Quantum dots-based "chemical tongue" for the discrimination of short-length Aβ peptides

A "chemical tongue" is proposed based on thiomalic acid-capped quantum dots (QDs) with signal enrichment provided by excitation-emission matrix (EEM) fluorescence spectroscopy for the determination of close structural analogs-short-length amyloid β (Aβ) peptides related to Alzheimer's disease. Excellent discrimination is obtained by principal component analysis (PCA) for seven derivatives: Aβ1-16, Aβ4-16, Aβ4-9, Aβ5-16, Aβ5-12, Aβ5-9, Aβ12-16. Detection of Aβ4-16, Aβ4-16, and Aβ5-9 in binary and ternary mixtures performed by QDs-based chemical tongue using partial least squares-discriminant analysis (PLS-DA) provided perfect 100% accuracy for the two studied peptides (Aβ4-16 and Aβ4-16), while for the third one (Aβ5-9) it was slightly lower (97.9%). Successful detection of Aβ4-16 at 1 pmol/mL (1.6 ng/mL) suggests that the detection limit of the proposed method for short-length Aβ peptides can span nanomolar concentrations. This result is highly promising for the development of simple and efficient methods for sequence recognition in short-length peptides and better understanding of mechanisms at the QD-analyte interface.

Dual mode of voltammetric studies on Cu(II) complexes of His2 peptides: phosphate and peptide sequence recognition

Copper(II) complexes of peptides with a histidine residue at the second position (His2 peptides) provide a unique profile of electrochemical behavior, offering signals of both Cu(II) reduction and Cu(II) oxidation. Furthermore, their structures with vacant positions in the equatorial coordination plane could facilitate interactions with other biomolecules. In this work, we designed a library of His2 peptides based on the sequence of Aβ_5-9 (RHDSG), an amyloid beta peptide derivative. The changes in the Aβ_5-9 sequence highly affect the Cu(II) oxidation signals, altered further by anionic species. As a result, Cu(II) complexes of Arg1 peptides without Asp residues were chosen as the most promising peptide-based molecular receptors for phosphates. The voltammetric data on Cu(II) oxidation for binary Cu(II)-His2 peptide complexes and ternary Cu(II)-His2 peptide/phosphate systems were also tested for His2 peptide recognition. We achieved a highly promising identification of subtle modifications in the peptide sequence. Thus, we introduce voltammetric measurement as a potential novel tool for peptide sequence recognition.

Ni(II) Ions May Target the Entire Melatonin Biosynthesis Pathway—A Plausible Mechanism of Nickel Toxicity

Nickel is toxic to humans. Its compounds are carcinogenic. Furthermore, nickel allergy is a severe health problem that affects approximately 10–20% of humans. The mechanism by which these conditions develop remains unclear, but it may involve the cleavage of specific proteins by nickel ions. Ni(II) ions cleave the peptide bond preceding the Ser/Thr-Xaa-His sequence. Such sequences are present in all four enzymes of the melatonin biosynthesis pathway, i.e., tryptophan 5-hydroxylase 1, aromatic-l-amino-acid decarboxylase, serotonin N-acetyltransferase, and acetylserotonin O-methyltransferase. Moreover, fragments prone to Ni(II) are exposed on surfaces of these proteins. Our results indicate that all four studied fragments undergo cleavage within tens of hours at pH 8.2 and 37 °C, corresponding with the conditions in the mitochondrial matrix. Since melatonin, a potent antioxidant and anti-inflammatory agent, is synthesized within the mitochondria of virtually all human cells, depleting its supply may be detrimental, e.g., by raising the oxidative stress level. Intriguingly, Ni(II) ions have been shown to mimic hypoxia through the stabilization of HIF-1α protein, but melatonin prevents the action of HIF-1α. Considering all this, the enzymes of the melatonin biosynthesis pathway seem to be a toxicological target for Ni(II) ions.

