten-globule is rich in β-sheet. These observations supports a hypothesis that oxidation of PrP destabilises the native fold of PrPC, making the transition to PrPSc more energetically favourable. This study gives a structural and thermodynamic explanation for the high levels of oxidised Met residues in scrapie isolates. Finally, in chapter 5 the interaction of PrPC with Aβ peptide, responsible for Alzheimer’s disease (AD), is investigated. In particular, the influence of full length PrP and fragments on the kinetics of Aβ fibril growth is investigated. The complete inhibition Aβ fibril formation is observed when as little as one-twentieth of the molar ratio of PrPC is used. The unstructured N-terminus of PrPC, residues 23 to115, is thought to be crucial for this inhibition, while, residues 116 to 231 have no influence on the fibril formation. Gel filtration chromatography indicates that the complex formed by PrPC with an Aβ oligomer is 12 to 24 monomers in size.
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