Role of amyloid-β–metal interactions in Alzheimer’s disease

There is an evolving field of metallobiology that has begun to describe a key role for bioavailable metals (particularly copper, zinc and iron) in the pathogenesis of Alzheimer’s disease (AD). In particular, there is an apparent failure in metal ion homeostasis, potentially caused by a pathological mislocalization of the metals in the brain, which appears to be an obligatory step in both the precipitation and potentiation of the disease. A number of both preclinical and clinical studies have also provided a strong burden of proof that normalizing metal ion homeostasis represents a valid therapeutic target, and may indeed represent the first disease-modifying strategy for AD. The role of metals in the pathophysiology of AD will be discussed in this article.

Role of metal ions in aggregation of intrinsically disordered proteins in neurodegenerative diseases

Neurodegenerative diseases constitute a set of pathological conditions originating from the slow, irreversible, and systematic cell loss within the various regions of the brain and/or the spinal cord. Depending on the affected region, the outcomes of the neurodegeneration are very broad and diverse, ranging from the problems with movements to dementia. Some neurodegenerative diseases are associated with protein misfolding and aggregation. Many proteins that misfold in human neurodegenerative diseases are intrinsically disordered; i.e., they lack a stable tertiary and/or secondary structure under physiological conditions in vitro. These intrinsically disordered proteins (IDPs) functionally complement ordered proteins, being typically involved in regulation and signaling. There is accumulating evidence that altered metal homeostasis may be related to the progression of neurodegenerative diseases. This review examines the effects of metal ion binding on the aggregation pathways of IDPs found in neurodegenerative diseases.

Copper(II) complexes of rat amylin fragments

The fragments of rat amylin rIAPP(17-29) (Ac-VRSSNNLGPVLPP-NH(2)), rIAPP(17-22) (Ac-VRSSNN-NH(2)), rIAPP(19-22) (Ac-SSNN-NH(2)) and rIAPP(17-20) (Ac-VRSS-NH(2)) together with the related mutant peptides (Ac-VASS-NH(2) and Ac-VRAA-NH(2)) have been synthesized and their copper(II) complexes studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. Despite the lack of any common strongly coordinating donor functions some of these fragments are able to bind copper(II) ions in the physiological pH range. The longest fragment rat amylin(17-29) keeps one equivalent copper(II) ion in solution in the whole pH range, while two other peptides Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) are also able to interact with copper(II) ions in the slightly alkaline pH range. According to the spectral parameters of the complexes, the peptides can be classified into two different categories: (i) the tetrapeptides Ac-VRSS-NH(2), Ac-VASS-NH(2) and Ac-VRAA-NH(2) can interact with copper(II) only under strongly alkaline conditions (pH > 10.0) and the formation of only one species with four amide nitrogen coordination can be detected; (ii) the peptides Ac-VRSSNNLGPVLPP-NH(2), Ac-VRSSNN-NH(2) and Ac-SSNN-NH(2) can form complexes above pH 6.0 with the major stoichiometries [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-). These data support that rIAPP(17-29) can interact with copper(II) ions under physiological conditions and the SSNN tetrapeptide fragment can be considered as the shortest sequence responsible for metal binding. Density functional theory (DFT) calculations provide some information on the possible coordination modes of Ac-SSNN-NH(2) towards the copper(II) ion and suggest that for [CuH(-2)L], [CuH(-3)L](-) and [CuH(-4)L](2-), the binding of two, three and four deprotonated amide nitrogens, with NH(-) of the side chain of asparagine as anchoring group, is probable. Moreover, these data reveal that peptides can be effective metal binding ligands even in the absence of anchoring groups, if more polar side chains are present in a specific sequence.

