Quantification of the Binding Properties of Cu²⁺ to the Amyloid Beta Peptide: Coordination Spheres for Human and Rat Peptides and Implication on Cu²⁺-Induced Aggregation

Abstract

SUMMARY There is no consensus on the coordinating ligands for Cu2+ by A β. Yet the differences in peptide sequence between human and rat have been hypothesized to alter metal ion binding in a manner that alters Cu2+-induced aggregation of A β. Herein, we employ isothermal titration calorimetry (ITC), circular dichroism (CD) and electron paramagnetic resonance (EPR) spectroscopy to examine the Cu2+ coordination spheres to human and rat A β and an extensive set of A β(16) mutants. EPR of the mutant peptides is consistent with a 3N1O binding geometry, like the native human peptide at pH 7.4. The thermodynamic data reveal an equilibrium between three coordination spheres, {NH2, O-, NIm His6, amide N−}, {NH2, O-, NIm His6, NIm His13} and {NH2, O-, NIm His6, NIm His14} for human A β(16) but only one for rat A β(16), { NH2, O-, NIm His6, N−}, at pH 7.4 -6.5. ITC and CD data establish that the mutation R5G is sufficient for reproducing this difference in Cu2+ binding properties at pH 7.4. The substitution of bulky and positively charged Arg by Gly is proposed to stabilize the coordination {NH2, O-, NIm His6, amide N−} that then results in one dominating coordination sphere for the case of the rat peptide. The differences in the coordination geometries for Cu2+ by the human and rat A β are proposed to contribute to the variation in the ability of Cu2+ to induce aggregation of A β peptides.

Calorimetric investigation of copper(II) binding to Aβ peptides: thermodynamics of coordination plasticity

“…This is indicative of a Cu 2? -to-peptide ratio of 1:1 and is in agreement with previous ITC studies on the Ab16 peptide [31][32][33]. Fitting the binding isotherm to a single set of sites model provided the buffer-and pH-dependent parameters listed in Table 1.…”

“…The affinity of Ab for Cu 2? has been obtained primarily by spectrofluorometric [18,[28][29][30] and calorimetric techniques [23,[31][32][33]. These values are still highly variable in the literature [10,34], with dissociation constants (K d ) varying from the high nanomolar (30-60 nM) [29,[34][35][36] to low nanomolar (0.1-1 nM) range [30][31][32]37].…”

Molecular Simulations of Human and Mouse Aβ1–16 at Different pH Values: Structural Characteristics toward Understanding Cu2+‐Coordinated Amyloid Beta Spheres

“…Nterminus and thus has al ower probability of 0.32 %. The results indicatet hat our CDSM predicts the same coordination sphere of Cu 2 + -mAb [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] as component sphere II m reportedb y Eury et al [9] The results of Hong et al [20] at pH 6.5-7.4,r evealed that His6 is the major His residue bound to Cu 2 + in mAb [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and the ratio between His6 and His14 was reported as 94:6, in excellent agreement with present value of 6.27:0.32. After pH > 6.7, no His residue is protonated in the mAb 1-16 structure.…”

“…Due to these inconsistencies, Silva et al [19] studied the contribution probabilities of three His residues in the two coordination spheresa nd suggested that both components I and II coexist at pH 7.4. With experimental studies, Hong et al [20] also arguedt hat both components Ia nd II can coexist at low pH values (6.5-7.4), but with somedivergence in coordination sphere number,r esidue content, and relative involvement of three His residues. As tudy by Jiang et al revealed that the sites, especially His residue(s), for Cu 2 + to bind might also be from another peptide chain, [21] indicatingt hat coordination of aC u 2 + ionc an link more than one Ab chain and thus generate oligomers;a saresult, the aggregation process accelerates.…”

“…The latter is consistent with the component proposed by Hong et al at pH 7.4. [20] However,G aggelli et al proposed mode for component II m (pH 7.5) is 3N [NH 2 (Asp1), N imi (His6), N imi (His14)]. [25] In their latest study,K im et al [26] argued that component II m is 2N2O [CO (Ala2),C O(Glu3 side chain), N imi (His6), N imi (His14)] at pH 8.5.…”