Oligopeptides Generated by Neprilysin Degradation of β-Amyloid Have the Highest Cu(II) Affinity in the Whole Aβ Family.

The catabolism of β-amyloid (Aβ) is carried out by numerous endopeptidases including neprilysin, which hydrolyzes peptide bonds preceding positions 4, 10, and 12 to yield Aβ and a minor Aβ species. Alternative processing of the amyloid precursor protein by β-secretase also generates the Aβ species. All these peptides contain a Xxx-Yyy-His sequence, also known as an ATCUN or NTS motif, making them strong chelators of Cu(II) ions. We synthesized the corresponding peptides, Phe-Arg-His-Asp-Ser-Gly-OH (Aβ), Glu-Val-His-His-Gln-Lys-am (Aβ), Val-His-His-Gln-Lys-am (Aβ), and pGlu-Val-His-His-Gln-Lys-am (pAβ), and investigated their Cu(II) binding properties using potentiometry, and UV-vis, circular dichroism, and electron paramagnetic resonance spectroscopies. We found that the three peptides with unmodified N-termini formed square-planar Cu(II) complexes at pH 7.4 with analogous geometries but significantly varied K values of 6.6 fM (Aβ), 9.5 fM (Aβ), and 1.8 pM (Aβ). Cyclization of the N-terminal Glu11 residue to the pyroglutamate species pAβ dramatically reduced the affinity (5.8 nM). The Cu(II) affinities of Aβ and Aβ are the highest among the Cu(II) complexes of Aβ peptides. Using fluorescence spectroscopy, we demonstrated that the Cu(II) exchange between the Phe-Arg-His and Val-His-His motifs is very slow, on the order of days. These results are discussed in terms of the relevance of Aβ, a major Cu(II) binding Aβ fragment generated by neprilysin, as a possible Cu(II) carrier in the brain.