Tuning Receptor Properties of Metal-Amyloid Beta Complexes. Studies on the Interaction between Ni(II)-Aβ5-9 and Phosphates/Nucleotides

Amyloid-beta (Aβ) peptides, potentially relevant in the pathology of Alzheimer's disease, possess distinctive coordination properties, enabling an effective binding of transition-metal ions, with a preference for Cu(II). In this work, we found that a N-truncated Aβ analogue bearing a His-2 motif, Aβ_5-9, forms a stable Ni(II) high-spin octahedral complex at a physiological pH of 7.4 with labile coordination sites and facilitates ternary interactions with phosphates and nucleotides. As the pH increased above 9, a spin transition from a high-spin to a low-spin square-planar Ni(II) complex was observed. Employing electrochemical techniques, we showed that interactions between the binary Ni(II)-Aβ_5-9 complex and phosphate species result in significant changes in the Ni(II) oxidation signal. Thus, the Ni(II)-Aβ_5-9 complex could potentially serve as a receptor in electrochemical biosensors for phosphate species. The obtained results could also be important for nickel toxicology.

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

Correction for 'Copper(ii) complex of N-truncated amyloid-β peptide bearing a His-2 motif as a potential receptor for phosphate anions' by Aleksandra Tobolska et al., Dalton Trans., 2021, DOI: 10.1039/d0dt03898a.

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.

How trimerization of CTR1 N-terminal model peptides tunes Cu-binding and redox-chemistry

Employing peptide-based models of copper transporter 1 (CTR1), we show that the trimeric arrangement of its N-terminus tunes its reactivity with Cu, promoting Cu(ii) reduction and stabilizing Cu(i). Hence, the employed multimeric models of CTR1 provide an important contribution to studies on early steps of Cu uptake by cells.

Aβ5-x Peptides: N-Terminal Truncation Yields Tunable Cu(II) Complexes

The Aβ_5-x peptides (x = 38, 40, 42) are minor Aβ species in normal brains but elevated upon the application of inhibitors of Aβ processing enzymes. They are interesting from the point of view of coordination chemistry for the presence of an Arg-His metal binding sequence at their N-terminus capable of forming a 3-nitrogen (3N) three-coordinate chelate system. Similar sequences in other bioactive peptides were shown to bind Cu(II) ions in biological systems. Therefore, we investigated Cu(II) complex formation and reactivity of a series of truncated Aβ_5-x peptide models comprising the metal binding site: Aβ_5-9, Aβ_5-12, Aβ_5-12Y10F, and Aβ_5-16. Using CD and UV-vis spectroscopies and potentiometry, we found that all peptides coordinated the Cu(II) ion with substantial affinities higher than 3 × 10¹² M^-1 at pH 7.4 for Aβ_5-9 and Aβ_5-12. This affinity was elevated 3-fold in Aβ_5-16 by the formation of the internal macrochelate with the fourth coordination site occupied by the imidazole nitrogen of the His13 or His14 residue. A much higher boost of affinity could be achieved in Aβ_5-9 and Aβ_5-12 by adding appropriate amounts of the external imidazole ligand. The 3N Cu-Aβ_5-x complexes could be irreversibly reduced to Cu(I) at about -0.6 V vs Ag/AgCl and oxidized to Cu(III) at about 1.2 V vs Ag/AgCl. The internal or external imidazole coordination to the 3N core resulted in a slight destabilization of the Cu(I) state and stabilization of the Cu(III) state. Taken together these results indicate that Aβ_5-x peptides, which bind Cu(II) ions much more strongly than Aβ_1-x peptides and only slightly weaker than Aβ_4-x peptides could interfere with Cu(II) handling by these peptides, adding to copper dyshomeostasis in Alzheimer brains.

Copper Transporters? Glutathione Reactivity of Products of Cu-Aβ Digestion by Neprilysin

Aβ_4–42 is the major subspecies of Aβ peptides characterized by avid Cu(II) binding via the ATCUN/NTS motif. It is thought to be produced in vivo proteolytically by neprilysin, but in vitro experiments in the presence of Cu(II) ions indicated preferable formation of C-terminally truncated ATCUN/NTS species including Cu^IIAβ_4–16, Cu^IIAβ_4–9, and also Cu^IIAβ_12–16, all with nearly femtomolar affinities at neutral pH. Such small complexes may serve as shuttles for copper clearance from extracellular brain spaces, on condition they could survive intracellular conditions upon crossing biological barriers. In order to ascertain such possibility, we studied the reactions of Cu^IIAβ_4–16, Cu^IIAβ_4–9, Cu^IIAβ_12–16, and Cu^IIAβ_1–16 with reduced glutathione (GSH) under aerobic and anaerobic conditions using absorption spectroscopy and mass spectrometry. We found Cu^IIAβ_4–16 and Cu^IIAβ_4–9 to be strongly resistant to reduction and concomitant formation of Cu(I)–GSH complexes, with reaction times ∼10 h, while Cu^IIAβ_12–16 was reduced within minutes and Cu^IIAβ_1–16 within seconds of incubation. Upon GSH exhaustion by molecular oxygen, the Cu^IIAβ complexes were reformed with no concomitant oxidative damage to peptides. These finding reinforce the concept of Aβ_4–x peptides as physiological trafficking partners of brain copper.

