Thermodynamic and structural characterization of the copper(II) complexes of peptides containing both histidyl and aspartyl residues

Metal coordination governs the antimicrobial efficacy of calcitermin derivatives

Antimicrobial peptides are promising alternatives to classical antibiotics. Their microbicidal activity can arise from different mechanisms, one of which is known as nutritional immunity and has metal micronutrients and metal-binding biomolecules as its main players. Calcitermin is an antimicrobial peptide and an effective metal chelator. Its properties as an antibacterial and anti-Candida agent have been recently studied both as a free peptide and in the presence of zinc and copper ions, with which it forms stable complexes. Calcitermin derivatives have also gained attention thanks to the possibility of improving their properties, like metal-binding affinity and/or stability in biological fluids, through ad hoc modifications of the native peptide sequence. In this work, the Ala-to-Ser substitutions close to the coordination site of calcitermin have been introduced to study the impact on the biological activity and metal-binding properties. Our results show that metal coordination has a clear impact on the bioactivity of the studied compounds, to the point that the truncated fragment of calcitermin, solely containing the main metal-binding residues, also shows antimicrobial activity.

Detailed investigation of the binding abilities of the heterodomain of a multiHis cyclopeptide toward Cu(II) ions

Cyclopeptides hold significant relevance in various fields of science and medicine, due to their unique structural properties and diverse biological activities. Cyclic peptides, characterized by intrinsically higher conformational order, exhibit remarkable stability and resistance to proteolytic degradation, making them attractive candidates for developing targeted drug delivery systems. The aim of this work is to elucidate the unique coordination properties of the multi-His cyclic peptide with c(HDHKHPHHKHHP) sequence (HDCP - heterodomain cyclopeptide). This peptide, indeed, is able to form homo- and hetero-dinuclear complexes in a wide pH range, being thus a good chelator for Cu(II) ions. Herein, we present the results of a combined study, involving potentiometric, spectroscopic (UV-Vis, CD, and EPR), and computational investigations, on its coordination properties. To better understand the interaction pattern with Cu(II) metal ions, two other peptides, each one bearing only one of the two binding domains of HDCP are also considered in this study: c(HDHKHPGGKGGP) = CP1, c(GKGGKPHHKHHP) = CP2, which share sequence fragments of HDCP and allow separate investigations of its coordination domains.

Peptides Derived from Angiogenin Regulate Cellular Copper Uptake

The angiogenin protein (ANG) is one of the most potent endogenous angiogenic factors. In this work we characterized by means of potentiometric, spectroscopic and voltammetric techniques, the copper complex species formed with peptide fragments derived from the N-terminal domain of the protein, encompassing the sequence 1-17 and having free amino, Ang1-17, or acetylated N-terminus group, AcAng1-17, so to explore the role of amino group in metal binding and cellular copper uptake. The obtained data show that amino group is the main copper anchoring site for Ang1-17. The affinity constant values, metal coordination geometry and complexes redox-potentials strongly depend, for both peptides, on the number of copper equivalents added. Confocal laser scanning microscope analysis on neuroblastoma cells showed that in the presence of one equivalent of copper ion, the free amino Ang1-17 increases cellular copper uptake while the acetylated AcAng1-17 strongly decreases the intracellular metal level. The activity of peptides was also compared to that of the protein normally present in the plasma (wtANG) as well as to the recombinant form (rANG) most commonly used in literature experiments. The two protein isoforms bind copper ions but with a different coordination environment. Confocal laser scanning microscope data showed that the wtANG induces a strong increase in intracellular copper compared to control while the rANG decreases the copper signal inside cells. These data demonstrate the relevance of copper complexes' geometry to modulate peptides' activity and show that wtANG, normally present in the plasma, can affect cellular copper uptake.

The Unusual Role of Pro in Cu(II) Binding by His2-Cyclopentapeptide

In this paper, we present findings from studying the interaction of copper(II) ions with the His₂-cyclopentapeptide and the role of proline used for the purpose of potentiometric titration and UV-Vis, CD and EPR spectroscopic measurements. Experiments of two homodetic peptides differing by one amino acid residue were conducted for a ligand to metal ratio of 1:1 in the pH range 2.5–11.0. The presented studies reveal that peptides form only mononuclear complexes, and the CuH₂L complex appears in the system first (for both L1 and L2). Study results show that the presence of Pro influences the structure of formed complexes and their stabilities and has a strong impact on the efficiency of copper(II) coordination.

The Role of the Disulfide Bridge in the Copper (II) Binding by the Cyclic His4-Peptide

In this paper, we present studies on the influence of the disulfide bridge on the copper (II) ions’ binding abilities by the cyclic His₄-peptide. The studied ligand HKHPHRHC_-S-S-C consists of nine amino acids. The cyclic structure was obtained through a disulfide bridge between two cysteinyl groups. Moreover, this peptide is characterized by the presence of four His residues in the sequence, which makes it an interesting ligand for transition metal ions. The potentiometric and spectroscopic (UV-Vis spectroscopy and circular dichroism spectroscopy (CD)) studies were carried out in various molar ligand to metal ratios: 2:1, 1:1, and 1:2, in the pH range of 2.5–11 at 25 °C. The results showed that the cyclic His₄-peptide promotes dinuclear complexes in each of these systems and forms the final dinuclear species with the {N_Im, 3N^-_amide}{N_Im, 3N^-_amide} coordination mode. The obtained data shows that cyclization by the formation of the disulfide bond has an impact on the peptide chain flexibility and appearance of additional potential donors for metal ions and influences the copper (II) ions’ coordination.

