Spectroscopic and Electronic Structure Studies of Copper(II) Binding to His111 in the Human Prion Protein Fragment 106−115: Evaluating the Role of Protons and Methionine Residues

The effect of histidine, histamine, and imidazole on electrochemical properties of Cu(II) complexes of Aβ peptides containing His-2 and His-3 motifs

The N-truncation of amyloid beta (Aβ) peptides could lead to peptide sequences with the histidine residue at the second and third positions, creating His-2 and His-3 motifs, known as high-affinity Cu(II) binding sites. In such complexes, the Cu(II) ion is arrested in a rigid structure of a square-planar arrangement of nitrogen donors, which highly limits its susceptibility to Cu(II) reduction. Cu(II) reduction fuels the Cu(II)/Cu(I) redox cycle, which is engaged in the production of reactive oxygen species (ROS). Employing electrochemical techniques, cyclic voltammetry (CV) and differential pulse voltammetry (DPV), together with UV-vis spectroscopy, we showed that low-molecular-weight (LMW) substances, such as imidazole, histamine, and histidine, could enhance the redox activity of Cu(II) complexes of three models of N-truncated Aβ peptides, Aβ4-9, Aβ5-9, and Aβ12-16, identifying three main mechanisms. LMW compounds could effectively compete with Aβ peptides for Cu(II) ions, forming Cu(II)/LMW species, which are more prone to Cu(II) reduction. LMW substances could also shift the equilibrium between the Cu(II)/Aβ species towards the species with higher susceptibility to Cu(II) reduction. Finally, the presence of LMW molecules could promote Cu(I) reoxidation in ternary Cu(II)/Aβ/LMW systems. The obtained results raise further questions regarding the Cu(II) redox activity in Alzheimer's disease.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Full Equilibrium Picture in Aqueous Binary and Ternary Systems Involving Copper(II), 1-Methylimidazole-Containing Hydrazonic Ligands, and the 103-112 Human Prion Protein Fragment

In this work, we describe two novel 1-methylimidazole N-acylhydyrazonic ligands and their interaction with copper(II) in solution. Binary systems constituted by each of these hydrazones and the metal ion were studied by potentiometric titrations. The magnitude of their affinities for zinc(II) was also determined for the sake of comparison. Additionally, a full evaluation of the copper(II) chelation profile of the new ligands in ternary systems containing a human prion protein fragment was performed. Mixed ligand complexes comprising the HuPrP103-112 fragment, copper(II) ions, and an N-acylhydrazone were characterized by potentiometry, ultraviolet-visible spectroscopy, and circular dichroism. Some of these species were also identified by electrospray ionization mass spectrometry and unequivocally assigned through their isotopic distribution pattern. To the best of our knowledge, this is the first report concerning the stability of ternary complexes involving a hydrazonic metal-protein interaction modulator, copper, and a peptide. The ability of N-acylhydrazones to prevent peptide oxidation was also examined. Both ligands can partially prevent the formation of the doubly oxidized product, a process mediated by copper(II) ions. Oxidative stress is considered an important hallmark of neurodegenerative diseases such as prion-related spongiform encephalopathies. In this context, active intervention with respect to the deleterious copper-catalyzed methionine oxidation could represent an interesting therapeutic approach.

EPR of copper centers in the prion protein

Most proteins implicated in neurodegenerative diseases bind metal ions, notably copper and zinc. Metal ion binding may be part of the protein's function or, alternatively, may promote a deleterious gain of function. With regard to Cu²⁺ ions, electron paramagnetic resonance techniques have proven to be instrumental in determining the biophysical characteristics of the copper binding sites, as well as structural features of the coordinating protein and how they are impacted by metal binding. Here, the most useful methods are described as they apply to the prion protein, which serves as a model for the broader spectrum of neurodegenerative proteins.

