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Description
The cellular prion protein (PrPC) is a membrane-bound glycoprotein found in the central nervous system of all mammal species and has emerged as an important copper binding protein. It is well established that the redox behaviour of copper bound to PrPC, plays an important role in both the physiological function of the protein, and in the pathogenesis of neurodegenerative diseases known as transmissible spongiform encephalopathies or prion diseases. They are accompanied by an accumulation of a misfolded, not soluble isoform (PrPSC) of the endogenous prion protein (PrPC) [1]. NMR studies on recombinant human PrPC demonstrate that the C-terminal region, which adopts a globular fold that is largely helical but with a small two-strand β-sheet [2], and the N-terminal region, which is unstructured and flexible in solution [3].
A hallmark of this region is the so-called octarepeat (OR) domain. Human PrPC has four OR repeats, which an interact with divalent ions such as Zn(II), Mn(II), Fe(II), although the affinity for Cu(II) is highest [4]. There is evidence that the OR domain is able to reduce Cu(II) via an electron transfer (ET) from a tryptophan (Trp) residue [4]. Density functional theory and molecular dynamics simulations demonstrated that the resulting Cu(I) [5] and reactive oxygen species [6] can lead to formation of the precursor of the pathogenic PrPC structures [7]. Despite the high relevance of the redox behaviour of Cu bound to PrPC, the exact mechanism of misfolding remains unclear.
Up-to-date, no direct measurement of a Trp* to Cu(II) ET and the structural changes accompanying it has been reported. Our approach here is to mimick the biological activity by inducing an ET via a photon within the OR-Cu2+ complex in aqueous solution.
We phototrigger the Cu2+ to Cu1+ reduction by electronically exciting Trp residue (280 nm), present in the N-terminal region of the protein. Preliminary results obtained using femtosecond-resolved X-ray absorption spectroscopy (fs-XAS) technique at the X-ray free electron lasers (XFELs) show unequivocally the ET taking place instead of energy transfer. We have used this technique, in combination with X-ray scattering, to gain deeper understanding of ET processes involving the Cu2+ active site and the subsequent structural changes of the nearby PrPC environment upon photoreduction.
