Nobody's Perfect: Choice of the Buffer and the Rate of Cu2+ Ion-Peptide Interaction

The choice of correct pH buffer is crucial in chemical studies modeling biological processes involving Cu2+ ions. Popular buffers for physiological pH are known to form Cu(II) complexes, but their impact on kinetics of Cu(II) complexation has not been considered. We performed a stopped-flow kinetic study of Cu2+ ion interactions with four popular buffers (phosphate, Tris, HEPES, and MOPS) and two buffers considered as nonbinding (MES and PIPPS). Next, we studied their effects on the rate of Cu2+ reaction with Gly-Gly-His (GGH), a tripeptide modeling physiological Cu(II) sites, which we studied previously at conditions presumably excluding the buffer interference [Kotuniak, R.; Angew. Chem., Int. Ed. 2020, 59, 11234-11239]. We observed that (i) all tested pH 7.4 buffers formed Cu(II) complexes within the stopped-flow instrument dead time; (ii) Cu(II)-peptide complexes were formed via ternary complexes with the buffers; (iii) nevertheless, Good buffers affected the observed rate of Cu(II)-GGH complex formation only slightly; (iv) Tris was a competitive inhibitor of Cu(II)-GGH complexation; while (v) phosphate was a reaction catalyst. This is particularly important as phosphate is a biological buffer.

Essential Role of Histidine for Rapid Copper(II)-Mediated Disassembly of Neurokinin B Amyloid

Neurokinin B is a tachykinin peptide involved in a diverse range of neuronal functions. It rapidly forms an amyloid, which is considered physiologically important for efficient packing into dense core secretory vesicles within hypothalamic neurons. Disassembly of the amyloid is thought to require the presence of copper ions, which interact with histidine at the third position in the peptide sequence. However, it is unclear how the histidine is involved in the amyloid structure and why copper coordination can trigger disassembly. In this work, we demonstrate that histidine contributes to the amyloid structure via π-stacking interactions with nearby phenylalanine residues. The ability of neurokinin B to form an amyloid is dependent on any aromatic residue at the third position in the sequence; however, only the presence of histidine leads to both amyloid formation and rapid copper-induced disassembly.

Interactions between copper (II) and β-amyloid peptide using capillary electrophoresis-ICP-MS: Kd measurements at the nanogram scale

Although the interaction between the β-amyloid peptide and copper (II) appears to play an important role in Alzheimer's disease, the affinity constant is still controversial and values are ranging from 10⁷ to 10¹¹ M^-1. With the aim of clarifying this point, a complementary method, based on the capillary electrophoresis-ICP-MS hyphenation, was developed and competitive binding experiments were conducted in the presence of nitrilotriacetic acid. The effect of the capillary surface (neutral or positively charged) and nature of the buffer (Tris or Hepes) have been studied. Tris buffer was found to be inappropriate for such determination as it enhances the dissociation of copper (II) complexes, already occurring in the presence of an electric field in capillary electrophoresis. Using Hepes, a value of 10¹⁰ M^-1 was found for the affinity of the small β-amyloid peptide 1-16 for copper (II), which is in agreement with the values obtained for other proteins involved in neurodegenerative diseases. These constants were also determined in conditions closer to those of biological media (higher ionic strength, presence of carbonates).

Copper(ii) complex of N-truncated amyloid-β peptide bearing a His-2 motif as a potential receptor for phosphate anions

Electrochemical and spectroscopic studies demonstrated that the N-truncated amyloid β peptide Aβ_5-9 (Arg-His-Asp-Ser-Gly-NH₂) possessing histidine at position 2 (His-2) formed ternary complexes with copper(ii) and phosphate anions or phosphate groups of biomolecules. The recognition ability of Cu(ii)-Aβ_5-9 toward phosphate species provided a new perspective on designing phosphate-selective molecular receptors.

