A survey of diamagnetic probes for copper2+binding to the prion protein.1H NMR solution structure of the palladium2+bound single octarepeat

Synthesis and Biological Activities of Some Metal Complexes of Peptides: A Review

Peptides, both natural and synthetic, are well suited for a wide range of purposes and offer versatile applications in different fields such as biocatalysts, injectable hydrogels, tumor treatment, and drug delivery. The research of the better part of the cited papers was conducted using various database platforms such as MetalPDB. The rising prominence of therapeutic peptides encompasses anticancer, antiviral, antimicrobial, and anti-neurodegenerative properties. The metals Na, K, Mg, Ca, Fe, Mn, Co, Cu, Zn, and Mo are ten of the twenty elements that are considered essential for life. Crucial for understanding the biological role of metals is the exploration of metal-bound proteins and peptides. Aside from essential metals, there are other non-essential metals that also interact biologically, exhibiting either therapeutic or toxic effects. Irregularities in metal binding contribute to diseases like Alzheimer's, neurodegenerative disorders, Wilson's, and Menkes disease. Certain metal complexes have potential applications as radiopharmaceuticals. The examination of these complexes was achieved by preforming UV-Vis, IR, EPR, NMR spectroscopy, and X-ray analysis. This summary, although unable to cover all of the studies in the field, offers a review of the ongoing experimentation and is a basis for new ideas, as well as strategies to explore and gain knowledge from the extensive realm of peptide-chelated metals and biotechnologies.

Studying Peptide-Metal Ion Complex Structures by Solution-State NMR

Metal chelation can provide structural stability and form reactive centers in metalloproteins. Approximately one third of known protein structures are metalloproteins, and metal binding, or the lack thereof, is often implicated in disease, making it necessary to be able to study these systems in detail. Peptide-metal complexes are both present in nature and can provide a means to focus on the binding region of a protein and control experimental variables to a high degree. Structural studies of peptide complexes with metal ions by nuclear magnetic resonance (NMR) were surveyed for all the essential metal complexes and many non-essential metal complexes. The various methods used to study each metal ion are presented together with examples of recent research. Many of these metal systems have been individually reviewed and this current overview of NMR studies of metallopeptide complexes aims to provide a basis for inspiration from structural studies and methodology applied in the field.

Copper and Zinc Interactions with Cellular Prion Proteins Change Solubility of Full-Length Glycosylated Isoforms and Induce the Occurrence of Heterogeneous Phenotypes

Prion diseases are characterized biochemically by protein aggregation of infectious prion isoforms (PrP^Sc), which result from the conformational conversion of physiological prion proteins (PrP^C). PrP^C are variable post-translationally modified glycoproteins, which exist as full length and as aminoterminally truncated glycosylated proteins and which exhibit differential detergent solubility. This implicates the presence of heterogeneous phenotypes, which overlap as protein complexes at the same molecular masses. Although the biological function of PrP^C is still enigmatic, evidence reveals that PrP^C exhibits metal-binding properties, which result in structural changes and decreased solubility. In this study, we analyzed the yield of PrP^C metal binding affiliated with low solubility and changes in protein banding patterns. By implementing a high-speed centrifugation step, the interaction of zinc ions with PrP^C was shown to generate large quantities of proteins with low solubility, consisting mainly of full-length glycosylated PrP^C; whereas unglycosylated PrP^C remained in the supernatants as well as truncated glycosylated proteins which lack of octarepeat sequence necessary for metal binding. This effect was considerably lower when PrP^C interacted with copper ions; the presence of other metals tested exhibited no effect under these conditions. The binding of zinc and copper to PrP^C demonstrated differentially soluble protein yields within distinct PrP^C subtypes. PrP^C–Zn²⁺-interaction may provide a means to differentiate glycosylated and unglycosylated subtypes and offers detailed analysis of metal-bound and metal-free protein conversion assays.

Effects of metal binding on solubility and resistance of physiological prions depend on tissues and glycotypes

Prion diseases entail the conversion of a normal host-encoded prion protein (PrP(C)) into an infectious isoform (PrP(Sc)). Various PrP(C) types differing in banding profiles and detergent solubility are present in different tissues, but only few PrP(Sc) types have been generated although PrP(C) acts as substrate. We hypothesize that distinct PrP(C) subtypes may be converted more efficiently to PrP(Sc) than others. One prerequisite for the analysis is the identification of the PrP(C) subtypes present in the protein complexes. Metal binding to PrP(C) is one of the most prominent features of the protein which induces increased proteolysis resistance and structural changes which might play an important role in the conversion process. Here we analyzed the metal-induced structural PrP(C) transformation of two different Triton X-100 soluble PrP(C) types derived from human platelets and brains by changes in protein solubility. We found that zinc and copper rendered approximately half of total PrP(C) and mainly un- and low-glycosylated PrP(C) to the Triton insoluble fraction. Our results indicate the presence of at least two distinct PrP(C) subtypes by metal interactions. The differentiation of high and low soluble metal bound PrP(C) offers precious information about PrP(C) protein composition and provides approaches for analyzing the transformation efficiency to PrP(Sc).

