Ternary Cu(II) Complex with GHK Peptide and Cis-Urocanic Acid as a Potential Physiologically Functional Copper Chelate

Glycyl-l-histidyl-l-lysine prevents copper- and zinc-induced protein aggregation and central nervous system cell death in vitro

Common features of neurodegenerative diseases are oxidative and inflammatory imbalances as well as the misfolding of proteins. An excess of free metal ions can be pathological and contribute to cell death, but only copper and zinc strongly promote protein aggregation. Herein we demonstrate that the endogenous copper-binding tripeptide glycyl-l-histidyl-l-lysine (GHK) has the ability to bind to and reduce copper redox activity and to prevent copper- and zinc-induced cell death in vitro. In addition, GHK prevents copper- and zinc-induced bovine serum albumin aggregation and reverses aggregation through resolubilizing the protein. We further demonstrate the enhanced toxicity of copper during inflammation and the ability of GHK to attenuate this toxicity. Finally, we investigated the effects of copper on enhancing paraquat toxicity and report a protective effect of GHK. We therefore conclude that GHK has potential as a cytoprotective compound with regard to copper and zinc toxicity, with positive effects on protein solubility and aggregation that warrant further investigation in the treatment of neurodegenerative diseases.

Metal Ion-Peptide-Intermediated Ligand Transfer for Serum Detection of the Omicron Variant of SARS-Cov-2 in Echinococcosis Patients

A method is developed to electrochemically induce target-specific covalent capturing of the spike protein of SARS-Cov-2, forming a covalent peptide-protein complex fit for working with such complicated clinical samples. Specifically, peptide-coordinated copper ions can be electrochemically controlled to induce cross-linkage between certain amino acids on the peptide probe and the target protein. Therefore, target specificity can be tuned electrochemically, realizing highly specific targeting of the omicron S protein or broader specificity toward all variants of the virus. Using this method, with electrochemically catalyzed generation of signal-enhancing molecules, the sensitivity and covalent detection allow their application in both serum and fecal samples. These results may point to their possible use in screening new variants of the virus in the near future.

Chelator PBT2 Forms a Ternary Cu2+ Complex with β-Amyloid That Has High Stability but Low Specificity

The metal chelator PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) acts as a terdentate ligand capable of forming binary and ternary Cu²⁺ complexes. It was clinically trialed as an Alzheimer's disease (AD) therapy but failed to progress beyond phase II. The β-amyloid (Aβ) peptide associated with AD was recently concluded to form a unique Cu(Aβ) complex that is inaccessible to PBT2. Herein, it is shown that the species ascribed to this binary Cu(Aβ) complex in fact corresponds to ternary Cu(PBT2)N_Im^Aβ complexes formed by the anchoring of Cu(PBT2) on imine nitrogen (N_Im) donors of His side chains. The primary site of ternary complex formation is His6, with a conditional stepwise formation constant at pH 7.4 (Kc [M^-1]) of logKc = 6.4 ± 0.1, and a second site is supplied by His13 or His14 (logKc = 4.4 ± 0.1). The stability of Cu(PBT2)N_Im^H13/14 is comparable with that of the simplest Cu(PBT2)N_Im complexes involving the N_Im coordination of free imidazole (logKc = 4.22 ± 0.09) and histamine (logKc = 4.00 ± 0.05). The 100-fold larger formation constant for Cu(PBT2)N_Im^H6 indicates that outer-sphere ligand-peptide interactions strongly stabilize its structure. Despite the relatively high stability of Cu(PBT2)N_Im^H6, PBT2 is a promiscuous chelator capable of forming a ternary Cu(PBT2)N_Im complex with any ligand containing an N_Im donor. These ligands include histamine, L-His, and ubiquitous His side chains of peptides and proteins in the extracellular milieu, whose combined effect should outweigh that of a single Cu(PBT2)N_Im^H6 complex regardless of its stability. We therefore conclude that PBT2 is capable of accessing Cu(Aβ) complexes with high stability but low specificity. The results have implications for future AD therapeutic strategies and understanding the role of PBT2 in the bulk transport of transition metal ions. Given the repurposing of PBT2 as a drug for breaking antibiotic resistance, ternary Cu(PBT2)N_Im and analogous Zn(PBT2)N_Im complexes may be relevant to its antimicrobial properties.

Chemistry towards Biology

Although it may not seem like it, chemical biology has existed for a long time from today's perspective [...].

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.

