Aleksandra
Hecel
a,
Sara
Draghi
b,
Daniela
Valensin
*b and
Henryk
Kozlowski
c
aFaculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14., 50-383 Wroclaw, Poland
bDepartment of Biotechnology, Chemistry and Pharmacy, University of Siena, Via A. Moro 2, 53100 Siena, Italy. E-mail: daniela.valensin@unisi.it
cPublic Higher Medical Professional School in Opole, Katowicka 68, 45060 Opole, Poland
First published on 22nd May 2017
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.
It is well accepted that Cu2+ ions bind to the cellular form of human PrP (hPrPC) in vivo.9 Four Cu2+ ions are bound within the His-containing sequence, –PHGGGWGQ– that is repeated four times between residues 60–91.10–16 Two additional copper binding sites are located at His96 and His111, outside the octarepeat domain.17–25 This region, encompassing residues from 91 to 127 in hPrPC, seems to be essential for amyloid formation and infectivity of prion disease.26,27 Prion is not only a characteristic protein for mammals, it is also found in many other species including avians, fishes, reptiles and amphibians. Chicken prion protein (chPrP) was extracted from the brain of domestic fowl as a homolog of hPrP.28 chPrP shows around 30% identity with mammalian prion proteins but exhibits a very similar 3D structure.29 Despite low sequence homology, the essential features of mammalian prion proteins are also conserved in avian PrP.30
chPrP is able to interact with Cu2+ through the tandem hexapeptide (residues 53–94) and amyloidogenic (residues 105–140) regions. The repeats consist of –PHNPGY– sequences which anchor copper via His imidazoles.10,31–38 The amyloidogenic domain of chPrP shares several common features with hPrP91–127:39,40
i. They both have two His residues spaced by almost the same number of amino acids. In chPrP they correspond to His110 and His124 (Scheme 1).
ii. They have an identical hydrophobic tail (residues 113–127 and 126–140, in hPrP and chPrP respectively) of 15 amino acids rich in Ala and Gly residues (Scheme 1).
iii. They both bind two copper ions at each His residue. Either 3N1O or 4N complexes are formed at physiological pH. The formation of 3N {Nim, 2N−} or 4N {Nim, 3N−} copper species is strongly dependent on the pH value and the analyzed peptide sequences. Generally, it is well accepted that an equilibrium between 3N1O and 4N complexes is present at neutral pH for the hPrP91–127 system.17,18,41,42 In contrast, for chPrP105–140 at physiological pH, 4N species only are detected.39
It is well known that the hydrophobic 112–125 region of hPrP undergoes random coil to α-helix transition in the presence of membrane like environments (surfactant or lipid micelles) and structuring solvents.43–47 Similarly to what happens to other amyloidogenic proteins, like Aβ and αS, this structural rearrangement strongly impacts Cu2+ binding modes in terms of both donor atoms and affinity.47,48 As we recently demonstrated, the interaction of hPrP91–127 with anionic micelles facilitates simultaneous copper coordination to both His96 and His111, which on the other hand behaves as separate metal anchoring sites in the absence of micelle environment.47 Since chPrP105–140 contains the same C-terminal tail (Scheme 1) analogous structural changes are expected when this fragment interacts with micelles. In addition, this behaviour might affect Cu2+ interaction with chPrP105–140 as well.
In this work we have investigated Cu2+ binding features of the chPrP fragment spanning residues 105–140 in the presence of micelles formed by anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethyl ammonium chloride (DTAC) surfactants. We focused on exploring the impact of the hydrophobic tail on conformation, Cu2+ binding affinity and tertiary structure of Cu2+ complexes with the amyloidogenic chPrP fragment. Copper coordination to human and avian systems was compared as well.
The preparation of surfactant solutions should be performed carefully. It is very important to not shake and stir the solution during dissolution in order to avoid foaming of the solution. The dissolving process of surfactants was performed by using Ultrasonic Baths at around 60 °C. The stock solution (1 M SDS or 1 M DTAC) in water was prepared and only a few microliters were added to the samples in order to achieve a final concentration of 40 mM SDS or 100 mM DTAC.