β-cleavage of the human prion protein impacts Cu(II) coordination at its non-octarepeat region

The cellular prion protein (PrP^C) is a membrane-anchored copper binding protein that undergoes proteolytic processing. β-cleavage of PrP^C is associated with a pathogenic condition and it yields two fragments: N2 with residues 23-89, and C2 including residues 90-231. The membrane-bound C2 fragment retains the Cu binding sites at His96 and His111, but it also has a free N-terminal NH₂ group. In this study, the impact of β-cleavage of PrP^C in its Cu(II) binding properties was evaluated, using the peptide of the human prion protein hPrP(90-115) as a model for the C2 fragment. The Cu(II) coordination properties of hPrP(90-115) were studied using circular dichroism (CD) and electron paramagnetic resonance (EPR); while the H96A and H111A substitutions and its acetylated variants were also studied. Cu binding to hPrP(90-115) is dependent on metal ion concentration: At low copper concentrations the participation of His96 and free NH₂-terminus is evident, while at high copper concentrations the His111 site is populated without participation of the N-terminal NH₂ group. The presence of a free NH₂-terminal group in the C2 fragment significantly impacts the Cu(II) coordination properties of the His96 site, where the NH₂ group also anchors the metal ion. This study provides further insights into the impact of proteolytic processing of PrP^C in the Cu binding properties of this important neuronal protein.

Intermediate Detection in the Casiopeina-Cysteine Interaction Ending in the Disulfide Bond Formation and Copper Reduction

A strategy to improve the cancer therapies involves agents that cause the depletion of the endogenous antioxidant glutathione (GSH), increasing its efflux out of cells and inducing apoptosis in tumoral cells due to the presence of reactive oxygen species. It has been shown that Casiopeina copper complexes caused a dramatic intracellular GSH drop, forming disulfide bonds and reducing Cu^II to Cu^I. Herein, through the determination of the [Cu^II]-SH bond before reduction, we present evidence of the adduct between cysteine and one Casiopeina as an intermediate in the cystine formation and as a model to understand the anticancer activity of copper complexes. Evidence of such an intermediate has never been presented before.

Amyloid β Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms

Amyloid β (Aβ) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that Aβ disrupts the Cu-dependent interaction between cellular prion protein (PrP^C) and N-methyl-d-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that Aβ competes for Cu with PrP^C; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrP^C is cleaved by α-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the α-cleaved His111. In this study, the competition for Cu(II) between Aβ(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of Aβ(1-16) on Cu(II) coordination to PrP is highly site-specific: Aβ(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-Aβ(1-16) complex. In contrast, Aβ(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the α-cleaved His111 site, Aβ(1-16) yields at least two different ternary complexes depending on the ratio of PrP/Cu(II)/Aβ. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of Aβ, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of Aβ on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which Aβ might affect Cu-dependent functions of PrP^C, such as NMDAR modulation.

Combination of Copper Ions and Nucleotide Generates Aggregates from Prion Protein Fragments in the N-Terminal Domain

BACKGROUND

It has been previously found that PrP_23-98, which contains four highly conserved octarepeats (residues 60-91) and one partial repeat (residues 92-96), polymerizes into amyloid-like and proteinase K-resistant spherical aggregates in the presence of NADPH plus copper ions.

OBJECTIVE

We aimed to determine the requirements for the formation of these aggregates.

METHODS

In this study, we performed an aggregation experiment using N-acetylated and Camidated PrP fragments of the N-terminal domain, Octa1, Octa2, Octa3, Octa4, PrP_84-114, and PrP_76-114, in the presence of NADPH with copper ions, and focused on the effect of the number of copper-binding sites on aggregation.

RESULTS

Among these PrP fragments, Octa4, containing four copper-binding sites, was particularly effective in forming aggregates. We also tested the effect of other pyridine nucleotides and adenine nucleotides on the aggregation of Octa4. ATP was equally effective, but NADH, NADP, ADP, and AMP had no effect.

CONCLUSION

The phosphate group on the adenine-linked ribose moiety of adenine nucleotides and pyridine nucleotides is presumed to be essential for the observed effect on aggregation. Efficient aggregation requires the presence of the four octarepeats. These insights may be helpful in the eventual development of therapeutic agents against prion-related disorders.

