Cu²⁺ Binding to A[beta] peptides : Detailed Heat Capacity Studies Provide Structural Insight into Complex Formation

Abstract

Alzheimer's disease (AD), an irreversible, progressive and devastating neurodegenerative disease, is the most common cause of dementia. It is characterized by the extracellular amyloid plaques and the intracellular neurofibrillary tangles. Amyloid plaques are majorly constituted of A[beta] peptides, which are cleaved from the membrane bound amyloid precursor protein (APP). A[beta] peptides consist of either 40 or 42 amino acid residues. Elevated concentrations of copper, zinc, and iron have been measured in amyloid plaques and recent in vitro studies have shown that zinc and copper ions promote the aggregation of these peptides. Furthermore, it was found that high affinity metal ion chelators can be used in the dissolution of aggregated A[beta] peptides. Taken together, these studies show the important role metal ions play in AD. In vitro studies have shown that aggregation of human A[beta] peptides increases due to the presence of Cu, but this effect is much less profound for rat A[beta] peptides. Previous studies indicate that Cu²⁺ binding occurs within the first 16 amino acid residues of the full length A[beta]16, A[beta]28 and three variant peptides using isothermal titration calorimetry (ITC) and ACES buffer as a weak competing ligand. In this study, the binding of Cu²⁺ to A[beta]16 was studied at three different concentrations of ACES buffer (20mM, 50mM, and 100mM). Buffer independent binding constants (K) are then extracted to test for ternary complex formation. According to the thermodynamic studies of Cu²⁺ binding to A[beta]28 and A[beta]28 mutants, a model was proposed, which was further tested by obtaining the heat capacity ([delta]C[subscript]p) data for Cu²⁺ binding to A[beta]16 A[beta]28 and A[beta]28 mutants. In addition, heat capacity ([delta]C[subscript]p) data for Cu²⁺ binding to Rat A[beta]28 is obtained and compared to human A[beta]28. Taken together, this data will be used to shed light on why the rat peptides do not form fibrils.