Ligand and metal complex titrations were carried out in 4.0 mM HCl water solutions containing 0.1 M KCl or 40 mM SDS or 100 mM DTAC ionic strength. During potentiometric measurements an appropriate stirring speed (medium or low) was selected to avoid foaming of the samples. For measurements in water we used a Mettler Toledo InLab semi-micro electrode. The same electrode was not employed for solutions containing SDS and DTAC, since they both form insoluble salts with K+ ions. For these reasons the KCl electrolyte in the calomel electrode was exchanged for NaCl.47,50 The ligand concentration was 0.5 mM and the Cu2+ to ligand molar ratio was 1:1.2. The SUPERQUAD program was used for stability constant calculations.51 Reported logβ values refer to the overall equilibria:
pCu + qH + rL ↔ CupHqLr | (1) |
(2) |
CupHq−1Lr + H ↔ CupHqLr | (3) |
logβ | logK | logβ* | |
---|---|---|---|
A | |||
HL | 10.99(2) | 10.99 | |
H2L | 21.49(1) | 10.50 | |
H3L | 31.56(2) | 10.07 | |
H4L | 41.30(1) | 9.74 | |
H5L | 50.45(2) | 9.15 | |
H6L | 56.94(2) | 6.49 | |
H7L | 62.62(3) | 5.72 | |
B | |||
H2L | 22.97(5) | ||
H3L | 33.99(2) | 11.02 | |
H4L | 44.86(3) | 10.87 | |
H5L | 54.27(3) | 9.41 | |
H6L | 62.14(4) | 7.87 | |
H7L | 69.07(4) | 6.93 | |
C | |||
HL | 10.75(2) | 10.75 | |
H2L | 20.57(2) | 9.82 | |
H3L | 30.43(3) | 9.86 | |
H4L | 39.95(1) | 9.52 | |
H5L | 48.95(3) | 9.00 | |
H6L | 55.35(3) | 6.40 | |
H7L | 61.04(4) | 5.69 |
Potentiometric titrations of Cu2+–chPrP complexes in water and in the presence of SDS and DTAC micelles were carried out to evaluate the corresponding complex formation constants and the distribution diagrams (Table 2, Fig. 1). The data shown in Table 2 indicate that the same species are formed independently in the investigated environment. On the other hand, stability constants distinctly differ. CuH6L species results from His imidazole deprotonation. The difference between logK* values (logβ*CuH6L − logβ*H6L) measured in water (4.41), SDS (5.36) and DTAC (3.87) is explained by the fact that the imidazole nitrogen atoms are more and less basic in SDS and in DTAC solutions, respectively. In addition, imidazole protonation constants greatly differ when metal bound and free peptides are compared. This behavior supports His binding to Cu2+ in all three cases. Further deprotonation results in the formation of CuH5L species in which two imidazole nitrogen atoms of His residues are deprotonated. The most significant differences are observed for logβ* values. logβ* of CuH6L (5.36) and CuH5L (7.44) in SDS (Table 2B) differs by more than two units, while they are much closer for water and DTAC systems. These values strongly indicate that the Cu2+ ion coordinates to both imidazoles in the case of SDS, while only one His is bound to copper in water and DTAC solutions.
Fig. 1 Species distribution diagram for Cu2+–chPrP105–140 complexes (A) in water, (B) SDS and (C) DTAC solution at 1:1.2 Cu2+/peptide ratio; T = 298.2; cpeptide = 5 × 10−4 M. |
logβ | logK | logβ*a | |
---|---|---|---|
a A logβ* = logβ(CuHjL) − logβ(HnL) (where the index j corresponds to the number of protons in the coordinated ligand to the metal ion and n corresponds to the number of protons coordinated to the ligand). | |||
A | |||
CuH6L | 61.35(5) | 4.41 | |
CuH5L | 55.81(7) | 5.51 | 5.36 |
CuH4L | 50.87(4) | 4.94 | |
CuH3L | 44.81(4) | 6.06 | |
CuH2L | 38.42(5) | 6.39 | |
CuHL | 30.22(8) | 8.20 | |
CuL | 20.38(11) | 9.84 | |
CuH−1L | 10.76(9) | 9.62 | |
CuH−2L | 0.44(11) | 10.32 | |
B | |||
CuH6L | 67.50(4) | 5.36 | |
CuH5L | 61.71(3) | 5.79 | 7.44 |
CuH4L | 55.50(4) | 6.21 | |
CuH3L | 48.71(4) | 6.79 | |
CuH2L | 40.84(6) | 7.87 | |
CuHL | 31.95(7) | 8.89 | |
CuL | 21.88(8) | 10.07 | |
CuH−1L | 10.86(9) | 11.02 | |
CuH−2L | 0.10(10) | 10.76 | |
C | |||
CuH6L | 59.22(9) | 3.87 | |
CuH5L | 53.71(9) | 5.51 | 4.76 |
CuH4L | 48.60(5) | 5.11 | |
CuH3L | 42.10(6) | 6.50 | |
CuH2L | 35.90(5) | 6.20 | |
CuHL | 28.12(8) | 7.78 | |
CuL | 18.82(9) | 9.30 | |
CuH−2L | −0.92(10) |
In addition, the distribution diagrams reported in Fig. 1 clearly show that:
(i) the pH values at which CuH4L, CuH3L, CuH2L and CuHL predominates are the highest ones in the presence of SDS micelles;
(ii) CuH5L exists as main species at pH 6.0 for SDS systems only and
(iii) DTAC micelles do not significantly influence the speciation profiles observed in water in the pH range of 4.0–9.0.