Quantitative prediction of electronic absorption spectra of copper(II)-bioligand systems: Validation and applications

The visible region of the electronic absorption spectra of Cu(II) complexes was studied by time-dependent density functional theory (TD-DFT). The performance of twelve functionals in the prediction of absorption maxima (λ_max) was tested on eleven compounds with different geometry, donors and charge. The ranking of the functionals for λ_max was determined in terms of mean absolute percent deviation (MAPD) and standard deviation (SD) and it is as follows: BHandHLYP > M06 ≫ CAM-B3LYP ≫ MPW1PW91 ~ B1LYP ~ BLYP > HSE06 ~ B3LYP > B3P86 ~ ω-B97x-D ≫ TPSSh ≫ M06-2X (MAPD) and BHandHLYP > M06 ~ HSE06 > ω-B97x-D ~ CAM-B3LYP ~ MPW1PW91 > B1LYP ~ B3LYP > B3P86 > BLYP ≫ TPSSh ≫ M06-2X (SD). With BHandHLYP functional the MAPD is 3.1% and SD is 2.3%, while with M06 the MAPD is 3.7% and SD is 3.7%. The protocol validated in the first step of the study was applied to: i) calculate the number of transitions in the spectra and relate them to the geometry of Cu(II) species; ii) determine the coordination of axial water(s); iii) predict the electronic spectra of the systems where Cu(II) is bound to human serum albumin (HSA) and to the regions 94-97 and 108-112 of prion protein (PrP). The results indicate that the proposed computational protocol allows a successful prediction of the electronic spectra of Cu(II) species and to relate an experimental spectrum to a specific structure.

The Thermodynamic and pH Metric Binding Studies of Cu+2 Ions with Egg Protein by Spectrometric and Diffusion Current Techniques

Transition metals have unique efficacy in catalyzing various industrial reactions and also in living system, the redox reaction and redox changes in the metal ions catalyzed valence changes in the substrate molecule. The survey of the existing literature revealed that the binding of Molybdenum, Vanadium, Zinc, Cadmium, Copper, Nickel and Cobalt with the protein is well known but no binding studies of copper metal with egg protein are reported. With a view to extend the existing knowledge of ecological nature of metal-protein system, it was thought of interest to investigate the properties of metal-protein mixture. Investigations on the aspects of these binding problems were planned and their bindings constants have been determined using suitable physico-chemical methods. The pH metric, diffusion current measurements, spectrophotometric methods have been used on the binding of copper ions with albumin. The effect of physico-chemical factors on interaction between divalent metal ion i.e. copper with albumin has been discussed. On the basis of observed results, it is found that the binding data were dependent on pH and temperature. From scatchard plots, the intrinsic association constants (k) and the number of binding sites (n) were calculated and found high at lower pH and temperatures. Therefore, a lower temperature and lower pH offered more sites in the protein molecule for interaction with copper (II) ions. The enthalpy (ΔH), entropy (ΔS) changes, free energy change (ΔG°) have been calculated.

Structural Determinants of the Prion Protein N-Terminus and Its Adducts with Copper Ions

The N-terminus of the prion protein is a large intrinsically disordered region encompassing approximately 125 amino acids. In this paper, we review its structural and functional properties, with a particular emphasis on its binding to copper ions. The latter is exploited by the region’s conformational flexibility to yield a variety of biological functions. Disease-linked mutations and proteolytic processing of the protein can impact its copper-binding properties, with important structural and functional implications, both in health and disease progression.

The effect of a membrane-mimicking environment on the interactions of Cu2+ with an amyloidogenic fragment of chicken prion protein