CuII Binding Properties of N-Truncated Aβ Peptides: In Search of Biological Function

As life expectancy increases, the number of people affected by progressive and irreversible dementia, Alzheimer's Disease (AD), is predicted to grow. No drug designs seem to be working in humans, apparently because the origins of AD have not been identified. Invoking amyloid cascade, metal ions, and ROS production hypothesis of AD, herein we share our point of view on Cu(II) binding properties of Aβ_4-x, the most prevalent N-truncated Aβ peptide, currently known as the main constituent of amyloid plaques. The capability of Aβ_4-x to rapidly take over copper from previously tested Aβ_1-x peptides and form highly stable complexes, redox unreactive and resistant to copper exchange reactions, prompted us to propose physiological roles for these peptides. We discuss the new findings on the reactivity of Cu(II)Aβ_4-x with coexisting biomolecules in the context of synaptic cleft; we suggest that the role of Aβ_4-x peptides is to quench Cu(II) toxicity in the brain and maintain neurotransmission.

On the ability of CuAβ1-x peptides to form ternary complexes: Neurotransmitter glutamate is a competitor while not a ternary partner

In the light of conflicting reports on the ability of copper(II) complexes of amyloid beta (Aβ) peptides to form ternary complexes with small molecules co-present in the biological milieu, we performed a study of coordination equilibria in the system containing Cu(II) ions, the Aβ1-16 peptide, glutamic acid and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid, HEPES) buffer. Using potentiometry, isothermal titration calorimetry (ITC), UV-visible spectroscopy and EPR, we concluded that glutamic acid was not able to form such a ternary complex, but can efficiently compete for the Cu(II) ion with the Aβ peptide at Glu concentrations relevant for the synaptic cleft. We also found that the literature constants for Cu(II) complexes with Glu were overestimated, but this effect was partially compensated by the formation of a ternary Cu(Glu)(HEPES) complex. Our results indicate that small molecules co-present with Cu(II) ions and Aβ peptides in the synaptic cleft are not very likely to enhance Cu(II)/Aβ interactions, but instead should be considered as a Cu(II) buffering system that may help prevent these interactions and participate in Cu(II) clearance from the synaptic cleft.

Dissecting ITC data of metal ions binding to ligands and proteins

BACKGROUND

ITC is a powerful technique that can reliably assess the thermodynamic underpinnings of a wide range of binding events. When metal ions are involved, complications arise in evaluating the data due to unavoidable solution chemistry that includes metal speciation and a variety of linked equilibria.

SCOPE OF REVIEW

This paper identifies these concerns, provides recommendations to avoid common mistakes, and guides the reader through the mathematical treatment of ITC data to arrive at a set of thermodynamic state functions that describe identical chemical events and, ideally, are independent of solution conditions. Further, common metal chromophores used in biological metal sensing studies are proposed as a robust system to determine unknown solution competition.

MAJOR CONCLUSIONS

Metal ions present several complications in ITC experiments. This review presents strategies to avoid these pitfalls and proposes and experimentally validates mathematical approaches to deconvolute complex equilibria that exist in these systems.

GENERAL SIGNIFICANCE

This review discusses the wide range of complications that exists in metal-based ITC experiments. It provides a starting point for scientists new to this field and articulates concerns that will help experienced researchers troubleshoot experiments.

Metals and cholesterol: two sides of the same coin in Alzheimer's disease pathology

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease. It begins years prior to the onset of clinical symptoms, such as memory loss and cognitive decline. Pathological hallmarks of AD include the accumulation of β-amyloid in plaques and hyperphosphorylated tau in neurofibrillary tangles. Copper, iron, and zinc are abnormally accumulated and distributed in the aging brain. These metal ions can adversely contribute to the progression of AD. Dysregulation of cholesterol metabolism has also been implicated in the development of AD pathology. To date, large bodies of research have been carried out independently to elucidate the role of metals or cholesterol on AD pathology. Interestingly, metals and cholesterol affect parallel molecular and biochemical pathways involved in AD pathology. The possible links between metal dyshomeostasis and altered brain cholesterol metabolism in AD are reviewed.

The missing link in the amyloid cascade of Alzheimer's disease - metal ions

Progressive deposition of amyloid beta (Aβ) peptides into amyloid plaques is the pathological hallmark of Alzheimer's disease (AD). The amyloid cascade hypothesis pins this deposition as the primary cause of the disease, but the mechanisms that causes this deposition remain elusive. An increasing amount of evidence shows that biometals Zn(II) and Cu(II) can interact with Aβ, thus influencing the fibrillization and toxicity. This review focuses on the role of Zn(II) and Cu(II) in AD, and revisits the amyloid cascade hypothesis demonstrating the possible roles of Zn(II) and Cu(II) in the disease pathogenesis.