Cu(II) mediates kinetically distinct, non-amyloidogenic aggregation of amyloid-beta peptides

Cu(II) ions are implicated in the pathogenesis of Alzheimer disease by influencing the aggregation of the amyloid-β (Aβ) peptide. Elucidating the underlying Cu(II)-induced Aβ aggregation is paramount for understanding the role of Cu(II) in the pathology of Alzheimer disease. The aim of this study was to characterize the qualitative and quantitative influence of Cu(II) on the extracellular aggregation mechanism and aggregate morphology of Aβ(1-40) using spectroscopic, microelectrophoretic, mass spectrometric, and ultrastructural techniques. We found that the Cu(II):Aβ ratio in solution has a major influence on (i) the aggregation kinetics/mechanism of Aβ, because three different kinetic scenarios were observed depending on the Cu(II):Aβ ratio, (ii) the metal:peptide stoichiometry in the aggregates, which increased to 1.4 at supra-equimolar Cu(II):Aβ ratio; and (iii) the morphology of the aggregates, which shifted from fibrillar to non-fibrillar at increasing Cu(II):Aβ ratios. We observed dynamic morphological changes of the aggregates, and that the formation of spherical aggregates appeared to be a common morphological end point independent on the Cu(II) concentration. Experiments with Aβ(1-42) were compatible with the conclusions for Aβ(1-40) even though the low solubility of Aβ(1-42) precluded examination under the same conditions as for the Aβ(1-40). Experiments with Aβ(1-16) and Aβ(1-28) showed that other parts than the Cu(II)-binding His residues were important for Cu(II)-induced Aβ aggregation. Based on this study we propose three mechanistic models for the Cu(II)-induced aggregation of Aβ(1-40) depending on the Cu(II):Aβ ratio, and identify key reaction steps that may be feasible targets for preventing Cu(II)-associated aggregation or toxicity in Alzheimer disease.

Iron(II) binding to amyloid-β, the Alzheimer's peptide

Iron has been implicated in Alzheimer's disease, but until now no direct proof of Fe(II) binding to the amyloid-β peptide (Aβ) has been reported. We used NMR to evidence Fe(II) coordination to full-length Aβ40 and truncated Aβ16 peptides at physiological pH and to show that the Fe(II) binding site is located in the first 16 amino-acid residues. Fe(II) caused selective broadening of some NMR peaks that was dependent on the Fe:Aβ stoichiometry and temperature. Analysis of Fe(II) broadening effect in the (1)H, (13)C, and 2D NMR data established that Asp1, Glu3, the three His, but not Tyr10 nor Met35 are the residues mainly involved in Fe(II) coordination.

MRI probes for sensing biologically relevant metal ions

Given the important role of metal ions in fundamental biological processes, the visualization of their concentration in living animals by repeatable, noninvasive imaging techniques, such as MRI, would be highly desirable. A large number of metal-responsive MRI contrast agents, the majority based on Gd(3+) complexes, have been reported in recent years. The contrast-enhancing properties (relaxivity) of a Gd(3+) complex can be most conveniently modulated by interaction with the sensed metal cation via changes in the number of water molecules bound directly to Gd(3+) or changes in the size of the complex, which represent the two major strategies to develop metal sensitive MRI probes. Here, we survey paramagnetic lanthanide complexes involving Gd(3+) agents and paramagnetic chemical exchange saturation transfer probes designed to detect the most important endogenous metal ions: calcium, zinc, iron and copper. Future work will likely focus on extending applications of these agents to living animals, as well as on exploring new ways of creating molecular MRI probes in order to meet requirements such as higher specificity or lower detection limits.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Substantial contribution of the two imidazole rings of the His13-His14 dyad to Cu(II) binding in amyloid-β(1-16) at physiological pH and its significance

The interaction of amyloid-β (Aβ) peptide with Cu(II) appears to play an important role in the etiology of Alzheimer's disease. At physiological pH, the Cu(II) coordination in Aβ is heterogeneous, and there exist at least two binding modes in which Cu(II) is coordinated by histidine residues. Electron spin resonance studies have revealed a picture of the Cu(II) binding at a higher or lower pH, where only one of the two binding modes is almost exclusively present. We describe a procedure to directly examine the coordination of Cu(II) to each histidine residue in the dominant binding mode at physiological pH. We use nonlabeled and residue-specifically (15)N-labeled Aβ(1-16). For quantitative analysis, the intensities of three-pulse electron spin-echo envelope modulation (ESEEM) spectra are analyzed. Spectral simulations show that ESEEM intensities provide information about the contribution of each histidine residue. Indeed, the ESEEM experiments at pH 6.0 confirm the dominant contribution of His6 to the Cu(II) coordination as expected from the work of other researchers. Interestingly, however, the ESEEM data obtained at pH 7.4 reveal that the contributions of the three residues to the Cu(II) coordination are in the order of His14 ≈ His6 > His13 in the dominant binding mode. The order indicates a significant contribution from the simultaneous coordination by His13 and His14 at physiological pH, which has been underappreciated. These findings are supported by hyperfine sublevel correlation spectroscopy experiments. The simultaneous coordination by the two adjacent residues is likely to be present in a non-β-sheet structure. The coexistence of different secondary structures is possibly the molecular origin for the formation of amorphous aggregates rather than fibrils at relatively high concentrations of Cu(II). Through our approach, precise and useful information about Cu(II) binding in Aβ(1-16) at physiological pH is obtained without any side-chain modification, amino acid residue replacement, or pH change, each of which might lead to an alteration in the peptide structure or the coordination environment.