Fig. 2 CD spectra of (A) chPrP105–140 and (B) Cu2+–chPrP105–140 in 40 mM SDS solutions. 0.1 cm quartz cell, T = 298.2 K; concentration of peptide = 1 × 10−4 M, 1:1.2 Cu2+/peptide ratio. |
In order to better investigate the conformational behavior of the chPrP105–140 fragment in the presence of SDS, NMR studies were carried out with the final aim to determine the 3D structure. As expected, no trivial correlations were detected in NOESY spectra, strongly demonstrating the presence of an α-helix structure (Fig. 3). A structural preliminary analysis was performed by evaluating the set of inter proton distances calculated from NOEs which clearly anticipates α-helix elements between residues 120–131 (Fig. 2S†).
The obtained distances were then used as constraints for simulated annealing calculations by using the DYANA program. The best 20 structures are shown in Fig. 4. As expected residues 120–131 adopt a nice α helical conformation, while, the N-terminal part is still flexible and disordered. Similarly to what happened for hPrP, these structural changes strongly affect His positions as well: His110 (shown in blue) has a completely random rearrangement, while His124 (cyan) being inside the α-helix has its own defined orientation.
By considering that hPrP and chPrP amyloidogenic domains share the same hydrophobic tail and taking into account that this region is responsible for the α-helix structuring upon interaction with SDS, we expected that the two peptides possess very similar structures. However, their comparison indicates that the α-helix in chPrP includes His124 as well, while it starts immediately after His111 in hPrP. This could be due to the presence of two point mutations. In fact, the two Met present in hPrP are substituted by Phe and Val residues in chPrP (Scheme 1). Besides that, the two α-helixes are very similar (Fig. 3S†).
The effect of DTAC micelles was also investigated. Both chPrP105–140 and its copper complexes (Fig. 4S†) show prevalent random coil conformation in the pH range of 3–8. Starting from acid pH to pH 8.7 we observed a gradual shift of the CD band at 197 nm to a longer wavelength (203 nm). At pH 9.2, CD spectra begin to change dramatically to assume typical features of α-helix spectra at pH higher than 10. As shown by potentiometric measurements (see Table 1C), the deprotonation of lysine and tyrosine residues occurs at pH around 9. This may strongly impact peptide interaction with positive charged DTAC micelles, which in turn causes the observed conformational changes.
Species | UV-Vis | CD | ||
---|---|---|---|---|
λ [nm] | ε [cm−1 M−1] | λ [nm] | Δε [cm−1 M−1] | |
H 2 O | ||||
CuH6L | ||||
CuH5L | 581 | 39.76 | 590 | −3.67 |
501 | 1.46 | |||
337.5 | 9.25 | |||
305 | −9.90 | |||
CuH4L | 566 | 59.13 | 591 | −5.66 |
501.5 | 1.50 | |||
334.5 | 9.02 | |||
304 | −8.16 | |||
CuH3L | 563 | 72.68 | 718.5 | 2.25 |
569 | −5.34 | |||
328.5 | 9.38 | |||
304.5 | −2.51 | |||
CuH2L | 554 | 91.63 | 643.5 | 13.31 |
532.5 | −15.29 | |||
320.5 | 17.07 | |||
CuHL | 549 | 106.78 | 639.5 | 18.36 |
534 | −19.68 | |||
320 | 20.92 | |||
CuL | 540 | 118.49 | 639 | 19.82 |
530 | −20.61 | |||
321.5 | 22.37 | |||
CuH−1L | 535 | 139.82 | 642 | 20.95 |
534 | −22.11 | |||
318.5 | 23.03 | |||
CuH−2L | 529 | 141.69 | 641 | 21.40 |
533.5 | −23.48 | |||
318 | 22.02 | |||
SDS | ||||
CuH6L | ||||
CuH5L | ||||
CuH4L | 577 | 55.42 | 596 | −3.41 |
478 | 1.17 | |||
335.5 | 13.95 | |||
CuH3L | 555 | 97.44 | 586 | −6.87 |
487 | 1.73 | |||
