Zinc binding to amyloid-β:: Isothermal titration calorimetry and Zn competition experiments with Zn sensors

Aggregation of the peptide amyloid-beta (A beta) to amyloid plaques is a key event in Alzheimer's disease. According to the amyloid cascade hypothesis, A beta aggregates are toxic to neurons via the production of reactive oxygen species and are hence directly involved in the cause of the disease. Zinc ions play an important role, because they are able to bind to A beta and influence the aggregation properties. In the present work isothermal titration calorimetry and Zn sensors (zincon, Newport Green, and zinquin) were used to investigate the interaction of Zn with the full-length A beta 1-40 and A beta 1-42, as well as the truncated A beta 1-16 and A beta 1-28. The results suggest that Zn binding to A beta induces a release of similar to 0.9 proton by the peptide. This correspond to the expected value upon Zn binding to the three histidines and indicates that further ligands are not deprotonated upon Zn binding. Such behavior is expected for carboxylates, but not the N-terminus. Moreover, the apparent dissociation constant (K-d,K-app) of Zn binding to all forms of A beta is in the low micromolar range (1-20 mu M) and rather independent of the aggregation state including soluble A beta, A beta fibrils, or Zn-induced A beta aggregates. Finally, Zn in the soluble or aggregated Zn-A beta form is well accessible for Zn chelators. The potential repercussions on metal chelation therapy are discussed.