Aβ₄₋₁₆, Aβ₄₋₉, and Aβ₁₂₋₁₆, oligopeptide products of β-amyloid degradation by neprilysin, bind Cu^II ions very tightly and are considered as possible Cu^II carriers in the brain. We demonstrated that Cu^II(Aβ_4−x) complexes, but not Cu^II(Aβ₁₂₋₁₆), are kinetically resistant to reduction by glutathione. No covalent Aβ peptide modifications were observed during the copper reduction and reoxidation by ambient oxygen, yielding the original complexes. These features suggest that Cu^II(Aβ_4−x) complexes might be able to cross the blood−brain barrier.

Peptide Bond Cleavage by Ni(II) Ions within the Nuclear Localization Signal Sequence

Nickel is harmful to humans, being both carcinogenic and allergenic. However, the mechanisms of this toxicity are still unresolved. We propose that Ni(II) ions disintegrate proteins by hydrolysis of peptide bonds preceding the Ser/Thr-Xaa-His sequences. Such sequences occur in nuclear localization signals (NLSs) of human phospholipid scramblase 1, Sam68-like mammalian protein 2, and CLK3 kinase. We performed spectroscopic experiments showing that model nonapeptides derived from these NLSs bind Ni(II) at physiological pH. We also proved that these sequences are prone to Ni(II) hydrolysis. Thus, the aforementioned NLSs may be targets for nickel toxicity. This implies that Ni(II) ions disrupt the transport of some proteins from cytoplasm to cell nucleus.

The N-terminal 14-mer model peptide of human Ctr1 can collect Cu(ii) from albumin. Implications for copper uptake by Ctr1

Human cells acquire copper primarily via the copper transporter 1 protein, hCtr1. We demonstrate that at extracellular pH 7.4 CuII is bound to the model peptide hCtr11-14via an ATCUN motif and such complexes are strong enough to collect CuII from albumin, supporting the potential physiological role of CuII binding to hCtr1.

Cu(II) Binding to the N-Terminal Model Peptide of the Human Ctr2 Transporter at Lysosomal and Extracellular pH

It was shown that His3 of human copper transporter 1 (hCtr1) prompts the ATCUN-like Cu(II) coordination for model peptides of the hCtr1 N-terminus. Its high Cu(II) affinity is a potential driving force for the transfer of Cu(II) from extracellular Cu(II) carriers to hCtr1. Having a sequence similar to that of hCtr1, hCtr2 has been proposed as another human copper transporter. However, the N-terminal domain of hCtr2 is much shorter than that of hCtr1, with different copper binding motifs at its N-terminus. Employing a model peptide of the hCtr2 N-terminus, MAMHF-am, we demonstrated that His4 provides a unique pattern of Cu(II) complexes, involving Met sulfurs in their Cu(II) coordination sphere. The affinity of Cu(II) for MAMHF-am is a few orders of magnitude lower than that reported for the hCtr1 model peptides at the extracellular pH of 7.4, suggesting a maximal complementary role of Cu(II) binding to hCtr2 in the import of copper from the extracellular space to the cytoplasm. On the other hand, the ability of the hCtr2 model peptide to capture Cu(II) from amino acids and short peptides (potential degradation products of proteins) at pH 5.0 and the known predominant lysosomal localization of hCtr2 support an important potential role of the Cu(II)-hCtr2 interaction in the recovery of copper from lysosomes.

Triggering Cu-coordination change in Cu(ii)-Ala-His-His by external ligands

A switch in copper(ii)-coordination between the two motifs Xxx-Zzz-His (ATCUN) and Xxx-His in the hybride peptide Ala-His-His is induced by adding a ligand such as imidazole.