Minimal Zn(2+) binding site of amyloid-β

Zinc-induced aggregation of amyloid-β peptide (Aβ) is a hallmark molecular feature of Alzheimer's disease. Here we provide direct thermodynamic evidence that elucidates the role of the Aβ region 6-14 as the minimal Zn(2+) binding site wherein the ion is coordinated by His(6), Glu(11), His(13), and His(14). With the help of isothermal titration calorimetry and quantum mechanics/molecular mechanics simulations, the region 11-14 was determined as the primary zinc recognition site and considered an important drug-target candidate to prevent Zn(2+)-induced aggregation of Aβ.

Zinc-induced dimerization of the amyloid-β metal-binding domain 1-16 is mediated by residues 11-14

Analysis of complex formation between amyloid-β fragments using surface plasmon resonance biosensing and electrospray mass spectrometry reveals that region 11-14 mediates zinc-induced dimerization of amyloid-β and may serve as a potential drug target for preventing development and progression of Alzheimer's disease.

Novel drug targets based on metallobiology of Alzheimer's disease

IMPORTANCE OF THE FIELD

Increased localization of Zn, Fe, Cu and Al within the senile plaques (SP) exacerbates amyloid beta (Aβ)-mediated oxidative damage, and acts as catalyst for Aβ aggregation in Alzheimer's disease (AD). Thus, disruption of aberrant metal-peptide interactions via chelation therapy holds considerable promise as a rational therapeutic strategy against Alzheimer's amyloid pathogenesis.

AREAS COVERED IN THIS REVIEW

The complexities of metal-induced genesis of SP are reviewed. The recent advances in the molecular mechanism of action of metal chelating agents are discussed with critical assessment of their potential to become drugs.

WHAT THE READER WILL GAIN

Taking into consideration the interaction of metals with the metal-responsive elements on the Alzheimer's amyloid precursor protein (APP), readers will gain understanding of several points to bear in mind when developing a screening campaign for AD-therapeutics.

TAKE HOME MESSAGE

A functional iron-responsive element (IRE) RNA stem loop in the 5' untranslated region (UTR) of the APP transcript regulates neural APP translation. Desferrioxamine, clioquinol, tetrathiolmolybdate, dimercaptopropanol, VK-28, and natural antioxidants, such as curcumin and ginko biloba need critical evaluation as AD therapeutics. There is a necessity for novel screens (related to metallobiology) to identify therapeutics effective in AD.

Affinity of Cu+ for the copper-binding domain of the amyloid-β peptide of Alzheimer's disease

The role of metal ions in Alzheimer's disease etiology is unresolved. For the redox-active metal ions iron and copper, the formation of reactive oxygen species by metal amyloid complexes has been proposed to contribute to Alzheimer's disease neurodegeneration. For copper, reactive oxygen species are generated by copper redox cycling between its 1+ and 2+ oxidation states. Thus, the AβCu(I) complex is potentially a critical reactant associated with Alzheimer's disease etiology. Through competitive chelation, we have measured the affinity of the soluble copper-binding domain of the amyloid-β peptide for Cu(I). The dissociation constants are in the femtomolar range for both wild-type and histidine-to-alanine mutants. These results indicate that Cu(I) binds more tightly to monomeric amyloid-β than Cu(II) does, which leads us to propose that Cu(I) is a relevant in vivo oxidation state.

