Mixed metal copper(II)-nickel(II) and copper(II)-zinc(II) complexes of multihistidine peptide fragments of human prion protein

Modulating the aggregation of human prion protein PrP106-126 by an indole-based cyclometallated palladium complex

The spontaneous aggregation of infectious or misfolded forms of prion protein is known to be responsible for neurotoxicity in brain cells, which ultimately leads to the progression of prion disorders. Bovine spongiform encephalopathy (BSE) in animals and Creutzfeldt-Jakob disease (CJD) in humans are glaring examples in this regard. Square-planar complexes with labile ligands and indole-based compounds are found to be efficiently inhibitory against protein aggregation. Herein, we report the synthesis of an indole-based cyclometallated palladium complex. The ligand and complex were characterized by various spectroscopic techniques such as UV-visible, NMR, IR, and HRMS. The molecular structure of the complex was confirmed by single-crystal X-ray crystallography. The interaction of the complex with PrP106-126 was studied using UV-visible spectroscopy, CD spectroscopy, MALDI-TOF MS, and molecular docking. The inhibition effects of the complex on the PrP106-126 aggregation, fibrillization and amyloid formation phenomena were analysed through the ThT assay, CD, TEM and AFM. The effect of the complex on the aggregation process of PrP106-126 was determined kinetically through the ThT assay. The complex presented high binding affinity with the peptide and influenced the peptide's conformation and aggregation in different modes of binding. Furthermore, the MTT assay on neuronal HT-22 cells showed considerable protective properties of the complex against PrP106-126-mediated cytotoxicity. These findings suggest that the compound influences peptide aggregation in different ways, and the anti-aggregation action is primarily associated with the metal's physicochemical properties and the reactivity rather than the ligand. As a result, we propose that this compound be investigated as a potential therapeutic molecule in metallopharmaceutical research to treat prion disease (PD).

Zn(II) binding causes interdomain changes in the structure and flexibility of the human prion protein

The cellular prion protein (PrPC) is a mainly α-helical 208-residue protein located in the pre- and postsynaptic membranes. For unknown reasons, PrPC can undergo a structural transition into a toxic, β-sheet rich scrapie isoform (PrPSc) that is responsible for transmissible spongiform encephalopathies (TSEs). Metal ions seem to play an important role in the structural conversion. PrPC binds Zn(II) ions and may be involved in metal ion transport and zinc homeostasis. Here, we use multiple biophysical techniques including optical and NMR spectroscopy, molecular dynamics simulations, and small angle X-ray scattering to characterize interactions between human PrPC and Zn(II) ions. Binding of a single Zn(II) ion to the PrPC N-terminal domain via four His residues from the octarepeat region induces a structural transition in the C-terminal α-helices 2 and 3, promotes interaction between the N-terminal and C-terminal domains, reduces the folded protein size, and modifies the internal structural dynamics. As our results suggest that PrPC can bind Zn(II) under physiological conditions, these effects could be important for the physiological function of PrPC.

Binding Selectivity of Methanobactin from Methylosinus trichosporium OB3b for Copper(I), Silver(I), Zinc(II), Nickel(II), Cobalt(II), Manganese(II), Lead(II), and Iron(II)

Methanobactin (Mb) from Methylosinus trichosporium OB3b is a member of a class of metal binding peptides identified in methanotrophic bacteria. Mb will selectively bind and reduce Cu(II) to Cu(I), and is thought to mediate the acquisition of the copper cofactor for the enzyme methane monooxygenase. These copper chelating properties of Mb make it potentially useful as a chelating agent for treatment of diseases where copper plays a role including Wilson's disease, cancers, and neurodegenerative diseases. Utilizing traveling wave ion mobility-mass spectrometry (TWIMS), the competition for the Mb copper binding site from Ag(I), Pb(II), Co(II), Fe(II), Mn(II), Ni(II), and Zn(II) has been determined by a series of metal ion titrations, pH titrations, and metal ion displacement titrations. The TWIMS analyses allowed for the explicit identification and quantification of all the individual Mb species present during the titrations and measured their collision cross-sections and collision-induced dissociation patterns. The results showed Ag(I) and Ni(II) could irreversibly bind to Mb and not be effectively displaced by Cu(I), whereas Ag(I) could also partially displace Cu(I) from the Mb complex. At pH ≈ 6.5, the Mb binding selectivity follows the order Ag(I)≈Cu(I)>Ni(II)≈Zn(II)>Co(II)>>Mn(II)≈Pb(II)>Fe(II), and at pH 7.5 to 10.4 the order is Ag(I)>Cu(I)>Ni(II)>Co(II)>Zn(II)>Mn(II)≈Pb(II)>Fe(II). Breakdown curves of the disulfide reduced Cu(I) and Ag(I) complexes showed a correlation existed between their relative stability and their compact folded structure indicated by their CCS. Fluorescence spectroscopy, which allowed the determination of the binding constant, compared well with the TWIMS analyses, with the exception of the Ni(II) complex. Graphical abstract ᅟ.

