ESEEM Analysis of Multi-Histidine Cu(II)-Coordination in Model Complexes, Peptides, and Amyloid-β

Abstract

We validate the use of ESEEM to predict the number of 14N nuclei coupled to a Cu(II) ion by the use of model complexes and two small peptides with well-known Cu(II) coordination. We apply this method to gain new insight into less explored aspects of Cu(II) coordination in amyloid-β (Aβ). Aβ has two coordination modes of Cu(II) at physiological pH. A controversy has existed regarding the number of histidine residues coordinated to the Cu(II) ion in component II, which is dominant at high pH (∼8.7) values. Importantly, with an excess amount of Zn(II) ions, as is the case in brain tissues affected by Alzheimer’s disease, component II becomes the dominant coordination mode, as Zn(II) selectively substitutes component I bound to Cu(II). We confirm that component II only contains single histidine coordination, using ESEEM and set of model complexes. The ESEEM experiments carried out on systematically 15N-labeled peptides reveal that, in component II, His 13 and His 14 are more favored as equatorial ligands compared to His 6. Revealing molecular level details of subcomponents in metal ion coordination is critical in understanding the role of metal ions in Alzheimer’s disease etiology.

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

“…In the structureo fh A b 1-16 at pH 6.4-7.4, His6, His13 and His14 have as imilar probability ( % 0.3 %) of coordinating Cu 2 + ions (Table 2), which is in good agreement with the observations of Silva et al which support the coexistence of components Ia nd II and give equalp robability for bindingo fC u 2 + by the three His residuesa tp H7.4. [19] An analysiso ft he structure in Figure 3r evealed that there is as table helix structure in the region (shown in blue) hAb 3-7 (EFRHD), in which side chains extend outward and hindert he coordination of His6 with Cu 2 + .T he helix structure also disfavors the approach of the Cterminus to the Nterminus, even though the salt bridge Arg5-Glu11d raws His13 and His14 and the Nterminus closer. As ar esult, the probability that His6/His13/His14 approach the Nterminus becomes very low (0.3 %).…”

“…[2,4,9,[17][18][19] There are some inconsistences and uncertainties as wella sv ariables for the Cu 2 + -coordinated Ab [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] sphere because the coordination sphere, including both the sites andt he number of Cu 2 + ions binding to an Ab [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] peptide,d epends strictly on the acidityo f the solution. [11][12][13]19] Another contribution to the variable sphere would be the Ab 1-16 peptidei tself, au nstable and disordered Ab fragment. Even though in af ibril generated by af ull-length Ab 1-42 ,A b 1-16 is also found to be of disordered structure morphology.…”

“…2 + + -hAb [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Coordination Sphere Eury [9] and Silva [19] argued that componentsIand II of Cu 2 + -hAb 1-16 coexist at nearly physiological pH. Ac lear distinction both in pH range and binding site (Table 2) suggests that component Ic orresponds to pH < 6.4.…”

“…Figure 3r eveals that salt bridge Arg5-Asp1 (Table S1) prevents His6 approaching the Nterminus;w hereas the bend segment (Asp7) in the middle of the hAb [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] peptidef avors the approach of His13 to the Nterminus, indicating that Cu 2 + -hAb 1-16 coordinationa tp H> 7.4 involves NH 2 (Asp1), N( Asp1-Ala2), CO (Ala2-Glu3), and N imi (His13), corresponding to coordination sphere II. Eury et al suggested that sphere II is generated at pH > 7.7, [9] although they did not distinguish whichH is residue (6, 13 or 14) binds the Cu 2 + ion;S ilva et al [19] argued that coordination sphere II is exclusive at pH 8.7, but coexisted with Ia tp H7.4. His13 and His14 in II have equal percentage probability (40 %) to bind Cu 2 + ion.…”

“…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.…”