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.

Octarepeat peptides of prion are essential for multidrug resistance in gastric cancer cells

OBJECTIVE

In previous studies cellular prion protein (PrPc) is confirmed to be involved in multidrug resistance (MDR) of gastric cancer. Although octarepeat peptides are important functional domains of PrPc and are closely related to the transport of Cu2+/Zn2+ and antioxidative function, the significance in MDR remains unknown. We aimed to investigate the role of octarepeat peptides in gastric cancer MDR.

METHODS

Small interfering RNA (siRNA) against PrPc were transfected into adriamycin-resistant gastric cancer cell lines to inhibit the expression of wild type PrPc, and then constructs encoding PrPc without octarepeat peptides and PrPc without the fifth repeat peptide were transfected, respectively, to establish the cell models. In vitro drug sensitivity, cell apoptosis, measurement of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and glutathione (GSH), as well as changes in glutathione S-transferase (GST) were detected.

RESULTS

In vitro drug sensitivity test showed that octarepeat peptides could modulate the drug resistance of gastric cancer cells, but the deletion of the fifth repeat peptide had no effect. Specifically, the anti-apoptotic capacity of gastric cancer cells decreased significantly when the octarepeat peptides of PrPc was absent. Moreover, the activities of total SOD, Cu2+/Zn2+-SOD, GSH-Px, GSH, and GST detected in different stressing periods revealed that cells lacking octarepeat peptides of PrPc exhibited weakened responses to stress. However, absence of the fifth repeat peptide did not exert any effect on stress response.

CONCLUSION

The octarepeat peptides of prion is responsible for MDR in gastric cancer cells while the fifth repeat peptide is not.

Palladium(II) and platinum(II) bind strongly to an engineered blue copper protein

Studies of palladium(II) and platinum(II) binding to well-characterized proteins contribute to understanding the influence of these metals in the environment and body. The well-characterized apoprotein of azurin has a soft-metal binding site that may be exposed to solvent by mutation of a coordinating His-117 residue to glycine (H117G). Palladium(II) and platinum(II) form strong 1:1 adducts with the apo form of H117G azurin. A combination of UV-vis, circular dichroism, and inductively coupled plasma mass spectrometry techniques suggests that the metal binds specifically at His-46 and Cys-112 of the protein.

Structure of the flexible amino-terminal domain of prion protein bound to a sulfated glycan

The intrinsically disordered amino-proximal domain of hamster prion protein (PrP) contains four copies of a highly conserved octapeptide sequence, PHGGGWGQ, that is flanked by two polycationic residue clusters. This N-terminal domain mediates the binding of sulfated glycans, which can profoundly influence the conversion of PrP to pathological forms and the progression of prion disease. To investigate the structural consequences of sulfated glycan binding, we performed multidimensional heteronuclear ((1)H, (13)C, (15)N) NMR (nuclear magnetic resonance), circular dichroism (CD), and fluorescence studies on hamster PrP residues 23-106 (PrP 23-106) and fragments thereof when bound to pentosan polysulfate (PPS). While the majority of PrP 23-106 remain disordered upon PPS binding, the octarepeat region adopts a repeating loop-turn structure that we have determined by NMR. The beta-like turns within the repeats are corroborated by CD data demonstrating that these turns are also present, although less pronounced, without PPS. Binding to PPS exposes a hydrophobic surface composed of aligned tryptophan side chains, the spacing and orientation of which are consistent with a self-association or ligand binding site. The unique tryptophan motif was probed by intrinsic tryptophan fluorescence, which displayed enhanced fluorescence of PrP 23-106 when bound to PPS, consistent with the alignment of tryptophan side chains. Chemical-shift mapping identified binding sites on PrP 23-106 for PPS, which include the octarepeat histidine and an N-terminal basic cluster previously linked to sulfated glycan binding. These data may in part explain how sulfated glycans modulate PrP conformational conversions and oligomerizations.

Can copper binding to the prion protein generate a misfolded form of the protein?

