Copper-zinc cross-modulation in prion protein binding

In this paper we report a systematic XAS study of a set of samples in which Cu(II) was progressively added to complexes in which Zn(II) was bound to the tetra-octarepeat portion of the prion protein. This work extends previous EPR and XAS analysis in which, in contrast, the effect of adding Zn(II) to Cu(II)-tetra-octarepeat complexes was investigated. Detailed structural analysis of the XAS spectra taken at both the Cu and Zn K-edge when the two metals are present at different relative concentrations revealed that Zn(II) and Cu(II) ions compete for binding to the tetra-octarepeat peptide by cross-regulating their relative binding modes. We show that the specific metal-peptide coordination mode depends not only, as expected, on the relative metal concentrations, but also on whether Zn(II) or Cu(II) was first bound to the peptide. In particular, it seems that the Zn(II) binding mode in the absence of Cu(II) is able to promote the formation of small peptide clusters in which triplets of tetra-octarepeats are bridged by pairs of Zn ions. When Cu(II) is added, it starts competing with Zn(II) for binding, disrupting the existing peptide cluster arrangement, despite the fact that Cu(II) is unable to completely displace Zn(II). These results may have a bearing on our understanding of peptide-aggregation processes and, with the delicate cross-regulation balancing we have revealed, seem to suggest the existence of an interesting, finely tuned interplay among metal ions affecting protein binding, capable of providing a mechanism for regulation of metal concentration in cells.