Structural and thermodynamical properties of CuII amyloid-β16/28 complexes associated with Alzheimer's disease

The aggregation of the peptide amyloid-beta (A beta) to form amyloid plaques is a key event in Alzheimer's disease. It has been shown that Cull can bind to soluble A beta and influence its aggregation properties. Three histidines and the N-terminal amine have been proposed to be involved in its coordination. Here, for the first time, we show isothermal titration calorimetry (ITC) measurements of the Cull binding to A beta 16 and A beta 28, models of the soluble A beta. Moreover, different spectroscopic methods were applied. The studies revealed new insights into these Cu-II-A beta complexes: (1) ITC showed two Cull binding sites, with an apparent K-d of 10(-7) and 10(-5) M, respectively; (2) the high-affinity site has a smaller enthalpic contribution but a larger entropic contribution than the low-affinity binding site; (3) azide did not bind to Cull in the higher-affinity binding site, suggesting the absence of a weak, labile ligand; (4) azide could bind to the Cull in the low-affinity binding site in A beta 28 but not in A beta 16; (5) H-1-NMR suggests that the carboxylate of aspartic acid in position 1 is involved in the ligation to Cull in the high-affinity binding site; (6) the pK(a) of 11.3 of tyrosine in position 10 was not influenced by the binding of 2 equivalents of Cu-II.