Cu K-edge X-ray absorption spectroscopy reveals differential copper coordination within amyloid-β oligomers compared to amyloid-β monomers

The fatal neurological disorder Alzheimer's disease has been linked to soluble neurotoxic oligomers of amyloid-β (Aβ) peptides. Herein we demonstrate that Cu(1+) ligated within Aβ(42) oligomers (Aβ sequence: DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA) possesses a highly dioxygen sensitive tetrahedral coordination geometry. The biological implications of these findings are discussed.

Quantification of the binding properties of Cu2+ to the amyloid beta peptide: coordination spheres for human and rat peptides and implication on Cu2+-induced aggregation

There is no consensus on the coordinating ligands for Cu(2+) by Abeta. However, the differences in peptide sequence between human and rat have been hypothesized to alter metal ion binding in a manner that alters Cu(2+)-induced aggregation of Abeta. Herein, we employ isothermal titration calorimetry (ITC), circular dichroism (CD), and electron paramagnetic resonance (EPR) spectroscopy to examine the Cu(2+) coordination spheres to human and rat Abeta and an extensive set of Abeta(16) mutants. EPR of the mutant peptides is consistent with a 3N1O binding geometry, like the native human peptide at pH 7.4. The thermodynamic data reveal an equilibrium between three coordination spheres, {NH(2), O, N(Im)(His6), N(-)}, {NH(2), O, N(Im)(His6), N(Im)(His13)}, and {NH(2), O, N(Im)(His6), N(Im)(His14)}, for human Abeta(16) but one dominant coordination for rat Abeta(16), {NH(2), O, N(Im)(His6), N(-)}, at pH 7.4-6.5. ITC and CD data establish that the mutation R5G is sufficient for reproducing this difference in Cu(2+) binding properties at pH 7.4. The substitution of bulky and positively charged Arg by Gly is proposed to stabilize the coordination {NH(2), O-, N(Im)(His6), N(-)} that then results in one dominating coordination sphere for the case of the rat peptide. The differences in the coordination geometries for Cu(2+) by the human and rat Abeta are proposed to contribute to the variation in the ability of Cu(2+) to induce aggregation of Abeta peptides.

The role of zinc in Alzheimer's disease

Zinc, the most abundant trace metal in the brain, has numerous functions, both in health and in disease. Zinc is released into the synaptic cleft of glutamatergic neurons alongside glutamate from where it interacts and modulates NMDA and AMPA receptors. In addition, zinc has multifactorial functions in Alzheimer's disease (AD). Zinc is critical in the enzymatic nonamyloidogenic processing of the amyloid precursor protein (APP) and in the enzymatic degradation of the amyloid-β (Aβ) peptide. Zinc binds to Aβ promoting its aggregation into neurotoxic species, and disruption of zinc homeostasis in the brain results in synaptic and memory deficits. Thus, zinc dyshomeostasis may have a critical role to play in the pathogenesis of AD, and the chelation of zinc is a potential therapeutic approach.

Hepatocyte targeting and intracellular copper chelation by a thiol-containing glycocyclopeptide

Metal overload plays an important role in several diseases or intoxications, like in Wilson's disease, a major genetic disorder of copper metabolism in humans. To efficiently and selectively decrease copper concentration in the liver that is highly damaged, chelators should be targeted at the hepatocytes. In the present work, we synthesized a molecule able to both lower intracellular copper, namely Cu(I), and target hepatocytes, combining within the same structure a chelating unit and a carbohydrate recognition element. A cyclodecapeptide scaffold displaying a controlled conformation with two independent faces was chosen to introduce both units. One face displays a cluster of carbohydrates to ensure an efficient recognition of the asialoglycoprotein receptors, expressed on the surface of hepatocytes. The second face is devoted to metal ion complexation thanks to the thiolate functions of two cysteine side-chains. To obtain a chelator that is active only once inside the cells, the two thiol functions were oxidized in a disulfide bridge to afford the glycopeptide P(3). Two simple cyclodecapeptides modeling the reduced and complexing form of P(3) in cells proved a high affinity for Cu(I) and a high selectivity with respect to Zn(II). As expected, P(3) becomes an efficient Cu(I) chelator in the presence of glutathione that mimics the intracellular reducing environment. Finally, cellular uptake and ability to lower intracellular copper were demonstrated in hepatic cell lines, in particular in WIF-B9, making P(3) a good candidate to fight copper overload in the liver.