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.

Why the Ala-His-His Peptide Is an Appropriate Scaffold to Remove and Redox Silence Copper Ions from the Alzheimer’s-Related Aβ Peptide

The progressive, neurodegenerative Alzheimer’s disease (AD) is the most widespread dementia. Due to the ageing of the population and the current lack of molecules able to prevent or stop the disease, AD will be even more impactful for society in the future. AD is a multifactorial disease, and, among other factors, metal ions have been regarded as potential therapeutic targets. This is the case for the redox-competent Cu ions involved in the production of reactive oxygen species (ROS) when bound to the Alzheimer-related Aβ peptide, a process that contributes to the overall oxidative stress and inflammation observed in AD. Here, we made use of peptide ligands to stop the Cu(Aβ)-induced ROS production and we showed why the AHH sequence is fully appropriate, while the two parents, AH and AAH, are not. The AHH peptide keeps its beneficial ability against Cu(Aβ)-induced ROS, even in the presence of Zn^II-competing ions and other biologically relevant ions. The detailed kinetic mechanism by which AHH could exert its action against Cu(Aβ)-induced ROS is also proposed.

Kinetics of Cu(II) complexation by ATCUN/NTS and related peptides: a gold mine of novel ideas for copper biology

Cu(II)-peptide complexes are intensely studied as models for biological peptides and proteins and for their direct importance in copper homeostasis and dyshomeostasis in human diseases. In particular, high-affinity ATCUN/NTS (amino-terminal copper and nickel/N-terminal site) motifs present in proteins and peptides are considered as Cu(II) transport agents for copper delivery to cells. The information on the affinities and structures of such complexes derived from steady-state methods appears to be insufficient to resolve the mechanisms of copper trafficking, while kinetic studies have recently shown promise in explaining them. Stopped-flow experiments of Cu(II) complexation to ATCUN/NTS peptides revealed the presence of reaction steps with rates much slower than the diffusion limit due to the formation of novel intermediate species. Herein, the state of the field in Cu(II)-peptide kinetics is reviewed in the context of physiological data, leading to novel ideas in copper biology, together with the discussion of current methodological issues.

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.

Intermediate Cu(II)-Thiolate Species in the Reduction of Cu(II)GHK by Glutathione: A Handy Chelate for Biological Cu(II) Reduction

Gly-His-Lys (GHK) is a tripeptide present in the human bloodstream that exhibits a number of biological functions. Its activity is attributed to the copper-complexed form, Cu(II)GHK. Little is known, however, about the molecular aspects of the mechanism of its action. Here, we examined the reaction of Cu(II)GHK with reduced glutathione (GSH), which is the strongest reductant naturally occurring in human plasma. Spectroscopic techniques (UV–vis, CD, EPR, and NMR) and cyclic voltammetry helped unravel the reaction mechanism. The impact of temperature, GSH concentration, oxygen access, and the presence of ternary ligands on the reaction were explored. The transient GSH-Cu(II)GHK complex was found to be an important reaction intermediate. The kinetic and redox properties of this complex, including tuning of the reduction rate by ternary ligands, suggest that it may provide a missing link in copper trafficking as a precursor of Cu(I) ions, for example, for their acquisition by the CTR1 cellular copper transporter.

Gly-His-Lys (GHK) is a human bioactive tripeptide thought to be activated by Cu(II) binding, but little is known about the molecular aspects of its action. UV−vis, circular dichroism (CD), EPR, and NMR spectroscopies, and cyclic voltammetry were used to examine the reduction of Cu(II)GHK with glutathione (GSH), the most abundant biological thiol. A semistable GSH-Cu(II)GHK reaction intermediate was discovered, with properties suitable for delivering Cu(I) to biological transport proteins.

Ternary Cu2+ Complexes of Human Serum Albumin and Glycyl-l-histidyl-l-lysine

Human serum albumin (HSA) and the growth factor glycyl-l-histidyl-l-lysine (GHK) bind Cu2+ as part of their normal functions. GHK is found at its highest concentration in the albumin-rich fraction of plasma, leading to speculation that HSA and GHK form a ternary Cu2+ complex. Although preliminary evidence was presented 40 years ago, the structure and stability of such a complex have remained elusive. Here, we show that two ternary Cu(GHK)NImHSA complexes are formed between GHK and the imino nitrogen (NIm) of His side chains of HSA. We identified His3 as one site of ternary complex formation (conditional binding constant cKCu(GHK)NImHis3Cu(GHK) = 2900 M-1 at pH 7.4), with the second site (cKCu(GHK)NImHisXCu(GHK) = 1700 M-1) likely being supplied by either His128 or His510. Together with the established role of HSA as a molecular shuttle in the blood, these complexes may aid the transport of the exchangeable Cu2+ pool and the functional form of GHK.

