Copper–zinc cross-modulation in prion protein binding

Abstract

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.

X-Ray Absorption Spectroscopy Measurements of Cu-ProIAPP Complexes at Physiological Concentrations

“…X-ray Absorption Spectroscopy (XAS) is the technique of election to selectively obtain, at atomic resolution, information about the metals environment even in the non-crystalline systems of biological interest. Indeed, XAS has been proven to be particularly useful for the study of the metal binding mode in a number of cases related to amyloidogenic proteins [17][18][19][20].…”

Cu(II)–Zn(II) Cross-Modulation in Amyloid–Beta Peptide Binding: An X-ray Absorption Spectroscopy Study

“…In the context of the Alzheimer’s disease (AD) 3–5 , the possible role of Cu(II) and Zn(II) in aggregation has been extensively studied 6–7 . Recent Electron Spin Resonance (ESR) data 8 and X-ray Absorption Spectroscopy (XAS) 9–10 measurements, carried out in the related case of the prion protein (PrP), confirmed that there is a competition for PrP binding between the two ions, thus suggesting the existence of a general mechanism of fine regulation of metal binding possibly selected to prevent cell damage from accumulated free ions. In this general framework, it appears to be of the utmost importance to understand and clarify whether and how Cu(II) and Zn(II) cross-interact with amyloidogenic peptides.…”

Molecular Features of the Zn2+ Binding Site in the Prion Protein Probed by 113Cd NMR

“…Stellato et al probed the competition between Zn 2þ and Cu 2þ coordination and showed that at low Cu 2þ occupancy, Zn 2þ partially displaces His side chains from the copper centers (48). In a separate study, this group found that Zn 2þ does not fully coordinate to all OR His residues but instead may facilitate OR peptide clustering (47). Given that this work was performed exclusively on OR peptides, it is not clear whether this type of metal ion-assisted crosslinking applies to full-length PrP C .…”