Recent Development of Bifunctional Small Molecules to Study Metal-Amyloid-β Species in Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease related to the deposition of aggregated amyloid-β (Aβ) peptides in the brain. It has been proposed that metal ion dyshomeostasis and miscompartmentalization contribute to AD progression, especially as metal ions (e.g., Cu(II) and Zn(II)) found in Aβ plaques of the diseased brain can bind to Aβ and be linked to aggregation and neurotoxicity. The role of metal ions in AD pathogenesis, however, is uncertain. To accelerate understanding in this area and contribute to therapeutic development, recent efforts to devise suitable chemical reagents that can target metal ions associated with Aβ have been made using rational structure-based design that combines two functions (metal chelation and Aβ interaction) in the same molecule. This paper presents bifunctional compounds developed by two different design strategies (linkage or incorporation) and discusses progress in their applications as chemical tools and/or potential therapeutics.

Design of small molecules that target metal-A{beta} species and regulate metal-induced A{beta} aggregation and neurotoxicity

The accumulation of metal ions and amyloid-β (Aβ) aggregates found in the brain of patients with Alzheimer's disease (AD) has been suggested to be involved in AD pathogenesis. To investigate metal-Aβ-associated pathways in AD, development of chemical tools to target metal-Aβ species is desired. Only a few efforts, however, have been reported. Here, we report bifunctional small molecules, N-(pyridin-2-ylmethyl)aniline (L2-a) and N(1),N(1)-dimethyl-N(4)-(pyridin-2-ylmethyl)benzene-1,4-diamine (L2-b) that can interact with both metal ions and Aβ species, as determined by spectroscopic methods including high-resolution NMR spectroscopy. Using the bifunctional compound L2-b, metal-induced Aβ aggregation and neurotoxicity were modulated in vitro as well as in human neuroblastoma cells. Furthermore, treatment of human AD brain tissue homogenates containing metal ions and Aβ species with L2-b showed disassembly of Aβ aggregates. Therefore, our studies presented herein demonstrate the value of bifunctional compounds as chemical tools for investigating metal-Aβ-associated events and their mechanisms in the development and pathogenesis of AD and as potential therapeutics.

Mercury(II) promotes the in vitro aggregation of tau fragment corresponding to the second repeat of microtubule-binding domain: Coordination and conformational transition

The loss of metal homeostasis and the toxic effect of metal ion are important events in neurodegenerative and age-related diseases, such as Alzheimer's disease (AD). For the first time, we investigated the impacts of mercury(II) ions on the folding and aggregation of Alzheimer's tau fragment R2 (residues 275-305: VQIIN KKLDL SNVQS KCGSK DNIKH VPGGGS), corresponding to the second repeat unit of the microtubule-binding domain, which was believed to be pivotal to the biochemical properties of full tau protein. By ThS fluorescence assay and electron microscopy, we found that mercury(II) dramatically promoted heparin-induced aggregation of R2 at an optimum molar ratio of 1: 2 (metal: protein), and the resulting R2 filaments became smaller. Isothermal titration calorimetry (ITC) experiment revealed that the strong coordination of mercury(II) with R2 was an enthalpy-controlled, entropy-decreased thermodynamic process. The exceptionally large magnitude of heat release (ΔH₁ = -34.8 Kcal mol⁻¹) suggested that the most possible coordinating site on the R2 peptide chain was the thiol group of cysteine residue (Cys291), and this was further confirmed by a control experiment using Cys291 mutated R2. Circular dichroism spectrum demonstrated that this peptide underwent a significant conformational change from random coil to β-turn structure upon its binding to mercury(II) ion. This study was undertaken to better understand the mechanism of tau aggregation, and evaluate the possible role of mercury(II) in the pathogenesis of AD.

Fibrillation in human serum albumin is enhanced in the presence of copper(II)

The aggregation process in proteins is governed by several factors such as temperature, pH, presence of electrolytes, denaturants, and metal ions. Here, we report the role of Cu(II) in inducing rapid fibrillation in human serum albumin. We have monitored this process via UV-vis spectroscopy, fluorescence spectroscopy, circular dichroism, zeta-potential measurements, electron paramagnetic resonance studies, fluorescence microscopy, and field emission scanning electron microscopy. Images show a fibrillar network of human serum albumin in the presence of Cu(II) in 60% ethanol incubated at 65 degrees C at physiological pH. All other studies also support the enhanced fibrillation in presence of Cu(II).