Prion proteins (PrP) from different species have the ability to tightly bind Cu²⁺ ions. Copper coordination sites are located in the disordered and flexible N-terminal region which contains several His anchoring sites. Among them, two His residues are found in the so called amyloidogenic PrP region which is believed to play a key role in the process leading to oligomer and fibril formation. Both chicken and human amyloidogenic regions have a hydrophobic C-terminal region rich in Ala and Val amino acids. Recent findings revealed that this domain undergoes random coil to α-helix structuring upon interaction with membrane models. This interaction might strongly impact metal binding abilities either in terms of donor sets or affinity. In this study we investigated Cu²⁺ interaction with an amyloidogenic fragment, chPrP105-140, derived from chicken prion protein (chPrP), in different solution environments. The behavior of the peptide and its metal complexes was analyzed in water and in the presence of negative and positive charged membrane mimicking environments formed by sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium chloride (DTAC) micelles. The metal coordination sphere, the metal binding affinity and stoichiometry were evaluated by combining spectroscopic and potentiometric methods. Finally we compare copper(ii) interactions with human and chicken amyloidogenic fragments. Our results indicate that the chicken amyloidogenic fragment is a stronger copper ligand than the human amyloidogenic fragment.

Neuroprotective alpha-cleavage of the human prion protein significantly impacts Cu(ii) coordination at its His111 site

Alpha-cleavage proteolytic processing of human prion protein significantly impacts its Cu(ii) coordination properties at the His111 site.

Influence of the N-terminus acetylation of Semax, a synthetic analog of ACTH(4-10), on copper(II) and zinc(II) coordination and biological properties

Semax is a heptapeptide (Met-Glu-His-Phe-Pro-Gly-Pro) that encompasses the sequence 4-7 of N-terminal domain of the adrenocorticotropic hormone and a C-terminal Pro-Gly-Pro tripeptide. N-terminal amino group acetylation (Ac-Semax) modulates the chemical and biological properties of parental peptide, modifying the ability of Semax to form complex species with Cu(II) ion. At physiological pH, the main complex species formed by Ac-Semax, [CuLH_-2]^2-, consists in a distorted CuN₃O chromophore with a weak apical interaction of the methionine sulphur. Such a complex differs from the Cu(II)-Semax complex system, which exhibits a CuN₄ chromophore. The reduced ligand field affects the [CuLH_-2]^2- formal redox potential, which is more positive than that of Cu(II)-Semax corresponding species. In the amino-free form, the resulting complex species is redox-stable and unreactive against ascorbic acid, unlike the acetylated form. Semax acetylation did not protect from Cu(II) induced toxicity on a SH-SY5Y neuroblastoma cell line, thus demonstrating the crucial role played by the free NH₂ terminus in the cell protection. Since several brain diseases are associated either to Cu(II) or Zn(II) dyshomeostasis, here we characterized also the complex species formed by Zn(II) with Semax and Ac-Semax. Both peptides were able to form Zn(II) complex species with comparable strength. Confocal microscopy imaging confirmed that peptide group acetylation does not affect the Zn(II) influx in neuroblastoma cells. Moreover, a punctuate distribution of Zn(II) within the cells suggests a preferred subcellular localization that might explain the zinc toxic effect. A future perspective can be the use of Ac-Semax as ionophore in antibody drug conjugates to produce a dysmetallostasis in tumor cells.

Impact of SDS surfactant on the interactions of Cu(2+) ions with the amyloidogenic region of human prion protein

Prion diseases, known as Transmissible Spongiform Encephalopathies (TSEs), are a group of fatal neuronal, and to some extent infectious disorders, associated with a pathogenic protein agent called prion protein (PrP). The human prion protein (hPrP) fragment encompassing the 91-127 region, also known as the amyloidogenic domain, comprises two copper-binding sites corresponding to His-96 and His-111 residues that act as anchors for Cu(2+) binding. In this work, we investigated Cu(2+) interaction with hPrP91-127 in the presence of the anionic surfactant sodium dodecyl sulfate (SDS), which induces a partial α-helix folding of the peptide. Our data indicate that the Cu(2+) coordination ability of the amyloidogenic fragment in the presence of SDS micelles is significantly different to that observed in aqueous solution. This is mainly due to the fact that SDS micelles strongly stabilize the formation of the α-helical structure of the peptide backbone, which is well conserved also upon Cu(2+) binding, contrary to the random coil conformation mainly assumed by hPrP91-127 in aqueous solutions. Potentiometric and spectroscopic studies clearly indicate that in the case of SDS containing solutions, Cu(2+) ions coordinate simultaneously to both imidazoles, while in the case of water solutions, metal ion coordination involves only a single His side chain, which individually acts as an independent Cu(2+) anchoring site.