337 | 18.60 | |||
CuH2L | 550 | 116.04 | 566 | −9.36 |
487 | 2.69 | |||
328 | 15.06 | |||
CuHL | 538 | 144.68 | 653 | 8.27 |
548 | −15.47 | |||
324 | 19.40 | |||
CuL | 529 | 150.55 | 649 | 12.99 |
543 | −18.86 | |||
323 | 22.64 | |||
CuH−1L | 648.5 | 15.85 | ||
543 | −22.96 | |||
321.5 | 22.47 | |||
CuH−2L | ||||
DTAC | ||||
CuH6L | ||||
CuH5L | 565 | 32.60 | 595.5 | −4.62 |
338 | 7,0 | |||
304 | −11.03 | |||
CuH4L | 567 | 50.48 | 589 | −7.37 |
502 | 0.50 | |||
334.5 | 8.47 | |||
304.5 | −11.04 | |||
CuH3L | 563 | 66.82 | 701 | 1.12 |
568.5 | −6.93 | |||
331 | 8.49 | |||
305.5 | −5.30 | |||
CuH2L | 559 | 74.64 | 658 | 5.91 |
547.5 | −10.20 | |||
326 | 11.61 | |||
CuHL | 545 | 88.83 | 643 | 17.33 |
537.5 | −21.24 | |||
322 | 20.01 | |||
CuL | 544 | 93.73 | 638.5 | 17.99 |
537 | −22.77 | |||
323 | 21.23 | |||
CuH−2L | 542 | 90.54 | 634.5 | 17.89 |
535.5 | −22.28 | |||
324 | 22.22 |
The results obtained by spectroscopic studies (UV-Vis and CD spectra) of chPrP105–140 in water, SDS and DTAC solutions are shown in Fig. 5 and 6, respectively. The first absorptions in the UV-Vis and CD spectra appear at different pH values according to the investigated system. In particular, the first band at 580 nm is observed at pH 5.5 in the case of water and DTAC solutions, while in the presence of SDS, a pH increase of about 0.5–1.0 unit is necessary to observe the first band. This behavior is in agreement with potentiometric data showing that the pK values for the complex formed in SDS solutions are higher than those found for the analogues species in water and DTAC solutions.
Fig. 5 UV-Vis spectra of Cu2+–chPrP105–140 in water (A), SDS (B) and DTAC (C) solutions in a 1 cm quartz cell. 1:1.2 Cu2+/peptide ratio; T = 298.2; cpeptide = 1 × 10−3 M. |
Fig. 6 CD spectra for Cu2+ complexes of the chPrP105–140 in (A) water, (B) SDS and (C) DTAC solutions in a 1 cm quartz cell. 1:1.2 Cu2+/peptide ratio; T = 298.2; cpeptide = 1 × 10−3 M. |
In water and DTAC solutions, a UV-Vis band at 580 nm (Fig. 5A and C) is visible at pH around 5.50 (CuH4L is the main species). At this pH, on the CD spectra, we also observed d–d bands at 503 and 580 nm, respectively (Fig. 6A and C). These values support a 3N donor set {1Nim, 2N−}. The involvement of imidazole and amide nitrogens is also confirmed by the characteristic charge transfer transitions detected in CD spectra (Fig. 6A and C) which are in the 310–315 nm range for N− → Cu2+ (ligand to metal charge transfer LMCT transitions originating from amide nitrogens to Cu2+) or in the 280–345 nm for Nim → Cu2+ (one LMCT transition originating from the π1 orbital of the imidazole ring to Cu2+) absorptions.57–62 The CuH3L species, which dominates at pH around 6.2 (Fig. 1A and C), has a {1Nim, 2N−} copper binding mode as well, in fact we do not observe any significant changes on the CD and UV-Vis spectra. In water and DTAC solutions, CuH2L complexes are the main species at physiological pH (Fig. 1A and C). Spectrophotometric data recorded at that pH show the shift of the d–d band from 580 to 558 nm (Fig. 5A and C), strongly supporting a {Nim, 3N−} binding mode.17,18,25,63,64 At physiological pH, the CD d–d bands at 534 and 644 nm (Fig. 6A and C) indicate additional amide coordination. This coordination mode is also conserved at higher pH (8–11), where the UV-Vis d–d bands at 550–530 nm support a 4N donor set. Additionally, the distribution of d–d bands on CD spectra does not change (Fig. 6A and C) confirming the {1Nim, 3N−} binding mode.