Substoichiometric levels of Cu2+ ions accelerate the kinetics of fiber formation and promote cell toxicity of amyloid-{beta} from Alzheimer disease

A role for Cu(2+) ions in Alzheimer disease is often disputed, as it is believed that Cu(2+) ions only promote nontoxic amorphous aggregates of amyloid-β (Aβ). In contrast with currently held opinion, we show that the presence of substoichiometric levels of Cu(2+) ions in fact doubles the rate of production of amyloid fibers, accelerating both the nucleation and elongation of fiber formation. We suggest that binding of Cu(2+) ions at a physiological pH causes Aβ to approach its isoelectric point, thus inducing self-association and fiber formation. We further show that Cu(2+) ions bound to Aβ are consistently more toxic to neuronal cells than Aβ in the absence of Cu(2+) ions, whereas Cu(2+) ions in the absence of Aβ are not cytotoxic. The degree of Cu-Aβ cytotoxicity correlates with the levels of Cu(2+) ions that accelerate fiber formation. We note the effect appears to be specific for Cu(2+) ions as Zn(2+) ions inhibit the formation of fibers. An active role for Cu(2+) ions in accelerating fiber formation and promoting cell death suggests impaired copper homeostasis may be a risk factor in Alzheimer disease.

Bioinorganic chemistry of Parkinson's disease: structural determinants for the copper-mediated amyloid formation of alpha-synuclein

The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD). A central, unresolved question in the pathophysiology of PD relates to the role of AS-metal interactions in amyloid fibril formation and neurodegeneration. Our previous works established a hierarchy in alpha-synuclein-metal ion interactions, where Cu(II) binds specifically to the protein and triggers its aggregation under conditions that might be relevant for the development of PD. Two independent, non-interacting copper-binding sites were identified at the N-terminal region of AS, with significant difference in their affinities for the metal ion. In this work we have solved unknown details related to the structural binding specificity and aggregation enhancement mediated by Cu(II). The high-resolution structural characterization of the highest affinity N-terminus AS-Cu(II) complex is reported here. Through the measurement of AS aggregation kinetics we proved conclusively that the copper-enhanced AS amyloid formation is a direct consequence of the formation of the AS-Cu(II) complex at the highest affinity binding site. The kinetic behavior was not influenced by the His residue at position 50, arguing against an active role for this residue in the structural and biological events involved in the mechanism of copper-mediated AS aggregation. These new findings are central to elucidate the mechanism through which the metal ion participates in the fibrillization of AS and represent relevant progress in the understanding of the bioinorganic chemistry of PD.

Electrochemical and homogeneous electron transfers to the Alzheimer amyloid-beta copper complex follow a preorganization mechanism

Deciphering the electron transfer reactivity characteristics of amyloid β-peptide copper complexes is an important task in connection with the role they are assumed to play in Alzheimer's disease. A systematic analysis of this question with the example of the amyloid β-peptide copper complex by means of its electrochemical current-potential responses and of its homogenous reactions with electrogenerated fast electron exchanging osmium complexes revealed a quite peculiar mechanism: The reaction proceeds through a small fraction of the complex molecules in which the peptide complex is "preorganized" so as the distances and angles in the coordination sphere to vary minimally upon electron transfer, thus involving a remarkably small reorganization energy (0.3 eV). This preorganization mechanism and its consequences on the reactivity should be taken into account for reactions involving dioxygen and hydrogen peroxide that are considered to be important in Alzheimer's disease through the production of harmful reactive oxygen species.

Reaction rates and mechanism of the ascorbic acid oxidation by molecular oxygen facilitated by Cu(II)-containing amyloid-beta complexes and aggregates