A direct determination of the dissociation constant for the Cu(II) complex of amyloid beta 1-40 peptide

Interactions of amyloid beta (Abeta) peptides with Cu(II) are believed to play a crucial role in the molecular mechanisms of neurotoxicity of Alzheimer's disease. There is, however, a serious disagreement regarding the strength of Cu(II) binding to these peptides. We used recombinant amyloid beta peptide 1-40 (Abeta40) to determine the stoichiometry and dissociation constants of Cu(II)-Abeta40 complexes using fluorescence spectroscopy. A single Cu(Abeta40) complex, characterized with the conditional dissociation constant K(d)(cond) = 57 +/- 5 nM was identified. This complex does not bind Hepes buffer molecules, as indicated by the total lack of relationship between K(d)(cond) values and Hepes concentration. The differences between this and other determinations of this constant and its relevance for the understanding of Cu(II) interaction with Abeta peptides are discussed.

Copper and zinc binding to amyloid-beta: coordination, dynamics, aggregation, reactivity and metal-ion transfer

The metal ions copper, zinc and iron have been shown to be involved in Alzheimer's disease (AD). Cu, Zn and Fe ions are proposed to be implicated in two key steps of AD pathology: 1) aggregation of the peptide amyloid-beta (Abeta), and 2) production of reactive oxygen species (ROS) induced by Abeta. There is compelling evidence that Cu and Zn bind directly to Abeta in AD. This formation of Cu/Zn-Abeta complexes is thought to be aberrant as they have been detected only in AD, but not under healthy conditions. In this context, the understanding of how these metal ions interact with Abeta, their influence on structure and oligomerization become an important issue for AD. Moreover, the mechanism of ROS production by Cu-Abeta in relation to its aggregations state, as well as the metal-transfer reaction from and to Abeta are crucial in order to understand why Abeta oligomers are highly toxic and why Abeta seems to bind Cu and Zn only in AD.

Copper(II) binding to amyloid-beta fibrils of Alzheimer's disease reveals a picomolar affinity: stoichiometry and coordination geometry are independent of Abeta oligomeric form

Cu(2+) ions are found concentrated within senile plaques of Alzheimer's disease patients directly bound to amyloid-beta peptide (Abeta) and are linked to the neurotoxicity and self-association of Abeta. The affinity of Cu(2+) for monomeric Abeta is highly disputed, and there have been no reports of affinity of Cu(2+) for fibrillar Abeta. We therefore measured the affinity of Cu(2+) for both monomeric and fibrillar Abeta(1-42) using two independent methods: fluorescence quenching and circular dichroism. The binding curves were almost identical for both fibrillar and monomeric forms. Competition studies with free glycine, l-histidine, and nitrilotriacetic acid (NTA) indicate an apparent (conditional) dissociation constant of 10(-11) M, at pH 7.4. Previous studies of Cu-Abeta have typically found the affinity 2 or more orders of magnitude weaker, largely because the affinity of competing ligands or buffers has been underestimated. Abeta fibers are able to bind a full stoichiometric complement of Cu(2+) ions with little change in their secondary structure and have coordination geometry identical to that of monomeric Abeta. Electron paramagnetic resonance studies (EPR) with Abeta His/Ala analogues suggest a dynamic view of the tetragonal Cu(2+) complex, with axial as well as equatorial coordination of imidazole nitrogens creating an ensemble of coordination geometries in exchange between each other. Furthermore, the N-terminal amino group is essential for the formation of high-pH complex II. The Abeta(1-28) fragment binds an additional Cu(2+) ion compared to full-length Abeta, with appreciable affinity. This second binding site is revealed in Abeta(1-42) upon addition of methanol, indicating hydrophobic interactions block the formation of this weaker carboxylate-rich complex. A Cu(2+) affinity for Abeta of 10(11) M(-1) supports a modified amyloid cascade hypothesis in which Cu(2+) is central to Abeta neurotoxicity.