Measurement of Interpeptidic CuII Exchange Rate Constants of CuII-Amyloid-β Complexes to Small Peptide Motifs by Tryptophan Fluorescence Quenching

The interpeptidic Cu^II exchange rate constants were measured for two Cu amyloid-β complexes, Cu(Aβ_1-16) and Cu(Aβ_1-28), to fluorescent peptides GHW and DAHW using a quantitative tryptophan fluorescence quenching methodology. The second-order rate constants were determined at three pH values (6.8, 7.4, and 8.7) important to the two Cu(Aβ) coordination complexes, components Cu(Aβ)_I and Cu(Aβ)_II. The interpeptidic Cu^II exchange rate constant is approximately 10⁴ M^-1 s^-1 but varies in magnitude depending on many variables. These include pH, length of the Aβ peptide, location of the anchoring histidine ligand in the fluorescent peptide, number of amide deprotonations required in the tryptophan peptide to coordinate Cu^II, and interconversion between Cu(Aβ)_I and Cu(Aβ)_II. We also present EPR data probing the Cu^II exchange between peptides and the formation of ternary species between Cu(Aβ) and GHW. As the nonfluorescent GHK and DAHK peptides are important motifs found in the blood and serum, their ability to sequester Cu^II ions from Cu(Aβ) complexes may be relevant for the metal homeostasis and its implication in Alzheimer's disease. Thus, their kinetic Cu^II interpeptidic exchange rate constants are important chemical rate constants that can help elucidate the complex Cu^II trafficking puzzle in the synaptic cleft.

Ergothioneine in a peptide: Substitution of histidine with 2-thiohistidine in bioactive peptides

Ergothioneine (EGT) is the betaine of 2-thiohistidine (2-thioHis) and may be the last undiscovered vitamin. EGT cannot be incorporated into a peptide because the α-nitrogen is trimethylated, although this would be advantageous as an EGT-like moiety in a peptide would impart unique antioxidant and metal chelation properties. The amino acid 2-thioHis is an analogue of EGT and can be incorporated into a peptide, although there is only one reported occurrence of this in the literature. A likely reason is the harsh conditions reported for protection of the thione, with similarly harsh conditions used in order to achieve deprotection after synthesis. Here, we report a novel strategy for the incorporation of 2-thioHis into peptides in which we decided to leave the thione unprotected. This decision was based upon the reported low reactivity of EGT with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), a very electrophilic disulfide. This strategy was successful, and we report here the synthesis of 2-thioHis analogues of carnosine (βAH), GHK-tripeptide, and HGPLGPL. Each of these peptides contain a histidine (His) residue and possesses biological activity. Our results show that substitution of His with 2-thioHis imparts strong antioxidant, radical scavenging, and copper binding properties to the peptide. Notably, we found that the 2-thioHis analogue of GHK-tripeptide was able to completely quench the hydroxyl and ABTS radicals in our assays, and its antioxidant capacity was significantly greater than would be expected based on the antioxidant capacity of free 2-thioHis. Our work makes possible greater future use of 2-thioHis in peptides.

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.

Cu(II)-Binding N-Terminal Sequences of Human Proteins

Proteins anchor copper(II) ions mainly by imidazole from histidine residues located in different positions in the primary protein structures. However, the motifs with histidine in the first three N-terminal positions (His¹ , His² , and His³ ) show unique Cu(II)-binding properties, such as availability from the surface of the protein, high flexibility, and high Cu(II) exchangeability with other ligands. It makes such sequences beneficial for the fast exchange of Cu(II) between ligands. Furthermore, sequences with His¹ and His² , thus, non-saturating the Cu(II) coordination sphere, are redox-active and may play a role in Cu(II) reduction to Cu(I). All human protein sequences deposited in UniProt Knowledgebase were browsed for those containing His¹ , His² , or His³ . Proteolytically modified sequences (with the removal of a propeptide or Met residue) were taken for the analysis. Finally, the sequences were sorted out according to the subcellular localization of the proteins to match the respective sequences with the probability of interaction with divalent copper.

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.