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

Thermodynamic surprises of Cu(II)-amylin analogue complexes in membrane mimicking solutions

Membrane environment often has an important effect on the structure, and therefore also on the coordination mode of biologically relevant metal ions. This is also true in the case of Cu(II) coordination to amylin analogues—rat amylin, amylin_1–19, pramlintide and Ac-pramlintide, which offer N-terminal amine groups and/or histidine imidazoles as copper(II) anchoring sites. Complex stabilities are comparable, with the exception of the very stable Cu(II)–amylin_1–19, which proves that the presence of the amylin C-terminus lowers its affinity for copper(II); although not directly involved, its appropriate arrangement sterically prevents early metal binding. Most interestingly, in membrane-mimicking solution, the Cu(II) affinities of amylin analogues are lower than the ones in water, probably due to the crowding effect of the membrane solution and the fact that amide coordination occurs at higher pH, which happens most likely because the α-helical structure, imposed by the membrane-mimicking solvent, prevents the amides from binding at lower pH, requiring a local unwinding of the α-helix.

Metal specificity of the Ni(II) and Zn(II) binding sites of the N-terminal and G-domain of E. coli HypB

HypB is one of the chaperones required for proper nickel insertion into [NiFe]-hydrogenase. Escherichia coli HypB has two potential Ni(II) and Zn(II) binding sites-the N-terminal one and the so-called GTPase one. The metal-loaded HypB-SlyD metallochaperone complex activates nickel release from the N-terminal HypB site. In this work, we focus on the metal selectivity of the two HypB metal binding sites and show that (i) the N-terminal region binds Zn(II) and Ni(II) ions with higher affinity than the G-domain and (ii) the lower affinity G domain binds Zn(II) more effectively than Ni(II). In addition, the high affinity N-terminal domain, both in water and membrane mimicking SDS solution, has a larger affinity towards Zn(II) than Ni(II), while an opposite situation is observed at basic pH; at pH 7.4, the affinity of this region towards both metals is almost the same. The N-terminal HypB region is also more effective in Ni(II) binding than the previously studied SlyD metal binding regions. Considering that the nickel chaperone SlyD activates the release of nickel and blocks the release of zinc from the N-terminal high-affinity metal site of HypB, we may speculate that such pH-dependent metal affinity might modulate HypB interactions with SlyD, being dependent on both pH and the protein's metal status.

Impact of sphingosine and acetylsphingosines on the aggregation and toxicity of metal-free and metal-treated amyloid-β

Pathophysiological shifts in the cerebral levels of sphingolipids in Alzheimer's disease (AD) patients suggest a link between sphingolipid metabolism and the disease pathology. Sphingosine (SP), a structural backbone of sphingolipids, is an amphiphilic molecule that is able to undergo aggregation into micelles and micellar aggregates. Considering its structural properties and cellular localization, we hypothesized that SP potentially interacts with amyloid-β (Aβ) and metal ions that are found as pathological components in AD-affected brains, with manifesting its reactivity towards metal-free Aβ and metal-bound Aβ (metal–Aβ). Herein, we report, for the first time, that SP is capable of interacting with both Aβ and metal ions and consequently affects the aggregation of metal-free Aβ and metal–Aβ. Moreover, incubation of SP with Aβ in the absence and presence of metal ions results in the aggravation of toxicity induced by metal-free Aβ and metal–Aβ in living cells. As the simplest acyl derivatives of SP, N-acetylsphingosine and 3-O-acetylsphingosine also influence metal-free Aβ and metal–Aβ aggregation to different degrees, compared to SP. Such slight structural modifications of SP neutralize its ability to exacerbate the cytotoxicity triggered by metal-free Aβ and metal–Aβ. Notably, the reactivity of SP and the acetylsphingosines towards metal-free Aβ and metal–Aβ is determined to be dependent on their formation of micelles and micellar aggregates. Our overall studies demonstrate that SP and its derivatives could directly interact with pathological factors in AD and modify their pathogenic properties at concentrations below and above critical aggregation concentrations.

