The Prion protein (PrP) is a cell surface glycoprotein that has been directly implicated in the pathogenesis of a range of neurological disorders referred to as the transmissible spongiform encephalopathies (TSE’s). The protein has been shown to bind copper within its unstructured N-terminus but the affinity and stoichiometry of the association is a matter of some debate. In addition, the functional significance of this copper binding has yet to be elucidated. This study aimed to determine accurate metal binding parameters for PrP through the use of calorimetry and to provide insight into the potential redox implications of metal once bound. A method of analysis for complex binding to proteins is thoroughly assessed and found to be suitable. The study also aimed to qualify the involvement of metals in the proteins remarkable ability to survive in the environment.
This study confirms that PrP binds copper with an affinity relative to the amount of copper available to the protein. A high nanomolar affinity is reported within two regions on the protein, the octarepeat and the 5th site. Binding within the octarepeat region is found to be highest at low copper concentrations, reducing to micromolar affinity when copper levels exceed equivalents of 1. There is also strong evidence of a complex and cooperative binding mechanism. The 5th site also displays high nanomolar affinity for a single atom of copper. These two regions on the protein also interact in the coordination of copper (II). The copper bound protein is highly redox active and is capable of fully reversible cycling of electrons that are dependent mainly on the octarepeat. The protein does bind other divalent cations but none appear to be physiologically relevant considering the amount of these free metal ions in the body. When adsorbed to model clays, PrP is able to survive for long periods at room temperature. This longevity is increased significantly by the presence of metals in the soil, especially manganese.
These data provide confirmation of the precise parameters of divalent cation binding to PrP. It also confirms that the copper bound protein is capable of a physiological redox role.
|Date of Award||1 Apr 2009|
|Supervisor||David Brown (Supervisor)|