Aß5-x Peptides: N-Terminal Truncation Yields Tunable Cu(II) Complexes.

The Aß5-x peptides (x = 38, 40, 42) are minor Aß species in normal brains but elevated upon the application of inhibitors of Aß processing enzymes. They are interesting from the point of view of coordination chemistry for the presence of an Arg-His metal binding sequence at their N-terminus capable of forming a 3-nitrogen (3N) three-coordinate chelate system. Similar sequences in other bioactive peptides were shown to bind Cu(II) ions in biological systems. Therefore, we investigated Cu(II) complex formation and reactivity of a series of truncated Aß5-x peptide models comprising the metal binding site: Aß5-9, Aß5-12, Aß5-12Y10F, and Aß5-16. Using CD and UV-vis spectroscopies and potentiometry, we found that all peptides coordinated the Cu(II) ion with substantial affinities higher than 3 × 1012 M-1 at pH 7.4 for Aß5-9 and Aß5-12. This affinity was elevated 3-fold in Aß5-16 by the formation of the internal macrochelate with the fourth coordination site occupied by the imidazole nitrogen of the His13 or His14 residue. A much higher boost of affinity could be achieved in Aß5-9 and Aß5-12 by adding appropriate amounts of the external imidazole ligand. The 3N Cu-Aß5-x complexes could be irreversibly reduced to Cu(I) at about -0.6 V vs Ag/AgCl and oxidized to Cu(III) at about 1.2 V vs Ag/AgCl. The internal or external imidazole coordination to the 3N core resulted in a slight destabilization of the Cu(I) state and stabilization of the Cu(III) state. Taken together these results indicate that Aß5-x peptides, which bind Cu(II) ions much more strongly than Aß1-x peptides and only slightly weaker than Aß4-x peptides could interfere with Cu(II) handling by these peptides, adding to copper dyshomeostasis in Alzheimer brains.

Copper Exchange and Redox Activity of a Prototypical 8-Hydroxyquinoline: Implications for Therapeutic Chelation.

The N-truncated ß-amyloid (Aß) isoform Aß4-x is known to bind Cu(2+) via a redox-silent ATCUN motif with a conditional Kd = 30 fM at pH 7.4. This study characterizes the Cu(2+) interactions and redox activity of Aßx-16 (x = 1, 4) and 2-[(dimethylamino)-methyl-8-hydroxyquinoline, a terdentate 8-hydroxyquinoline (8HQ) with a conditional Kd(CuL) = 35 pM at pH 7.4. Metal transfer between Cu(Aß1-16), CuL, CuL2, and ternary CuL(NIm(Aß)) was rapid, while the corresponding equilibrium between L and Aß4-16 occurred slowly via a metastable CuL(NIm(Aß)) intermediate. Both CuL and CuL2 were redox-silent in the presence of ascorbate, but a CuL(NIm) complex can generate reactive oxygen species. Because the NIm(Aß) ligand will be readily exchangeable with NIm ligands of ubiquitous protein His side chains in vivo, this class of 8HQ ligand could transfer Cu(2+) from inert Cu(Aß4-x) to redox-active CuL(NIm). These findings have implications for the use of terdentate 8HQs as therapeutic chelators to treat neurodegenerative disease.

A Functional Role for Aß in Metal Homeostasis? N-Truncation and High-Affinity Copper Binding.

Accumulation of the ß-amyloid (Aß) peptide in extracellular senile plaques rich in copper and zinc is a defining pathological feature of Alzheimer's disease (AD). The Aß1-x (x=16/28/40/42) peptides have been the primary focus of Cu(II) binding studies for more than 15 years; however, the N-truncated Aß4-42 peptide is a major Aß isoform detected in both healthy and diseased brains, and it contains a novel N-terminal FRH sequence. Proteins with His at the third position are known to bind Cu(II) avidly, with conditional log K values at pH 7.4 in the range of 11.0-14.6, which is much higher than that determined for Aß1-x peptides. By using Aß4-16 as a model, it was demonstrated that its FRH sequence stoichiometrically binds Cu(II) with a conditional Kd value of 3×10(-14) M at pH 7.4, and that both Aß4-16 and Aß4-42 possess negligible redox activity. Combined with the predominance of Aß4-42 in the brain, our results suggest a physiological role for this isoform in metal homeostasis within the central nervous system.