Spectroscopic and Theoretical Study of Cu(I) Binding to His111 in the Human Prion Protein Fragment 106-115

The ability of the cellular prion protein (PrP(C)) to bind copper in vivo points to a physiological role for PrP(C) in copper transport. Six copper binding sites have been identified in the nonstructured N-terminal region of human PrP(C). Among these sites, the His111 site is unique in that it contains a MKHM motif that would confer interesting Cu(I) and Cu(II) binding properties. We have evaluated Cu(I) coordination to the PrP(106-115) fragment of the human PrP protein, using NMR and X-ray absorption spectroscopies and electronic structure calculations. We find that Met109 and Met112 play an important role in anchoring this metal ion. Cu(I) coordination to His111 is pH-dependent: at pH >8, 2N1O1S species are formed with one Met ligand; in the range of pH 5-8, both methionine (Met) residues bind to Cu(I), forming a 1N1O2S species, where N is from His111 and O is from a backbone carbonyl or a water molecule; at pH <5, only the two Met residues remain coordinated. Thus, even upon drastic changes in the chemical environment, such as those occurring during endocytosis of PrP(C) (decreased pH and a reducing potential), the two Met residues in the MKHM motif enable PrP(C) to maintain the bound Cu(I) ions, consistent with a copper transport function for this protein. We also find that the physiologically relevant Cu(I)-1N1O2S species activates dioxygen via an inner-sphere mechanism, likely involving the formation of a copper(II) superoxide complex. In this process, the Met residues are partially oxidized to sulfoxide; this ability to scavenge superoxide may play a role in the proposed antioxidant properties of PrP(C). This study provides further insight into the Cu(I) coordination properties of His111 in human PrP(C) and the molecular mechanism of oxygen activation by this site.

Roles of DMSO-type ruthenium complexes in disaggregation of prion neuropeptide PrP106–126

DMSO-type ruthenium complexes with aromatic ligands disaggregate the mature PrP106–126 fibrilsviametal coordination and hydrophobic interaction.

Calorimetric investigation of copper binding in the N-terminal region of the prion protein at low copper loading: evidence for an entropically favorable first binding event

Although the Cu²⁺-binding sites of the prion protein have been well studied when the protein is fully saturated by Cu²⁺, the Cu²⁺-loading mechanism is just beginning to come into view. Because the Cu²⁺-binding modes at low and intermediate Cu²⁺ occupancy necessarily represent the highest-affinity binding modes, these are very likely populated under physiological conditions, and it is thus essential to characterize them in order to understand better the biological function of copper–prion interactions. Besides binding-affinity data, almost no other thermodynamic parameters (e.g., ΔH and ΔS) have been measured, thus leaving undetermined the enthalpic and entropic factors that govern the free energy of Cu²⁺ binding to the prion protein. In this study, isothermal titration calorimetry (ITC) was used to quantify the thermodynamic parameters (K, ΔG, ΔH, and TΔS) of Cu²⁺ binding to a peptide, PrP(23–28, 57–98), that encompasses the majority of the residues implicated in Cu²⁺ binding by full-length PrP. Use of the buffer N-(2-acetomido)-aminoethanesulfonic acid (ACES), which is also a well-characterized Cu²⁺ chelator, allowed for the isolation of the two highest affinity binding events. Circular dichroism spectroscopy was used to characterize the different binding modes as a function of added Cu²⁺. The K_d values determined by ITC, 7 and 380 nM, are well in line with those reported by others. The first binding event benefits significantly from a positive entropy, whereas the second binding event is enthalpically driven. The thermodynamic values associated with Cu²⁺ binding by the Aβ peptide, which is implicated in Alzheimer’s disease, bear striking parallels to those found here for the prion protein.

The thermodynamics (K, ΔG, ΔH, and TΔS) of the two highest affinity Cu²⁺-binding events of the prion protein were investigated using isothermal titration calorimetry. Peptide PrP(23−28, 57−98) was used as a model system for the metal-binding region. The first binding event had a K_d of 7 nM and was entropically driven (+ΔS), whereas the second binding event had a K_d of 380 nm and was enthalpically driven (−ΔH).

