Zinc binding properties of the amyloid fragment Aβ(1–16) studied by electrospray-ionization mass spectrometry

Abstract

A major hallmark of Alzheimer’s disease (AD) is the strong accumulation in brain of senile plaques, mainly composed of the amyloid-β peptide (Aβ). Recent studies have suggested that the zinc cation would be a possible key mediating factor for the formation of amyloid extracellular deposits, by binding to Aβ and triggering the involved aggregation process. From a previous circular dichroism (CD) study, we have proposed the N-terminal 1–16 region of Aβ(1–16), as the minimal fragment able to specifically bind zinc. Here we investigate the Zn 2+ binding properties of Aβ(1–16) by electrospray-ionization mass spectrometry (ESI-MS). The stoichiometry of Aβ(1–16)/Zn 2+ association and the relative affinity of different cations towards Aβ(1–16) are investigated by analyzing the mass spectra of Aβ(1–16) in the presence of different cations, introduced alone or in competition. Zn 2+ binding sites are determined from collision-induced dissociation (CID) experiments conducted on the Aβ(1–16) cationized species. From these data, Aβ(1–16) is shown to form a 1:1 complex with Zn 2+ and to bind up to three cations upon increasing the Zn 2+ concentration. Under CID, zinc binding induces specific cleavages after the three histidines of the Aβ(1–16) sequence (H 6, H 13 and H 14), showing their simultaneous implication in the Zn 2+ coordination sphere. The binding of Aβ(1–16) to several Zn 2+ cations appears less specific, but still implicates the three histidines, each of them behaving thus as an autonomous binding site. A model is proposed to explain both the specific and the aspecific interactions of Zn 2+ with Aβ(1–16) that is confirmed here to behave as the minimal zinc-binding region of Aβ.