The reactivity of sphingosine and acetylsphingosines towards both metal-free and metal-treated amyloid-β is demonstrated showing a correlation of their micellization properties.

Using NMR spectroscopy to investigate the role played by copper in prion diseases

Prion diseases are a group of rare neurodegenerative disorders that develop as a result of the conformational conversion of normal prion protein (PrPC) to the disease-associated isoform (PrPSc). The mechanism that actually causes disease remains unclear. However, the mechanism underlying the conformational transformation of prion protein is partially understood-in particular, there is strong evidence that copper ions play a significant functional role in prion proteins and in their conformational conversion. Various models of the interaction of copper ions with prion proteins have been proposed for the Cu (II)-binding, cell-surface glycoprotein known as prion protein (PrP). Changes in the concentration of copper ions in the brain have been associated with prion diseases and there is strong evidence that copper plays a significant functional role in the conformational conversion of PrP. Nevertheless, because copper ions have been shown to have both a positive and negative effect on prion disease onset, the role played by Cu (II) ions in these diseases remains a topic of debate. Because of the unique properties of paramagnetic Cu (II) ions in the magnetic field, their interactions with PrP can be tracked even at single atom resolution using nuclear magnetic resonance (NMR) spectroscopy. Various NMR approaches have been utilized to study the kinetic, thermodynamic, and structural properties of Cu (II)-PrP interactions. Here, we highlight the different models of copper interactions with PrP with particular focus on studies that use NMR spectroscopy to investigate the role played by copper ions in prion diseases.

Membrane Binding Strongly Affecting the Dopamine Reactivity Induced by Copper Prion and Copper/Amyloid-β (Aβ) Peptides. A Ternary Copper/Aβ/Prion Peptide Complex Stabilized and Solubilized in Sodium Dodecyl Sulfate Micelles

The combination between dyshomeostatic levels of catecholamine neurotransmitters and redox-active metals such as copper and iron exacerbates the oxidative stress condition that typically affects neurodegenerative diseases. We report a comparative study of the oxidative reactivity of copper complexes with amyloid-β (Aβ₄₀) and the prion peptide fragment 76-114 (PrP_76-114), containing the high-affinity binding site, toward dopamine and 4-methylcatechol, in aqueous buffer and in sodium dodecyl sulfate micelles, as a model membrane environment. The competitive oxidative and covalent modifications undergone by the peptides were also evaluated. The high binding affinity of Cu/peptide to micelles and lipid membranes leads to a strong reduction (Aβ₄₀) and quenching (PrP_76-114) of the oxidative efficiency of the binary complexes and to a stabilization and redox silencing of the ternary complex Cu^II/Aβ₄₀/PrP_76-114, which is highly reactive in solution. The results improve our understanding of the pathological and protective effects associated with these complexes, depending on the physiological environment.

Complete observation of all structural, conformational, and fibrillation transitions of monomeric globular proteins at submicellar sodium dodecyl sulfate concentrations

Although considerable information is available regarding protein-sodium dodecyl sulfate (SDS) interactions, it is still unclear as to how much SDS is needed to denature proteins. The role of protein charge and micellar surfactant concentration on amyloid fibrillation is also unclear. This study reports on equilibrium measurements of SDS interaction with six model proteins and analyzes the results to obtain a general understanding of conformational breakdown, reorganization and restructuring of secondary structure, and entry into the amyloid fibrillar state. Significantly, all of these responses are entirely resolved at much lower than the critical micellar concentration (CMC) of SDS. Electrostatic interaction of the dodecyl sulfate anion (DS^- ) with positive surface potential on the protein can completely unfold both secondary and tertiary structures, which is followed by protein chain restructuration to α-helices. All SDS-denatured proteins contain more α-helices than the corresponding native state. SDS interaction stochastically drives proteins to the aggregated fibrillar state.

How copper ions and membrane environment influence the structure of the human and chicken tandem repeats domain?