Insights into the oxygen-based ligand of the low pH component of the Cu(2+)-amyloid-β complex

In spite of significant experimental effort dedicated to the study of Cu(2+) binding to the amyloid beta (Aβ) peptide, involved in Alzheimer's disease, the nature of the oxygen-based ligand in the low pH component of the Cu(2+)-Aβ(1-16) complex is still under debate. This study reports density-functional-theory-based calculations that explore the potential energy surface of Cu(2+) complexes including N and O ligands at the N-terminus of the Aβ peptide, with a focus on evaluating the role of Asp1 carboxylate in copper coordination. Model conformers including 3, 6, and 17 amino acids have been used to systematically study several aspects of the Cu(2+)-coordination such as the Asp1 side chain conformation, local peptide backbone geometry, electrostatic and/or hydrogen bond interactions, and number and availability of Cu(2+) ligands. Our results show that the Asp1 peptide carbonyl binds to Cu(2+) only if the coordination number is less than four. In contrast, if four ligands are available, the most stable structures include the Asp1 carboxylate in equatorial position instead of the Asp1 carbonyl group. The two lowest energy Cu(2+)-Aβ(1-17) models involve Asp1 COO(-), the N-terminus, and His6 and His14 as equatorial ligands, with either a carbonyl or a water molecule in the axial position. These models are in good agreement with experimental data reported for component I of the Cu(2+)-Aβ(1-16) complex, including EXAFS- and X-ray-derived Cu(2+)-ligand distances, Cu(2+) EPR parameters, and (14)N and (13)C superhyperfine couplings. Our results suggest that at low pH, Cu(2+)-Aβ species with Asp1 carboxylate equatorial coordination coexist with species coordinating the Asp1 carbonyl. Understanding the bonding mechanism in these species is relevant to gain a deeper insight on the molecular processes involving copper-amyloid-β complexes, such as aggregation and redox activity.

Enantioselective recognition mechanism of ofloxacin via Cu(II)-modulated DNA

The specific interactions of Cu(2+) with self-complementary DNA sequences involving d[G4C4(GC)2G4C4], d[(GC)10], and d[(AT)10], as well as the chiral recognition mechanism of ofloxacin enantiomers via the Cu(II)-modulated DNAs, were investigated using characterizations of circular dichroism, gel electrophoresis, FT-IR spectroscopy, UV melting measurement, electron paramagnetic resonance, and HPLC. The Cu(II)-coordinated GC-rich DNAs exhibit amplified enantioselectivity toward the S-enantiomer of ofloxacin. Especially in the case of d[G4C4(GC)2G4C4], ofloxacin enantiomers intercalate into the two adjacent guanine bases through the minor groove mediated by Cu(2+), which leads to a more favorable binding between S-ofloxacin and DNA. The highest ee value of ofloxacin enantiomers in the permeate after being adsorbed by the Cu(II)-DNA complex is obtained as 49.2% in the R-enantiomer at the [Cu(2+)]/[base] molar ratio of 0.25, while at the [Cu(2+)]/[base] molar ratio of 0.05 the highest ee value of ofloxacin enantiomers in the retentate reaches 26.3% in the S-enantiomer. This work illustrates a novel promising route to construct DNA-based chiral selectors toward certain drug enantiomers through the programmable enantioselective recognition on the basis of DNA chirality and the specific binding of transition metal ions.