Nerve Growth Factor Peptides Bind Copper(II) with High Affinity: A Thermodynamic Approach to Unveil Overlooked Neurotrophin Roles

Nerve growth factor (NGF) is a protein essential to neurons survival, which interacts with its receptor as a non-covalent dimer. Peptides belonging to NGF N-terminal domain are able to mimic the activity of the whole protein. Such activity is affected by the presence of copper ions. The metal is released in the synaptic cleft where proteins, not yet identified, may bind and transfer to human copper transporter 1 (hCtr1), for copper uptake in neurons. The measurements of the stability constants of copper complexes formed by amyloid beta and hCtr1 peptide fragments suggest that beta-amyloid (Aβ) can perform this task. In this work, the stability constant values of copper complex species formed with the dimeric form of N-terminal domain, sequence 1–15 of the protein, were determined by means of potentiometric measurements. At physiological pH, NGF peptides bind one equivalent of copper ion with higher affinity of Aβ and lower than hCtr1 peptide fragments. Therefore, in the synaptic cleft, NGF may act as a potential copper chelating molecule, ionophore or chaperone for hCtr1 for metal uptake. Copper dyshomeostasis and mild acidic environment may modify the balance between metal, NGF, and Aβ, with consequences on the metal cellular uptake and therefore be among causes of the Alzheimer’s disease onset.

Identification of a copper ion recognition peptide sequence in the subunit II of cytochrome c oxidase: a combined theoretical and experimental study

The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion binding loop of the subunit II of cytochrome c oxidase on the maturation of the binuclear purple CuA center of the enzyme has been investigated using several experimental and computational methods. The copper ion was found to form 1:1 complex of the pentapeptide with a binding constant ~ 10⁴ M^-1 to 10⁵ M^-1, where a 4 ligand coordination from the peptide in a type 2 copper center was revealed. The pH dependence of the metal-peptide was associated with a [Formula: see text] of ~ 10 suggesting deprotonation of the N-terminal amine. EXAFS studies as well as DFT calculations of the metal-peptide complexes revealed pH dependent changes in the metal-ligand bond distances. Spectroscopic properties of the metal peptides calculated from TDDFT studies agreed with the experimental results. Restrained molecular dynamics (RMD) simulations indicated coordination of a carbonyl oxygen from the asparagine (N) side chain and of water molecules apart from histidine (H), methionine (M) and terminal amine of asparagine (N) in a distorted square planar geometry of Cu-NHSFM. Analyses of the backbone distances as well as B-factors for the metal peptide suggested that the peptide backbone becomes more compact and rigid on binding of the metal ion. This indicated that binding of copper ion to this pentapeptide in the protein possibly cause movement of the protein backbone bringing other coordinating residues closer to the copper ion, and thus helping in sequential uptake of copper ions to the protein.

Cu(II) complexes with peptides from FomA protein containing -His-Xaa-Yaa-Zaa-His and -His-His-motifs. ROS generation and DNA degradation

Mono- and dinuclear Cu(II) complexes with Ac-PTVHNEYH-NH₂ (L1) and Ac-NHHTLND-NH₂ (L2) peptides from FomA protein of Fusobacterium nucleatum were studied by potentiometry, spectroscopic methods (UV-Vis, CD, EPR) and MS technique. The dominant mononuclear complexes for L1 ligand are: CuHL (pH range 5.0-6.0) with 2N {2N_im}, CuH_-2L (pH range 8.0-8.5) and CuH_-3L species (above pH 9.0) with 4N {N_im, 3N^-} coordination modes. The complexes: CuH_-1L with 3N {2N_im, N^-}, CuH_-2L with 3N {N_im, 2N^-} and CuH_-3L with 4N {N_im, 3N^-} binding sites are proposed for the L2 ligand. Probably in the CuH_-2L complex for CuL2 system the second His residue in His-His sequence is bound to Cu(II) ion, while the first His residue may stabilize this complex by His-His and/or His-Cu(II) interactions. The dominant dinuclear Cu₂L1 complexes in the pH range 6.5-10.5 are: the Cu₂H_-4L and Cu₂H_-6L species with 3N{N_im, 2N^-}4N{N_im, 3N^-} and 4N{N_im, 3N^-}4N{N_im, 3N^-} binding sites, respectively. In the case of the Cu₂L2 complex in the pH range 7.2-10.5, the Cu₂H_-4L and Cu₂H_-7L species dominate with 2N{N_im, N^-}4N{N_im, 3N^-} and (Cu(OH)₄^2-4N{N_im, 3N^-}) coordination modes, respectively. The ability to generate reactive oxygen species (ROS) by uncomplexed Cu(II) ions, ligands and their complexes at pH 7.4 in the presence of hydrogen peroxide or ascorbic acid was studied. UV-Vis, luminescence, EPR spin trapping and gel electrophoresis methods were used. Both complexes produce higher level of ROS compared to those of their ligands. ROS produced by Cu(II) complexes are hydroxyl radical and singlet oxygen, which contribute to oxidative DNA cleavage.

