On the ability of CuAβ1-x peptides to form ternary complexes: Neurotransmitter glutamate is a competitor while not a ternary partner

Abstract

In the light of conflicting reports on the ability of copper(II) complexes of amyloid beta (Aβ) peptides to form ternary complexes with small molecules co-present in the biological milieu, we performed a study of coordination equilibria in the system containing Cu(II) ions, the Aβ1-16 peptide, glutamic acid and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid, HEPES) buffer. Using potentiometry, isothermal titration calorimetry (ITC), UV-visible spectroscopy and EPR, we concluded that glutamic acid was not able to form such a ternary complex, but can efficiently compete for the Cu(II) ion with the Aβ peptide at Glu concentrations relevant for the synaptic cleft. We also found that the literature constants for Cu(II) complexes with Glu were overestimated, but this effect was partially compensated by the formation of a ternary Cu(Glu)(HEPES) complex. Our results indicate that small molecules co-present with Cu(II) ions and Aβ peptides in the synaptic cleft are not very likely to enhance Cu(II)/Aβ interactions, but instead should be considered as a Cu(II) buffering system that may help prevent these interactions and participate in Cu(II) clearance from the synaptic cleft.

CuII Binding Properties of N-Truncated Aβ Peptides: In Search of Biological Function

“…Instead, we found that these two ligands only competed for Cu(II) at the physiological range of its concentrations. 122 These studies confirmed a pleiotropic and hardly predictable character of Cu(II) interactions with Aβ 1−x peptides.…”

“…We checked whether glutamate, which may be co-present with Aβ peptides in glutamatergic synapses, might be a ternary partner for Aβ 1–16 in Cu­(II) binding. Instead, we found that these two ligands only competed for Cu­(II) at the physiological range of its concentrations . These studies confirmed a pleiotropic and hardly predictable character of Cu­(II) interactions with Aβ 1– x peptides.…”

“…This observation appears to be at odds with the lack of toxicity of monomeric Aβ 1– x . One ought to ask whether oxidative stress in AD brains can really be caused by the relatively weak CuAβ 1– x complex, so easy to silence by external chelators present in brain interstitial fluid and CSF, by way of ternary complex formation or even by simple competition, as was the case with glutamic acid?…”

The Case for Abandoning Therapeutic Chelation of Copper Ions in Alzheimer's Disease

“…The apparent dissociation constant of Aβ 1−x (x ≥ 16) at pH 7 is ~0.1 nM (Alies et al, 2013; Young et al, 2014) and thus better described as “moderate,” since other metal-binding species with comparable (or higher) capacity to bind Cu 2+ are also present in the central nervous system (CNS). For example, glutamate also reaches transiently high local concentration during synaptic signaling (Danbolt, 2001), making it competitive at relevant physiological concentrations (Frączyk et al, 2016), while other neurotransmitters such as histamine (HA) have even higher Cu 2+ affinity than glutamate (Dawson et al, 1990). Aside from one study concluding Aβ 1−42 oligomers have an enhanced affinity ( K d < 3 pM in HEPES pH 7.4; Jiang et al, 2013) that enables them to compete with human serum albumin (HSA), the latter has also been proposed as a major competitor with Aβ for copper ions within the CNS (Rózga and Bal, 2010), effectively competing for 99.9% of Cu 2+ (Perrone et al, 2010) and binding Cu + stronger than Aβ (Lu et al, 2015).…”