The native prion protein (PrP) has a two domain structure, with a globular folded alpha-helical C-terminal domain and a flexible extended N-terminal region. The latter can selectively bind Cu(2+) via four His residues in the octarepeat (OR) region, as well as two sites (His96 and His111) outside this region. In the disease state, the folded C-terminal domain of PrP undergoes a conformational change, forming amorphous aggregates high in beta-sheet content. Cu(2+) bound to the ORs can be redox active and has been shown to induce cleavage within the OR region, a process requiring conserved Trp residues. Using computational modeling, we have observed that electron transfer from Trp residues to copper can be favorable. These models also reveal that an indole-based radical cation or Cu(+) can initiate reactions leading to protein backbone cleavage. We have also demonstrated, by molecular dynamics simulations, that Cu(2+) binding to the His96 and His111 residues in the remaining PrP N-terminal fragment can induce localized beta-sheet structure, allowing us to suggest a potential mechanism for the initiation of beta-sheet misfolding in the C-terminal domain by Cu(2+).

The chemistry of copper binding to PrP: is there sufficient evidence to elucidate a role for copper in protein function?

There has been an enormous body of literature published in the last 10 years concerning copper and PrP (prion protein). Despite this, there is still no generally accepted role for copper in the function of PrP or any real consensus as to how and to what affinity copper associates with the protein. The present review attempts to look at all the evidence for the chemistry, co-ordination and affinity of copper binding to PrP, and then looks at what effect this has on the protein. We then connect this evidence with possible roles for PrP when bound to copper. No clear conclusions can be made from the available data, but it is clear from the present review what aspects of copper association with PrP need to be re-investigated.

On the Copper(II) Ion Coordination by Prion Protein HGGGW Pentapeptide Model

The interaction of the octapeptide domain of the prion protein with the transition-metal-ion Cu2+ was studied at the DFT level by using the HGGGW pentapeptide as a model to mimic the PHGGGWGQ octarepeat sequence. Ten complexes, in which the metal ion exhibits different coordinations, were considered. Our results indicate that the lowest-energy structure is characterized by a tetracoordinated metal center and that this tendency of the ion to assume the square planar geometry is strong enough to prevent the addition of a further water molecule in its coordination sphere. The role of tryptophan was found to cause a lowering of the system energy due to the stabilizing effect of the electrostatic interaction between the Trp aromatic indole and histidine imidazole rings.

Structural and thermodynamical properties of CuII amyloid-beta16/28 complexes associated with Alzheimer's disease

The aggregation of the peptide amyloid-beta (Abeta) to form amyloid plaques is a key event in Alzheimer's disease. It has been shown that CuII can bind to soluble Abeta and influence its aggregation properties. Three histidines and the N-terminal amine have been proposed to be involved in its coordination. Here, for the first time, we show isothermal titration calorimetry (ITC) measurements of the CuII binding to Abeta16 and Abeta28, models of the soluble Abeta. Moreover, different spectroscopic methods were applied. The studies revealed new insights into these CuII-Abeta complexes: (1) ITC showed two CuII binding sites, with an apparent Kd of 10(-7) and 10(-5) M, respectively; (2) the high-affinity site has a smaller enthalpic contribution but a larger entropic contribution than the low-affinity binding site; (3) azide did not bind to CuII in the higher-affinity binding site, suggesting the absence of a weak, labile ligand; (4) azide could bind to the CuII in the low-affinity binding site in Abeta28 but not in Abeta16; (5) 1H-NMR suggests that the carboxylate of aspartic acid in position 1 is involved in the ligation to CuII in the high-affinity binding site; (6) the pKa of 11.3 of tyrosine in position 10 was not influenced by the binding of 2 equivalents of CuII.

Transition metal complexes of terminally protected peptides containing histidyl residues

Histidine-containing peptide fragments of prion protein are efficient ligands to bind various transition metal ions and they have high selectivity in metal binding. The metal ion affinity follows the order: Pd(II)>>Cu(II)>>Ni(II)Zn(II)>Cd(II) approximately Co(II)>Mn(II). The high selectivity of metal binding is connected to the involvement of both imidazole and amide nitrogen atoms in metal binding for Pd(II), Cu(II) and Ni(II), while only the monodentate N(im)-coordination is possible with the other metal ions. The stoichiometry and binding mode of palladium(II) complexes show great variety depending on the metal ion to ligand ratio, pH and especially the presence of coordinating donor atoms in the side chains of peptide fragments. It is also clear from our data that the peptide fragments containing histidine outside the octarepeat (His96, His111 and His187) are more efficient ligands than the monomer peptide fragments of the octarepeat domain.