Interactions of α-Factor-1, a Yeast Pheromone, and Its Analogue with Copper(II) Ions and Low-Molecular-Weight Ligands Yield Very Stable Complexes

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.

Impact of the Histidine-Triazole and Tryptophan-Pyrene Exchange in the WHW Peptide: Cu(II) Binding, DNA/RNA Interactions and Bioactivity

In three novel peptidoids based on the tryptophan—histidine—tryptophan (WHW) peptide, the central histidine was replaced by Ala-(triazole), and two derivatives also had one tryptophan replaced with pyrene-alkyls of different lengths and flexibility. Pyrene analogues show strong fluorescence at 480–500 nm, attributed to intramolecular exciplex formation with tryptophan. All three peptidoids bind Cu2+ cation in water with strong affinity, with Trp- Ala-(triazole)-Trp binding comparably to the parent WHW, and the pyrene analogues even stronger, demonstrating that replacement of histidine with triazole in peptides does not hamper Cu2+ coordination. The studied peptidoids strongly bind to ds-DNA and ds-RNA, whereby their complexes with Cu2+ exhibit distinctively different interactions in comparison to metal-free analogues, particularly in the stabilization of ds-DNA against thermal denaturation. The pyrene peptidoids efficiently enter living cells with no apparent cytotoxic effect, whereby their red-shifted emission compared to the parent pyrene allows intracellular confocal microscopy imaging, showing accumulation in cytoplasmic organelles. However, irradiation with 350 nm light resulted in evident antiproliferative effect on cells treated with micromolar concentrations of the pyrene analogues, presumably attributed to pyrene-induced production of singlet oxygen and consecutive cellular damage.

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.

Electrospray-Induced Mass Spectrometry Is Not Suitable for Determination of Peptidic Cu(II) Complexes

The toolset of mass spectrometry (MS) is still expanding, and the number of metal ion complexes researched this way is growing. The Cu(II) ion forms particularly strong peptide complexes of biological interest which are frequent objects of MS studies, but quantitative aspects of some reported results are at odds with those of experiments performed in solution. Cu(II) complexes are usually characterized by fast ligand exchange rates, despite their high affinity, and we speculated that such kinetic lability could be responsible for the observed discrepancies. In order to resolve this issue, we selected peptides belonging to the ATCUN family characterized with high and thoroughly determined Cu(II) binding constants and re-estimated them using two ESI-MS techniques: standard conditions in combination with serial dilution experiments and very mild conditions for competition experiments. The sample acidification, which accompanies the electrospray formation, was simulated with the pH-jump stopped-flow technique. Our results indicate that ESI-MS should not be used for quantitative studies of Cu(II)-peptide complexes because the electrospray formation process compromises the entropic contribution to the complex stability, yielding underestimations of complex stability constants.

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.

Dielectric Spectroscopy Based Detection of Specific and Nonspecific Cellular Mechanisms

Using radiofrequency dielectric spectroscopy, we have investigated the impact of the interaction between a G protein-coupled receptor (GPCR), the sterile2 α-factor receptor protein (Ste2), and its cognate agonist ligand, the α-factor pheromone, on the dielectric properties of the plasma membrane in living yeast cells (Saccharomyces cerevisiae). The dielectric properties of a cell suspension containing a saturating concentration of α-factor were measured over the frequency range 40Hz–110 MHz and compared to the behavior of a similarly prepared suspension of cells in the absence of α-factor. A spherical three-shell model was used to determine the electrical phase parameters for the yeast cells in both types of suspensions. The relative permittivity of the plasma membrane showed a significant increase after exposure to α-factor (by 0.06 ± 0.05). The equivalent experiment performed on yeast cells lacking the ability to express Ste2 showed no change in plasma membrane permittivity. Interestingly, a large change also occurred to the electrical properties of the cellular interior after the addition of α-factor to the cell suspending medium, whether or not the cells were expressing Ste2. We present a number of different complementary experiments performed on the yeast to support these dielectric data and interpret the results in terms of specific cellular reactions to the presence of α-factor.

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.

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.

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

The tripeptide NH₂–Gly–His–Lys–COOH (GHK), cis-urocanic acid (cis-UCA) and Cu(II) ions are physiological constituents of the human body and they co-occur (e.g., in the skin and the plasma). While GHK is known as Cu(II)-binding molecule, we found that urocanic acid also coordinates Cu(II) ions. Furthermore, both ligands create ternary Cu(II) complex being probably physiologically functional species. Regarding the natural concentrations of the studied molecules in some human tissues, together with the affinities reported here, we conclude that the ternary complex [GHK][Cu(II)][cis-urocanic acid] may be partly responsible for biological effects of GHK and urocanic acid described in the literature.

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.

N-Terminal Cu-Binding Motifs (Xxx-Zzz-His, Xxx-His) and Their Derivatives: Chemistry, Biology and Medicinal Applications

Peptides and proteins with N-terminal amino acid sequences NH2 -Xxx-His (XH) and NH2 -Xxx-Zzz-His (XZH) form well-established high-affinity CuII -complexes. Key examples are Asp-Ala-His (in serum albumin) and Gly-His-Lys, the wound healing factor. This opens a straightforward way to add a high-affinity CuII -binding site to almost any peptide or protein, by chemical or recombinant approaches. Thus, these motifs, NH2 -Xxx-Zzz-His in particular, have been used to equip peptides and proteins with a multitude of functions based on the redox activity of Cu, including nuclease, protease, glycosidase, or oxygen activation properties, useful in anticancer or antimicrobial drugs. More recent research suggests novel biological functions, mainly based on the redox inertness of CuII in XZH, like PET imaging (with 64 Cu), chelation therapies (for instance in Alzheimer's disease and other types of neurodegeneration), antioxidant units, Cu transporters and activation of biological functions by strong CuII binding. This Review gives an overview of the chemical properties of Cu-XH and -XZH motifs and discusses the pros and cons of the vastly different biological applications, and how they could be improved depending on the application.