Prion proteins are infectious agents causing transmissible spongiform encephalopathies in a misfolded protease-resistant form of protein. Human PrP possesses 7 potential copper-binding sites. Notably, four of putative copper-binding sites are located in the octarepeat region (PrP 60-91). Recent studies have shown that peptides derived from human PrP effectively bind Cu2+ to form the Cu-centered catalytic complex required for generation of superoxide by coupling the oxidation of neurotransmitters and their analogues. In this study, we have studied the minimal motifs required for binding of metals within human PrP, by assessing (1) the peptide-dependent quenching of Tb3+ fluorescence and (2) the Cu2+-dependent quenching of intrinsic fluorescence in human PrP octarepeat-derived peptides. Assays with peptide-dependent quenching of Tb fluorescence supported the positive role of the His-ended X-X-H motif (in this case Q-P-H tripeptide sequence) rather than His-started H-G-G-G-W motif, as metal chelating motifs in short peptides. Controversially, the role of combination of Pro and His residues was supported by the Cu-dependent peptide fluorescence quenching assay. Above data suggested that there are two distinct modes of metal binding to His residues in the octarepeat regions in PrP, possibly by co-ordinations of His-started and His-ended motifs around the target metals depending on the conditions given.
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