Regulation of aggregation behavior and neurotoxicity of prion neuropeptides by platinum complexes

Prion diseases belong to a group of infectious, fatal neurodegenerative disorders. The conformational conversion of a cellular prion protein (PrP(C)) into an abnormal misfolded isoform (PrP(Sc)) is the key event in prion disease pathology. PrP106-126 resembles PrP(Sc) in some physicochemical and biological characteristics, such as apoptosis induction in neurons, fibrillar formation, and mediation of the conversion of native cellular PrP(C) to PrP(Sc). Numerous studies have been conducted to explore the inhibiting methods on the aggregation and neurotoxicity of prion neuropeptide PrP106-126. We showed that PrP106-126 aggregation, as assessed by fluorescence assay and atomic force microscopy, is inhibited by platinum complexes cisplatin, carboplatin, and Pt(bpy)Cl2. ESI-MS and NMR assessments of PrP106-126 and its mutant peptides demonstrate that platinum complexes bind to the peptides in coordination and nonbonded interactions, which rely on the ligand properties and the peptide sequence. In peptides, methionine residue is preferred as a potent binding site over histidine residue for the studied platinum complexes, implying a typical thiophile characteristic of platinum. The neurotoxicity induced by PrP106-126 is better inhibited by Pt(bpy)Cl2 and cisplatin. Furthermore, the ligand configuration contributes to both the binding affinity and the inhibition of peptide aggregation. The pursuit of novel platinum candidates that selectively target prion neuropeptide is noteworthy for medicinal inorganic chemistry and chemical biology.

Solvent microenvironments and copper binding alters the conformation and toxicity of a prion fragment

The secondary structures of amyloidogenic proteins are largely influenced by various intra and extra cellular microenvironments and metal ions that govern cytotoxicity. The secondary structure of a prion fragment, PrP(111-126), was determined using circular dichroism (CD) spectroscopy in various microenvironments. The conformational preferences of the prion peptide fragment were examined by changing solvent conditions and pH, and by introducing external stress (sonication). These physical and chemical environments simulate various cellular components at the water-membrane interface, namely differing aqueous environments and metal chelating ions. The results show that PrP(111-126) adopts different conformations in assembled and non-assembled forms. Aging studies on the PrP(111-126) peptide fragment in aqueous buffer demonstrated a structural transition from random coil to a stable β-sheet structure. A similar, but significantly accelerated structural transition was observed upon sonication in aqueous environment. With increasing TFE concentrations, the helical content of PrP(111-126) increased persistently during the structural transition process from random coil. In aqueous SDS solution, PrP(111-126) exhibited β-sheet conformation with greater α-helical content. No significant conformational changes were observed under various pH conditions. Addition of Cu²⁺ ions inhibited the structural transition and fibril formation of the peptide in a cell free in vitro system. The fact that Cu²⁺ supplementation attenuates the fibrillar assemblies and cytotoxicity of PrP(111-126) was witnessed through structural morphology studies using AFM as well as cytotoxicity using MTT measurements. We observed negligible effects during both physical and chemical stimulation on conformation of the prion fragment in the presence of Cu²⁺ ions. The toxicity of PrP(111-126) to cultured astrocytes was reduced following the addition of Cu²⁺ ions, owing to binding affinity of copper towards histidine moiety present in the peptide.

Molecular dynamics studies on 3D structures of the hydrophobic region PrP(109-136)

Prion diseases, traditionally referred to as transmissible spongiform encephalopathies, are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of mammalian species, manifesting as scrapie in sheep, bovine spongiform encephalopathy (or 'mad-cow' disease) in cattle, and Creutzfeldt-Jakob disease, Gerstmann-Strussler-Scheinker syndrome, fatal familial insomnia (FFI), and Kulu in humans, etc. These neurodegenerative diseases are caused by the conversion from a soluble normal cellular prion protein (PrP(C)) into insoluble abnormally folded infectious prions (PrP(Sc)). The hydrophobic region PrP(109-136) controls the formation of diseased prions: the normal PrP(113-120) AGAAAAGA palindrome is an inhibitor/blocker of prion diseases and the highly conserved glycine-xxx-glycine motif PrP(119-131) can inhibit the formation of infectious prion proteins in cells. This article gives detailed reviews on the PrP(109-136) region and presents the studies of its three-dimensional structures and structural dynamics.