Copper(II) Coordination Abilities of the Tau Protein's N-Terminus Peptide Fragments: A Combined Potentiometric, Spectroscopic and Mass Spectrometric Study

Copper(II) complexes of the N-terminal peptide fragments of tau protein have been studied by potentiometric and various spectroscopic techniques (UV-vis, CD, ESR and ESI-MS). The octapeptide Tau(9-16) (Ac-EVMEDHAG-NH₂ ) contains the H14 residue of the native protein, while Tau(26-33) (Ac-QGGYTMHQ-NH₂ ) and its mutants Tau(Q26K-Q33K) (Ac-KGGYTMHK-NH₂ ) and Tau(Q26K-Y29A-Q33K) (Ac-KGGATMHK-NH₂ ) include the H32 residue. To compare the binding ability of H14 and H32 in a single molecule the decapeptide Ac-EDHAGTMHQD-NH₂ (Tau(12-16)(30-34)) has also been synthesized and studied. The histidyl residue is the primary metal binding site for metal ions in all the peptide models studied. In the case of Tau(9-16) the side chain carboxylate functions enhance the stability of the M-N_im coordinated complexes compared to Tau(26-33) (logK(Cu-N_im )=5.04 and 3.78, respectively). Deprotonation and metal ion coordination of amide groups occur around the physiological pH range for copper(II). The formation of the imidazole- and amide-coordinated species changes the metal ion preference and the complexes formed with the peptides containing the H32 residue predominate over those of H14 at physiological pH values (90 %-10 %) and in alkaline samples (96 %-4 %).

Histidine-based copper tetrapeptides as enantioselective catalysts for aldol reactions

Copper(ii)-peptides are widely used as industrial catalysts such as in the aerobic oxidation of organic molecules, formation of new C–H bonds and in the azide–alkyne cycloaddition reaction. The length of peptides and the effect of adding copper metal into peptides were questioned in their field of applications. Five novel histidine-based tetrapeptides with the sequences HAAD (P1), HAFD (P2), HAVD (P3), AGHD (P4) and PGHD (P5) were synthesized using the solid phase peptide scheme and analysed with high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) with percentage purities as high as 99.5%. All the peptides were positively charged (+1) and the molecular weight calculated from m/z values of MS results coincided with the theoretical molecular weight of the peptides. Copper(ii)-peptides derived from these peptides and copper(ii) acetate monohydrate (CuP1–CuP5) in a 1 : 2 ratio was synthesised, purified and characterised by ultraviolet-visible spectroscopy (UV-Vis), ultraviolet-fluorescence spectroscopy (fluorescence) and fourier transform infrared spectroscopy (FTIR), circular dichroism spectroscopy (CD) and optical rotation polarimetry. It provided the necessary information on the secondary structure and the successful binding of copper(ii) to the specific amino acids, hence leading to the putative geometry of copper(ii)-peptides and the difference in the chirality of amino acids, peptides and copper(ii)-peptides. The catalytic activities of the synthesised complexes were evaluated. CuP1 & CuP3 catalysed both the asymmetric aldol reactions with high enantioselectivity of p-nitrobenzaldehyde with cyclohexanone (% ee = 87.3 & 80.3, respectively) and of p-anisaldehyde with cyclohexanone (% ee = 95.5 & 90.9, respectively).

P5 with the sequence H₂N-PGHD-CONH.

Copper(II) complexes with peptide fragments encompassing the sequence 122-130 of human doppel protein

Copper(II) complexes of the peptide fragment (Dpl122-130) encompassing the sequence 122-130 of human doppel protein were characterized by potentiometric, UV-Visible, CD and EPR spectroscopic methods. An analogous peptide, in which the aspartate residue was substituted by an asparagine amino acid, was synthesized in order to provide evidence on the possible role of carboxylate group in copper(II) coordination. It was found that the carboxylic group is directly involved in copper(II) coordination at acidic pH, forming the CuLH(2) species with Dpl122-130. This copper(II) complex displayed EPR parameters very similar to those of the analogous complex with the whole doppel protein. At pH higher than 7, the complexes showed magnetic parameters similar to those of the major species of protein formed in the pH range 7-8, with the metal coordination environment consisting of one imidazole and three amide nitrogen atoms. The comparison of Cu-Dpl122-130 binding constant values with those of the prion peptide fragments (PrP106-114), showed that doppel peptide had a higher metal binding affinity at acidic pH whereas the prion peptide fragment binds the metal tightly at physiological pH.