Cross talk between the octarepeats domain and the prion's fifth binding site driven by copper (II) interaction with the protein N-terminus

Prion diseases are a group of neurodegenerative diseases based on the conformational conversion of the normal form of protein (PrPC) to the disease-related scrapie isoform (PrPSc). Copper(II) coordination to the prion protein (PrPC) has attracted considerable interest for almost 20 years, mainly due to the possibility that such an interaction would be an important event for the physiological function of PrPC. In this work we report the copper(II) coordination features of the peptide fragment Ac(PEG11)3PrP(60-114) as a model of the whole N-terminus of PrPC metal binding domain. We studied the complexation properties of the peptide by mean of potentiometric, UV-Vis, circular dichroism (CD), and electrospray ionization mass spectrometry techniques. The results revealed that the preferred histidyl binding sites largely depend on pH and copper(II) to peptide ratio. Formation of macrochelate species occurs up to 2:1 metal to peptide ratio in the physiological pH range involving, at the same time, the histidyl residues present both inside and outside the octarepeat domain. At increased copper(II) to peptide ratios, however, amide bonded species form especially within the octarepeat domain. On the contrary, at basic pH, the amide bonded species predominate at any ratios and are preferably formed with the binding sites of His96 and His111 similarly to the metal binding affinity order observed in our previous studies.