On the other hand, in the case of SDS, the CuH3L is the dominant species at physiological pH (Fig. 1B). At pH 7, CD spectra have (i) a positive band in a 300–360 nm range which is commonly assigned to charge transfer transitions, like N− → Cu2+ and Nim → Cu2+ and (ii) positive and negative transition bands at 490 and 590 nm, respectively (Fig. 6B). A different distribution of the d–d band is also observed in UV-Vis spectra compared to the one recorded in water and DTAC solutions (Fig. 5B). This behavior is consistent with different donor sets, which takes into account the involvement of both His imidazole nitrogens.
In order to obtain more information on the copper(II)–chPrP105–140 coordination mode at physiological pH, we also carried out EPR experiments. The results obtained from EPR spectra are in agreement with those derived from potentiometric titrations, UV-Vis and CD spectroscopy. At physiological pH, the copper coordination sphere consists of four nitrogen atoms either in water (AII = 197.5, gII = 2.20) (Fig. 7A) or DTAC solutions (AII = 200, gII = 2.20) (Fig. 7C), as indicated by the EPR parameters. In the case of SDS, EPR spectra is broadened, probably due to the effect of the detergent, nevertheless it was possible to define EPR parameters. In SDS solutions, at pH 7.20, the copper ion is anchored by two imidazole nitrogen atoms from histidine residues and one backbone amide nitrogen forming a 3N binding mode (Fig. 7B). Above pH 8, additional amide is bound causing the formation of the 4N complex {2Nim, 2N−} (Fig. 7B). Interestingly, the EPR spectrum for the complex in SDS at pH 7.20 shows the presence of two different species. One is predominant and was described above (3N binding mode), but it was also possible to determine EPR parameters for the second (AII = 157.5, gII = 2.28), which indicate a 2N coordination sphere (less nitrogen content in the equatorial coordination shell). This might represent a Cu2+ ion bound to the two His residues, one of them being one (H111) inserted in the α-helical structure. The distribution diagram (Fig. 1B) clearly shows that at pH 7.20 two species occur (CuH3L and CuH2L), but the predominant one is CuH3L, which indicates that three nitrogen atoms are involved in the copper coordination sphere (3N).
Fig. 7 X-band EPR spectra of 1:1.2 Cu2+–chPrP105–140 frozen (A) water, (B) SDS and (C) DTAC solutions. cpeptide = 1 × 10−3 M. |
The progressive decrease of hyperfine splitting passing from EPR spectra in water, DTAC and SDS solutions supports a major involvement of amide nitrogen atoms in water and DTAC compared to SDS, where the binding of His imidazole is preferred. Nevertheless, comparing the EPR spectra at physiological pH (Fig. 5S†), we noticed that SDS micelles cause the simultaneous presence of different metal bound species. Additionally, in all of the EPR measurement range (0–5000 G), we do not observe any dimeric and bis-complex species.
In order to get more details of copper coordination to chPrP in the presence of SDS micelles, the induced copper line broadening effects on NMR spectra were analyzed as well. 1H titration experiments, recorded by increasing metal concentration, show that the most affected signals belong to both His (His110 and His124) and Tyr109 residues (Fig. 8). Upon the addition of 0.2 Cu2+ eq., their intensities decrease and completely vanish in the presence of a higher amount of the paramagnetic ions. Similar results are evident by comparing the 1H–1H TOCSY spectra of chPrP105–140 in the absence and in the presence of Cu2+ ions (Fig. 9). The cross-peaks belonging to Tyr109 and His110 are so broadened and can hardly be observed. Effects on Trp115 and His124 are visible as well. These findings strongly support metal binding to both His imidazoles. In addition the effects detected on the Tyr109 aromatic ring suggest its stabilizing role on copper complexation, similarly to what was observed in water solutions.39,65
Fig. 9 Selected region of 1H–1H TOCSY spectra of chPrP105–140 0.5 mM pH = 7.5, SDS 40 mM, T = 298, before (black) and after the addition of 0.2 equivalents of Cu2+ (pink). |
Cu2+ binding to chPrp105–133 in water was extensively investigated some years ago,39 the reported results indicated that the main copper binding site at physiological pH is located at His110 and it has a {Nim, 3N−} donor set. Our findings are in good agreement with this study. However, the comparison of the thermodynamic parameters obtained for the two copper complexes points out that the longest peptide, chPrP105–140, is a stronger ligand for Cu2+ in all of the pH range (Fig. 10), suggesting that the hydrophobic tail plays some stabilization effect on metal ion binding. Finally we compared the copper binding ability of human (hPrP91–127)47 and chicken (chPrP105–140) systems. Our findings indicate that chPrP105–140 is a better copper ligand than hPrP91–127 both in water and SDS solutions (Fig. 11). The chPrP105–140 fragment binds about 70% of Cu2+ at physiological pH. This behavior is possibly due to copper interaction with the aromatic ring of Tyr109, absent in the human PrP amyloidogenic sequence.