Resistance of Cu(Aβ4-16) to Copper Capture by Metallothionein-3 Supports a Function for the Aβ4-42 Peptide as a Synaptic Cu(II) Scavenger

Aβ4-42 is a major species of Aβ peptide in the brains of both healthy individuals and those affected by Alzheimer's disease. It has recently been demonstrated to bind Cu(II) with an affinity approximately 3000 times higher than the commonly studied Aβ1-42 and Aβ1-40 peptides, which are implicated in the pathogenesis of Alzheimer's disease. Metallothionein-3, a protein considered to orchestrate copper and zinc metabolism in the brain and provide antioxidant protection, was shown to extract Cu(II) from Aβ1-40 when acting in its native Zn7 MT-3 form. This reaction is assumed to underlie the neuroprotective effect of Zn7 MT-3 against Aβ toxicity. In this work, we used the truncated model peptides Aβ1-16 and Aβ4-16 to demonstrate that the high-affinity Cu(II) complex of Aβ4-16 is resistant to Zn7 MT-3 reactivity. This indicates that the analogous complex of the full-length peptide Cu(Aβ4-42) will not yield copper to MT-3 in the brain, thus supporting the concept of a physiological role for Aβ4-42 as a Cu(II) scavenger in the synaptic cleft.

A Functional Role for Aβ in Metal Homeostasis? N-Truncation and High-Affinity Copper Binding

Accumulation of the β-amyloid (Aβ) peptide in extracellular senile plaques rich in copper and zinc is a defining pathological feature of Alzheimer's disease (AD). The Aβ1-x (x=16/28/40/42) peptides have been the primary focus of Cu(II) binding studies for more than 15 years; however, the N-truncated Aβ4-42 peptide is a major Aβ isoform detected in both healthy and diseased brains, and it contains a novel N-terminal FRH sequence. Proteins with His at the third position are known to bind Cu(II) avidly, with conditional log K values at pH 7.4 in the range of 11.0-14.6, which is much higher than that determined for Aβ1-x peptides. By using Aβ4-16 as a model, it was demonstrated that its FRH sequence stoichiometrically binds Cu(II) with a conditional Kd value of 3×10(-14)  M at pH 7.4, and that both Aβ4-16 and Aβ4-42 possess negligible redox activity. Combined with the predominance of Aβ4-42 in the brain, our results suggest a physiological role for this isoform in metal homeostasis within the central nervous system.

Human annexins A1, A2, and A8 as potential molecular targets for Ni(II) ions

Nickel is harmful for humans, but molecular mechanisms of its toxicity are far from being fully elucidated. One of such mechanisms may be associated with the Ni(II)-dependent peptide bond hydrolysis, which occurs before Ser/Thr in Ser/Thr-Xaa-His sequences. Human annexins A1, A2, and A8, proteins modulating the immune system, contain several such sequences. To test if these proteins are potential molecular targets for nickel toxicity we characterized the binding of Ni(II) ions and hydrolysis of peptides Ac-KALTGHLEE-am (A1-1), Ac-TKYSKHDMN-am (A1-2), and Ac-GVGTRHKAL-am (A1-3), from annexin A1, Ac-KMSTVHEIL-am (A2-1) and Ac-SALSGHLET-am (A2-2), from annexin A2, and Ac-VKSSSHFNP-am (A8-1), from annexin A8, using UV-vis and circular dichroism (CD) spectroscopies, potentiometry, isothermal titration calorimetry, high-performance liquid chromatography (HPLC), and electrospray ionization mass spectrometry (ESI-MS). We found that at physiological conditions (pH 7.4 and 37 °C) peptides A1-2, A1-3, A8-1, and to some extent A2-2 bind Ni(II) ions sufficiently strongly in 4N complexes and are hydrolyzed at sufficiently high rates to justify the notion that these annexins can undergo nickel hydrolysis in vivo. These results are discussed in the context of specific biochemical interactions of respective proteins. Our results also expand the knowledge about Ni(II) binding to histidine peptides by determination of thermodynamic parameters of this process and spectroscopic characterization of 3N complexes. Altogether, our results indicate that human annexins A1, A2, and A8 are potential molecular targets for nickel toxicity and help design appropriate cellular studies.