A forefront of the research on Alzheimer's disease (AD) is the interaction of amyloid beta (Abeta) peptides with redox metal ions (e.g., Cu(II), Fe(III), and Fe(II)) and the biological relevance of the Abeta-metal complexes to neuronal cell loss and homeostasis of essential metals and other cellular species. This work is concerned with the kinetic and mechanistic studies of the ascorbic acid oxidation reaction by molecular oxygen that is facilitated by Cu(II) complexes with Abeta(1-16), Abeta(1-42), and aggregates of Abeta(1-42). The reaction rate was found to linearly increase with the concentrations of Abeta-Cu(II) and dissolved oxygen and be invariant with high ascorbic acid concentrations. The rate constants were measured to be 117.2 +/- 15.4 and 15.8 +/- 2.8 M(-1) s(-1) at low (<100 muM) and high AA concentrations, respectively. Unlike free Cu(II), in the presence of AA, Abeta-Cu(II) complexes facilitate the reduction of oxygen by producing H(2)O(2) as a major product. Such a conclusion is drawn on the basis that the reaction stoichiometry between AA and O(2) is 1:1 when the Abeta concentration is kept at a much greater value than that of Cu(II). A mechanism is proposed for the AA oxidation in which the oxidation states of the copper center in the Abeta complex alternates between 2+ and 1+. The catalytic activity of Cu(II) toward O(2) reduction was found to decrease in the order of free Cu(II) > Abeta(1-16)-Cu(II) > Abeta(1-42)-Cu(II) > Cu(II) complexed by the Abeta oligomer/fibril mixture > Cu(II) in Abeta fibrils. The finding that Cu(II) in oligomeric and fibrous Abeta aggregates possesses considerable activity toward H(2)O(2) generation is particularly significant, since in senile plaques of AD patients the coexisting copper and Abeta aggregates have been suggested to inflict oxidative stress through the production of reactive oxygen species (ROS). Although Cu(II) bound to oligomeric and fibrous Abeta aggregates is less effective than free Cu(II) and the monomeric Abeta-Cu(II) complex in producing ROS, in vivo the Cu(II)-containing Abeta oligomers and fibrils might be more biologically relevant given their stronger association with cell membranes and the closer proximity of ROS to cell membranes.

Copper(I) and copper(II) binding to β-amyloid 16 (Aβ16) studied by electrospray ionization mass spectrometry

Copper-β-amyloid 16 (Aβ16) complexes were investigated by electrospray ionization mass spectrometry (ESI-MS). Copper(i) and (ii) complexes were formed on-line in a microchip electrospray emitter by using a sacrificial copper electrode as the anode in positive ionization mode. In the presence of ascorbic acid in the peptide solution, the amount of Cu(i)-Aβ16 generated electrochemically was even higher. A kinetic model is proposed to account for the generation of copper complexes. The structure of Cu(i)-Aβ16 was investigated by tandem mass spectrometry (MS/MS), and the binding site of Cu(i) to Aβ16 was identified at the His13, His14 residues. Cu(ii)-Aβ16 was also investigated by MS/MS and, based on the unusual observations of a-ions, the two binding residues of His13 and His14 of Aβ16 to Cu(ii) were also confirmed. This approach provides direct information on Cu(i)-Aβ16 complexes generated in solution from metallic copper and gives evidence that both His13 and His14 are involved in the coordination of both Cu(i)- and Cu(ii)-Aβ16 complexes.

Zinc ions promote Alzheimer Abeta aggregation via population shift of polymorphic states

Although a key factor in Alzheimer's disease etiology is enrichment of Zn(2+) in aggregates, and there are data suggesting that zinc promotes aggregation, how Zn(2+)-Abeta coordination promotes aggregation is elusive. Here we probe the structures and mechanisms through which Zn(2+) can affect amyloidosis. By covalently linking fragments (that have experiment-based coordinates) we observed that, in oligomeric Zn(2+)-Abeta(42), Zn(2+) can simultaneously coordinate intra- and intermolecularly, bridging two peptides. Zinc coordination significantly decreases the solvation energy for large Zn(2+)-Abeta(42) oligomers and thus enhances their aggregation tendency. Zn(2+) binding does not change the beta-sheet association around the C-terminal hydrophobic region; however, it shifts the relative population of the preexisting amyloid polymorphic ensembles. As a result, although a parallel beta-sheet arrangement is still preferred, antiparallel and other less structured assemblies are stabilized, also becoming major species. Overall, Zn(2+) coordination promotes Abeta(42) aggregation leading to less uniform structures. Our replica exchange molecular dynamics simulations further reproduced an experimental observation that the increasing Zn(2+) concentration could slow down the aggregation rate, even though the aggregation rates are still much higher than in Zn(2+)-free solution.

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.