Fig. 10 Competition diagram between Cu2+–chPrP105–140 and Cu2+–chPrP105–133 complexes. [Cu2+] = [chPrP105–140] = [chPrP105–133] = 1 × 10−3 M. |
Fig. 11 Competition diagram between Cu2+–chPrP105–140 and Cu2+–hPrP91–127 complexes in (A) H2O solution and (B) SDS micelles. [Cu2+] = [chPrP105–140] = [hPrP91–127] = 1 × 10−3 M. |
In the present paper, we have studied copper(II) binding to the chPrP105–140 fragment to clarify the role played by the two His residues (H110 and H124) in the presence of anionic and cationic micelles. In fact, the metal binding mode observed at the N-terminal site (His110) might not be perturbed by SDS interaction at the C-terminal region. However, our findings clearly indicate that, for hPrP91–127, simultaneous or independent Cu2+ coordination His is strongly dependent on the α-helix conformation of the amyloidogenic PrP regions. At physiological pH, CD and UV-Vis absorption spectroscopy strongly support a {Nim, 3N−} binding mode in water and DTAC solutions (random coil) while a {2Nim, 2N−} copper donor set is suggested in SDS solutions (α-helix). The main chain nitrogens bound to copper belong to residues close to His110 imidazole, like His110 and Tyr109 amides. In fact, the rigid α-helix rearrangement around His124, probably hampers the binding of amide nitrogens which require the formation of five membered macrochelate rings.
In SDS micelles, near UV CD spectra and NMR analysis show nice helicoidal structuring of the peptide backbone (elements between residues 120–131), while in water solution, chPrP105–140 systems have CD spectra characterized by random coil conformation. The same results were observed for human analogues. Additionally, cationic micelles also cause the formation of the α-helix structure but at pH above 9.
One of the visible differences between human and chicken amyloidogenic sequences is the presence of a tyrosine residue in the chPrP105–140 fragment (Scheme 1). Our NMR findings point out that the most affected signals belong to both His (His110 and His124) and Tyr109 residues. The effects observed on the Tyr109 aromatic ring suggest its stabilizing role on copper ion binding and might explain the higher stability of chicken protein fragment complexes detected at pH higher than 5.5 (Fig. 11).
By bearing in mind that (i) the hPrP amyloidogenic region, considered critical for PrPC–PrPSc conformational transition, is highly conserved in chPrP and (ii) copper binding to the His111 site of the hPrP amyloidogenic region reduces the natural tendency of apo peptides to adopt β-sheet conformation, which on the contrary is stabilized by multi His binding,66 we might hypothesize a different impact of Cu2+ on hPrP and chPrP aggregation propensities according to the protein environment. Upon SDS interaction, Cu2+ coordination to both His sites might lead to the formation of a hairpin structure in both peptides, which in turn might stabilize the β-sheet structure. On the other hand, the different preferred His anchoring points found in hPrP and chPrP water solutions might result in different effects.39 The close proximity of the His binding site to the hydrophobic region in hPrP only, might induce different conformational changes in the two proteins, favoring or disfavoring the formation of fibrillar structures.
Interestingly, a comparison between the thermodynamic parameters obtained for the two copper complexes of chPrP105–133 and chPrP105–140 respectively, reveal that the longest peptide is a stronger ligand for Cu2+ for the whole pH range. This suggests that the hydrophobic tail may play some stabilization effect on metal ion binding.
Footnote |
† Electronic supplementary information (ESI) available: CD spectra of chPrP105–140 and Cu2+–chPrP105–140 in water solutions, CD spectra of chPrP105–140 and Cu2+–chPrP105–140 in DTAC solutions, survey of the NMR derived proton–proton constraints characterizing the chPrP105–140 fragment in SDS micelles, comparison of the NMR structure of human and chicken PrP amyloidogenic fragments in SDS micelles. See DOI: 10.1039/c7dt01069a |
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