Metal Ions and Intrinsically Disordered Proteins and Peptides: From Cu/Zn Amyloid-β to General Principles

Abstract

The interaction of d-block metal ions (Cu, Zn, Fe, etc.) with intrinsically disordered proteins (IDPs) has gained interest, partly due to their proposed roles in several diseases, mainly neurodegenerative. A prominent member of IDPs is the peptide amyloid-β (Aβ) that aggregates into metal-enriched amyloid plaques, a hallmark of Alzheimer's disease, in which Cu and Zn are bound to Aβ. IDPs are a class of proteins and peptides that lack a unique 3D structure when the protein is isolated. This disordered structure impacts their interaction with metal ions compared with structured metalloproteins. Metalloproteins either have a preorganized metal binding site or fold upon metal binding, resulting in defined 3D structure with a well-defined metal site. In contrast, for Aβ and likely most of the other IDPs, the affinity for Cu(I/II) and Zn(II) is weaker and the interaction is flexible with different coordination sites present. Coordination of Cu(I/II) with Aβ is very dynamic including fast Cu-exchange reactions (milliseconds or less) that are intrapeptidic between different sites as well as interpeptidic. This highly dynamic metal-IDP interaction has a strong impact on reactivity and potential biological role: (i) Due to the low affinity compared with classical metalloproteins, IDPs likely bind metals only at special places or under special conditions. For Aβ, this is likely in the neurons that expel Zn or Cu into the synapse and upon metal dysregulation occurring in Alzheimer's disease. (ii) Amino acid substitutions (mutations) on noncoordinating residues can change drastically the coordination sphere. (iii) Considering the Cu/Zn-Aβ aberrant interaction, therapeutic strategies can be based on removal of Cu/Zn or precluding their binding to the peptide. The latter is very difficult due to the multitude of metal-binding sites, but the fast koff facilitates removal. (iv) The high flexibility of the Cu-Aβ complex results in different conformations with different redox activity. Only some conformations are able to produce reactive oxygen species. (v) Other, more specific catalysis (like enzymes) is very unlikely for Cu/Zn-Aβ. (vi) The Cu/Zn exchange reactions with Aβ are faster than the aggregation process and can hence have a strong impact on this process. In conclusion, the coordination chemistry is fundamentally different for most of IDPs compared with the classical, structured metalloproteins or with (bio)-inorganic complexes. The dynamics is a key parameter to understand this interaction and its potential biological impact.

Crystal structure of catena-poly[[[dichloridocopper(II)]-{μ-tert-butyl N-methyl-N-[4-(6-{[4-(pyridin-2-yl-κN)-1H-1,2,3-triazol-1-yl-κN 3]methyl}-1,3-benzothiazol-2-yl)phenyl]carbamato}] acetonitrile monosolvate]

“…Metals are known to play a key role in this oxidative stress and also to be associated with peptide aggregation, at the core of the pathology (Faller et al, 2013;Viles, 2012). More specifically, Cu II has been found to form a complex with the amyloid peptide for which aggregation is one of the major hallmarks of AD (Eury et al, 2011;Faller et al, 2014). This has triggered significant ongoing interest in the development of chelators able to interact with metals in the context of AD (Santos et al, 2016;Conte-Daban et al, 2017).…”

Copper in Alzheimer’s disease: Implications in amyloid aggregation and neurotoxicity

“…24,27,28 On the other hand, it has been observed that substoichiometric amounts of Cu(II) can nucleate fibril formation and that the Cu(II)-generated fibers are capable of seeding fiber formation of metal-free Aβ (as indicated by a reduction in the lag time); See in line with the expected behaviour of intrinsically disordered proteins. 6 Though, surface-plasmon resonance (SPR) biosensing experiments have shown that these Cu(II)-induced aggregates are unstable. 25 Both the nucleation and elongation rates are accelerated by Cu(II) ions for Aβ 40 and Aβ 42 , independently of the pH, the enhancement being significantly higher for Aβ 42 for which the total fiber content is superior.…”

“…The content of defined structure depends strongly on external conditions and interactions with other biomolecules and metal ions. 6 An imbalance of metal-ion homeostasis in the brain is thought to play an important role in the pathogenesis of AD. The concentrations of Cu, Zn and Fe in the brain are tightly regulated in the central nervous system (CNS) at the level of the blood-brain barrier (BBB).…”

“…Only some of these conformations are able to produce reactive oxygen species. 6 The redox activity of copper can also play a role in the aggregation process, as copper can oxidize the Aβ protein itself. For instance, copper can mediate the formation of cross-linked dimeric species of Tyr10, 38 or the oxidation of Met35.…”

“…As a protein lacking a unique 3D structure, Aβ can adopt several conformations upon interaction with metals, even in its aggregated states. 6 Smith et al demonstrated that the generation of the Aβ toxic species is modulated by the concentration of Cu(II) ions and the ability to form intermolecular histidine bridges. 41 It has been suggested that a lower concentration of biologically available Cu(II) in the extracellular milieu of the brain could contribute to a more extensive and more aggressive amyloid pathology.…”