Prion proteins (PrPs) from different species have the enormous ability to anchor copper ions. The N-terminal domain of human prion protein (hPrP) contains four tandem repeats of the -PHGGGWGQ- octapeptide sequence. This octarepeat domain can bind up to four Cu²⁺ ions. Similarly to hPrP, chicken prion protein (chPrP) is able to interact with Cu²⁺ through the tandem hexapeptide -HNPGYP- region (residues 53-94). In this work, we focused on the human octapeptide repeat (human Octa₄, hPrP_60-91) (Ac-PHGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQ-NH₂) and chicken hexapeptide repeat (chicken Hexa₄, chPrP_54-77) (Ac-HNPGYPHNPGYPHNPGYPHNPGYP-NH₂) prion protein fragments. Due to the fact that PrP is a membrane-anchored glycoprotein and its unstructured and flexible N-terminal domain may interact with the lipid bilayer, our studies were carried out in presence of the surfactant sodium dodecyl sulfate (SDS) mimicking the membrane environment in vitro. The main objective of this work was to understand the effects of copper ion on the structural rearrangements of the human and chicken N-terminal repeat domain. The obtained results provide a fundamental first step in describing the thermodynamic (potentiometric titrations) and structural properties of Cu(II) binding (UV-Vis, NMR, CD spectroscopy) to both human Octa₄ and chicken Hexa₄ repeats in both a DMSO/water and SDS micelle environment. Interestingly, in SDS environment, both ligands indicate different copper coordination modes, which results of the conformational changes in micelle environment. Our results strongly support that copper binding mode strongly depends on the protein backbone structure. Moreover, we focused on previously obtained results for amyloidogenic human and chicken fragments in membrane mimicking environment.

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.

Histidine tracts in human transcription factors: insight into metal ion coordination ability

Consecutive histidine repeats are chosen both by nature and by molecular biologists due to their high affinity towards metal ions. Screening of the human genome showed that transcription factors are extremely rich in His tracts. In this work, we examine two of such His-rich regions from forkhead box and MAFA proteins-MB3 (contains 18 His) and MB6 (with 21 His residues), focusing on the affinity and binding modes of Cu2+ and Zn2+ towards the two His-rich regions. In the case of Zn2+ species, the availability of imidazole nitrogen donors enhances metal complex stability. Interestingly, an opposite tendency is observed for Cu2+ complexes at above physiological pH, in which amide nitrogens participate in binding.

Specific binding modes of Cu(I) and Ag(I) with neurotoxic domain of the human prion protein

Prion diseases are neurodegenerative disorders associated with a conformational change of the normal cellular isoform of the prion protein (PrP(C)) to an abnormal scrapie isoform (PrP(Sc)). human prion protein (hPrP(C)) is able to bind up to six Cu(II) ions. Four of them are distributed in the octarepeat domain, containing four tandem-repetitions of the sequence PHGGGWGQ. Immediately outside the octarepeat domain, in so called PrP amyloidogenic region, two additional and independent Cu(II) binding sites, encompassing His96 and His111 residues, respectively, are present. Considering the potential involvement of PrP in cellular redox homeostasis, investigations on Cu(I)-PrP interaction might be also biologically relevant. Interestingly, the amyloidogenic fragment of PrP contains a -M(X)nM- motif, known to act as Cu(I) binding site in different proteins. In order to shed more light on this issue, copper(I) and silver(I) interactions with model peptides derived from that region were analyzed. The results of our studies reveal that both metal ions are anchored to two thioether sulfurs of Met109 and Met112, respectively. Subsequent metal interaction and coordination to His96 and His111 imidazoles are primarily found for Cu(I) at physiological pH. Metal binding was also investigated in the presence of negatively charged micelles formed by the anionic surfactant, sodium dodecyl sulfate (SDS). Our results strongly support that metal binding mode strongly depends on the protein backbone structure. In particular we show that α-helix structuring of the amyloid PrP domain influences both the metal coordination sphere and the binding affinity.