Zn(II) Ions Substantially Perturb Cu(II) Ion Coordination in Amyloid-β at Physiological pH

The interaction of Cu(II) and Zn(II) ions with amyloid-beta (A beta) plays an important role in the etiology of Alzheimers disease. We describe the use of electron spin resonance (ESR) to measure metal-binding competition between Cu(II) and Zn(II) in amyloid-beta at physiological pH. Continuous wave ESR measurements show that the affinity of Cu(II) toward A beta(1-16) is significantly higher than that of Zn(II) at physiological pH. Importantly, of the two known Cu(II) coordination modes in A beta, component I and component II, Zn(II) displaces Cu(II) only from component I. Our results indicate that at excess amounts of Zn(II) component II becomes the most dominant coordination mode. This observation is important as A beta aggregates in the brain contain a high Zn(II) ion concentration. In order to determine details of the metal ion competition, electron spin echo envelope modulation experiments were carried out on A beta variants that were systematically N-15 labeled. In the presence of Zn(II), most peptides use His 14 as an equatorial ligand to bind Cu(II) ions. Interestingly, Zn(II) ions completely substitute Cu(II) ions that are simultaneously coordinated to His 6 and His 13. Furthermore, in the presence of Zn(II), the proportion of Cu(II) ions that are simultaneously coordinated to His 13 and His 14 is increased. On the basis of our results we suggest that His 13 plays a critical role in modulating the morphology of A beta aggregates.