Human islet amyloid polypeptide (hIAPP) is the major component of amyloid deposits found in pancreatic β-cells of patients with type 2 diabetes (T2D). Copper ions have an inhibitory effect on the amyloid aggregation of hIAPP, and they may play a role in the etiology of T2D. However, deeper knowledge of the structural details of the copper–hIAPP interaction is required to understand the molecular mechanisms involved. Here, we performed a spectroscopic study of Cu(II) binding to hIAPP and several variants, using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electronic absorption, and circular dichroism (CD) in the UV–vis region in combination with Born–Oppenheimer molecular dynamics (BOMD) and density functional theory geometry optimizations. We find that Cu(II) binds to the imidazole N1 of His18, the deprotonated amides of Ser19 and Ser20, and an oxygen-based ligand provided by Ser20, either via its hydroxyl group or its backbone carbonyl, while Asn22 might also play a role as an axial ligand. Ser20 plays a crucial role in stabilizing Cu(II) coordination toward the C-terminal, providing a potential link between the S20G mutation associated with early onset of T2D, its impact in Cu binding properties, and hIAPP amyloid aggregation. Our study defines the nature of the coordination environment in the Cu(II)–hIAPP complex, revealing that the amino acid residues involved in metal ion binding are also key residues for the formation of β-sheet structures and amyloid fibrils. Cu(II) binding to hIAPP may lead to the coexistence of more than one coordination mode, which in turn could favor different sets of Cu-induced conformational ensembles. Cu-induced hIAPP conformers would display a higher energetic barrier to form amyloid fibrils, hence explaining the inhibitory effect of Cu ions in hIAPP aggregation. Overall, this study provides further structural insights into the bioinorganic chemistry of T2D.

Metal ions have been implicated in several proteinopathies associated to degenerative and neurodegenerative diseases. While the molecular mechanisms for protein aggregation are still under investigation, recent findings from Cryo-EM point out to polymorphisms in aggregates obtained from patients, as compared to those formed in vitro, suggesting that several factors may impact aggregation in vivo. One of these factors could be the direct binding of metal ions to the proteins engaged in aggregate formation. In this opinion article, three case studies are discussed to address the question of how metal ion binding to a peptide or protein may impact its conformation, folding, and aggregation, and how this may be relevant in understanding the polymorphic nature of the aggregates related to disease. Specifically, the impact of Cu²⁺ ions in the amyloid aggregation of amyloid-β and amylin (or IAPP- islet amyloid polypeptide) are discussed and then contrasted to the case of Cu²⁺-induced non-amyloid aggregation of human lens γ-crystallin proteins. For the intrinsically disordered peptides amyloid-β and IAPP, the impact of Cu²⁺ ion binding is highly dependent on the relative location of the metal binding site and the hydrophobic regions involved in β-sheet folding and amyloid formation. Further structural studies of how Cu²⁺ binding impacts amyloid aggregation pathways and the molecular structure of the final amyloid fibril, both, in vitro and in vivo, will certainly shed light into the molecular origins of the polymorphisms observed in diseased tissue. Finally, contrasting these cases to that of Cu²⁺-induced non-amyloid aggregation of γ-crystallins, it is evident that, although the impact in aggregation – and the nature of the aggregate – may differ in each system, at the molecular level there is a competition between metal ion coordination and the stability of β-sheet structures. Considering the importance of the β-sheet fold in biology, it is fundamental to understand the energetics and molecular details behind such competition. This opinion article aims to highlight future research directions in the field that can help tackle the important question of how metal ion binding may impact protein folding and aggregation and how this relates to disease.

The N-terminus of the prion protein is a large intrinsically disordered region encompassing approximately 125 amino acids. In this paper, we review its structural and functional properties, with a particular emphasis on its binding to copper ions. The latter is exploited by the region’s conformational flexibility to yield a variety of biological functions. Disease-linked mutations and proteolytic processing of the protein can impact its copper-binding properties, with important structural and functional implications, both in health and disease progression.

The cellular prion protein (PrP^C) is a copper binding protein that undergoes post-translational modifications, such as endoproteolytic alpha cleavage, which occurs in the vicinity of the His111 Cu binding site. Alpha cleavage processing of PrP^C is considered to be neuroprotective since the cleavage site is located in a region that is key to the conversion of PrP^C into the infectious scrapie isoform (PrP^Sc), yielding a membrane bound C1 fragment of PrP^C that still contains His111. In this work, we use hPrP(111–115) fragment as a model peptide to evaluate the impact of alpha cleavage processing of PrP^C in its ability to coordinate Cu(II) ions at His111. By using different spectroscopic techniques such as electronic absorption, circular dichroism, nuclear magnetic resonance, and electron paramagnetic resonance, this study demonstrates that Cu(II) binding to the cleaved His111 site is highly dependent on Cu and proton concentrations. The imidazole group of His111 and its free NH₂ terminus emerge as the main anchoring sites for Cu(II) coordination, yielding very different complexes from those characterized for the intact His111 site in the full protein. Different Cu(II) coordination modes that could form with the alpha cleaved PrP^C under physiological conditions are identified and characterized. Overall, this study contributes to understand how alpha cleavage processing of PrP^C impacts its Cu(II) binding properties at His111. While the functional implications of Cu binding to the cleaved PrP^C remain to be discovered, proteolytic processing of PrP^C and its Cu binding features appear to be molecular events that might be strongly linked to its cellular function.