Measurement of Interpeptidic CuII Exchange Rate Constants of CuII-Amyloid-β Complexes to Small Peptide Motifs by Tryptophan Fluorescence Quenching

Abstract

The interpeptidic Cu^II exchange rate constants were measured for two Cu amyloid-β complexes, Cu(Aβ_1-16) and Cu(Aβ_1-28), to fluorescent peptides GHW and DAHW using a quantitative tryptophan fluorescence quenching methodology. The second-order rate constants were determined at three pH values (6.8, 7.4, and 8.7) important to the two Cu(Aβ) coordination complexes, components Cu(Aβ)_I and Cu(Aβ)_II. The interpeptidic Cu^II exchange rate constant is approximately 10⁴ M^-1 s^-1 but varies in magnitude depending on many variables. These include pH, length of the Aβ peptide, location of the anchoring histidine ligand in the fluorescent peptide, number of amide deprotonations required in the tryptophan peptide to coordinate Cu^II, and interconversion between Cu(Aβ)_I and Cu(Aβ)_II. We also present EPR data probing the Cu^II exchange between peptides and the formation of ternary species between Cu(Aβ) and GHW. As the nonfluorescent GHK and DAHK peptides are important motifs found in the blood and serum, their ability to sequester Cu^II ions from Cu(Aβ) complexes may be relevant for the metal homeostasis and its implication in Alzheimer's disease. Thus, their kinetic Cu^II interpeptidic exchange rate constants are important chemical rate constants that can help elucidate the complex Cu^II trafficking puzzle in the synaptic cleft.