Utilizing NMR and EPR spectroscopy to probe the role of copper in prion diseases

Copper is an essential nutrient for the normal development of the brain and nervous system, although the hallmark of several neurological diseases is a change in copper concentrations in the brain and central nervous system. Prion protein (PrP) is a copper-binding, cell-surface glycoprotein that exists in two alternatively folded conformations: a normal isoform (PrP(C)) and a disease-associated isoform (PrP(Sc)). Prion diseases are a group of lethal neurodegenerative disorders that develop as a result of conformational conversion of PrP(C) into PrP(Sc). The pathogenic mechanism that triggers this conformational transformation with the subsequent development of prion diseases remains unclear. It has, however, been shown repeatedly that copper plays a significant functional role in the conformational conversion of prion proteins. In this review, we focus on current research that seeks to clarify the conformational changes associated with prion diseases and the role of copper in this mechanism, with emphasis on the latest applications of NMR and EPR spectroscopy to probe the interactions of copper with prion proteins.

Studying metal ion-protein interactions: electronic absorption, circular dichroism, and electron paramagnetic resonance

Metal ions play a wide range of important functional roles in biology, and they often serve as cofactors in enzymes. Some of the metal ions that are essential for life are strongly associated with proteins, forming obligate metalloproteins, while others may bind to proteins with relatively low affinity. The spectroscopic tools presented in this chapter are suitable to study metal ion-protein interactions. Metal sites in proteins are usually low symmetry centers that differentially absorb left and right circularly polarized light. The combination of electronic absorption and circular dichroism (CD) in the UV-visible region allows the characterization of electronic transitions associated with the metal-protein complex, yielding information on the geometry and nature of the metal-ligand interactions. For paramagnetic metal centers in proteins, electron paramagnetic resonance (EPR) is a powerful tool that provides information on the chemical environment around the unpaired electron(s), as it relates to the electronic structure and geometry of the metal-protein complex. EPR can also probe interactions between the electron spin and nuclear spins in the vicinity, yielding valuable information on some metal-ligand interactions. This chapter describes each spectroscopic technique and it provides the necessary information to design and implement the study of metal ion-protein interactions by electronic absorption, CD, and EPR.

Thermodynamic and spectroscopic investigation on the role of Met residues in Cu(II) binding to the non-octarepeat site of the human prion protein

Among the common features shared by neurodegenerative diseases there is the central role played by specific proteins or peptides which accumulate in neurons as insoluble plaques or tangles, containing abnormal amounts of redox-active metal ions, like copper and iron. In the case of transmissible spongiform encephalopathies (TSE), the involved protein is known as "prion protein" (PrP(C)) since "prions" (proteinaceous and infectious) are the agents which make TSE transmissible. It is widely accepted that PrP(C), in its wild-type form, can bind up to six Cu(II) ions, four of them in the so-called "octarepeat domain" and the others in the "fifth (non-octarepeat) binding-site". The latter domain contains two His residues, acting as anchoring sites for Cu(II) ions, and other potential binding residues, such as Lys and Met. While it is widely accepted that Lys residues do not take part in complex-formation, the role of methionines is still debated. In order to shed light on this issue, some peptides have been synthesized, either directly mimicking the sequence of the second half of the fifth binding site of human-PrP(C) (apo-form) or analogues where Met residues have been substituted by n-leucine. In addition, a series of short peptides, containing both His and Met residues in different relative positions, have been investigated, for the sake of comparison. Spectroscopic results, including NMR spectra of systems containing Ni(II) as a probe for the paramagnetic Cu(II) ion, agree on the exclusion of any direct interaction between the sulphur atom of Met residues and the Cu(II) ion already bound to His-imidazole side-chains. However, thermodynamic data show that Met-109 somewhat contributes to stability of complex species and this can be attributed to different electronic and steric effects.

Copper(II) complexation to 1-octarepeat peptide from a prion protein: insights from theoretical and experimental UV-visible studies

The octarepeat domain in cellular prion protein (PrP(C)) has attracted much attention over the last 10 years because of its importance in the complexation of copper with PrP(C). The aim of this research was to study the UV-vis spectra of a peptide similar to the 1-repeat of the octarepeat region in PrP(C) using experimental and theoretical approaches and to gain insight into the complexation of the PrP(C) octarepeat domain with copper(II) ions in solution. We found that the copper atom was responsible for the peptide conformation, which allows for charge transfers between its two terminal residues.