A comparative antimicrobial and toxicological study of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles: synergistic activity and improved selectivity index of Au(iii)/Ag(i) complexes mixture

Combination therapy may be applied in the case of gold(iii) and silver(i) complexes with aromatic nitrogen-containing heterocycles to improve their antimicrobial activity and reduce toxic-side effects.

Zinc(ii) and cadmium(ii) complexes of N-terminally free peptides containing two separate cysteinyl binding sites

The novel synthesized cysteine peptides form stable zinc(ii) and cadmium(ii) complexes; the specific sequence makes possible metal induced amide deprotonation.

Combined EXAFS and DFT structure calculations provide structural insights into the 1:1 multi-histidine complexes of Cu(II) , Cu(I) , and Zn(II) with the tandem octarepeats of the mammalian prion protein

The metal-coordinating properties of the prion protein (PrP) have been the subject of intense focus and debate since the first reports of its interaction with copper just before the turn of the century. The picture of metal coordination to PrP has been improved and refined over the past decade, but structural details of the various metal coordination modes have not been fully elucidated in some cases. In the present study, we have employed X-ray absorption near-edge spectroscopy as well as extended X-ray absorption fine structure (EXAFS) spectroscopy to structurally characterize the dominant 1:1 coordination modes for Cu(II) , Cu(I) , and Zn(II) with an N-terminal fragment of PrP. The PrP fragment corresponds to four tandem repeats representative of the mammalian octarepeat domain, designated as OR4 , which is also the most studied PrP fragment for metal interactions, making our findings applicable to a large body of previous work. Density functional theory (DFT) calculations have provided additional structural and thermodynamic data, and candidate structures have been used to inform EXAFS data analysis. The optimized geometries from DFT calculations have been used to identify potential coordination complexes for multi-histidine coordination of Cu(II) , Cu(I) , and Zn(II) in an aqueous medium, modelled using 4-methylimidazole to represent the histidine side chain. Through a combination of in silico coordination chemistry as well as rigorous EXAFS curve-fitting, using full multiple scattering on candidate structures derived from DFT calculations, we have characterized the predominant coordination modes for the 1:1 complexes of Cu(II) , Cu(I) , and Zn(II) with the OR4 peptide at pH 7.4 at atomic resolution, which are best represented as square-planar [Cu(II) (His)4 ](2+) , digonal [Cu(I) (His)2 ](+) , and tetrahedral [Zn(II) (His)3 (OH2 )](2+) , respectively.

The specificity of interaction of Zn(2+), Ni(2+) and Cu(2+) ions with the histidine-rich domain of the TjZNT1 ZIP family transporter

The Zrt/Irt-like protein (ZIP) family contributes to the metal homeostasis by regulating the transport of divalent metal cations such as Fe(2+), Zn(2+), Mn(2+), Cd(2+) and sometimes even Cu(2+). Most ZIP members have a long variable loop between transmembrane domains (TMDs) III and IV; this region is predicted to be located in the cytoplasm and is postulated to be the metal ion binding site. In this study, we looked at the thermodynamic behavior and coordination chemistry of Zn(2+), Ni(2+) and Cu(2+) complexes with the histidine-rich domain, Ac-(185)RAHAAHHRHSH(195)-NH2 (HRD), from the yeast TjZNT1 protein, located between TMDs III and IV. The sequence is conserved also in higher species like Thlaspi japonicum. The stability of complexes increases in the series Ni(2+) < Zn(2+)≪ Cu(2+). The geometry of complexes is very different for each metal and in the case of Zn(2+) complexes, high specificity in binding is observed. Moreover, the stability of HRD-Cu(2+) complexes was compared with the five His residues containing peptide from Hpn protein (Helicobacter pylori). The results suggest a high ability of HRD in the binding of all three studied metals.

The copper(II) and zinc(II) coordination mode of HExxH and HxxEH motif in small peptides: the role of carboxylate location and hydrogen bonding network

Copper(II) and zinc(II) complexes with two hexapeptides encompassing HExxH and HxxEH motif were characterized by means of a combined experimental and theoretical approach. Parallel tempering and density functional theory (DFT) investigations show the presence of different hydrogen bonding networks between the copper(II) and zinc(II) complexes with the two peptides, suggesting a significant contribution of these non-covalent interactions to the stability constant values. The glutamate carboxylate group has a direct role in metal ion binding. The location of this amino acid along the sequence of the investigated peptides is critical to determine thermodynamic and spectroscopic features of the copper(II) complex species, whereas is less relevant in the zinc(II) complexes formation. Electrospray ionization mass spectrometry (ESI-MS) characterization of the zinc(II) complex species show that in the [ZnH-2L] two deprotonated amide nitrogen atoms are involved in the metal coordination environment, an uncommon behavior in zinc(II) complexes for multi-histidine ligands.

Zn impacts Cu coordination to amyloid-β, the Alzheimer's peptide, but not the ROS production and the associated cell toxicity

Combined coordination of Zn(II) and Cu(I) or Cu(II) to the amyloid-β peptide has been investigated using XANES, EPR and NMR spectroscopies. While Zn(II) does alter Cu(II) binding to Aβ, this has no effect on (Aβ)Cu induced ROS production and associated cell toxicity.