Kinetic Insights into Zn2+-Induced Amyloid β-Protein Aggregation Revealed by Stopped-Flow Fluorescence Spectroscopy.

Zn2+ has remarkable impacts on amyloid β-protein (Aβ) aggregation, which is crucial in the pathology of Alzheimer's disease. However, the Zn2+ concentration in human cerebrospinal fluid is commonly too low for interaction with Aβ, and only during neurotransmission is there a transient release of a high concentration of Zn2+. It is difficult to explore the details of the interaction between Zn2+ and Aβ within such a short time scale by using ordinary analytical methods. Herein, stopped-flow fluorescence spectroscopy was used to study the fast aggregation kinetics of Aβ42 in the presence of Zn2+ in the time scale from 1 ms to seconds. It was found that Zn2+ bound to Aβ42 within 1 ms; caused immediate conformational transition around Tyr10, which led to the enhancement of Aβ42 hydrophobicity; and then promoted fast aggregation of Aβ42 through enhanced hydrophobic interactions. Among the two Zn2+-binding sites on Aβ42 (KD = 107 nM and 5.2 μM), the first one of higher affinity had a greater impact on the aggregation of Aβ42. Three kinetic phases were observed in the Zn2+-induced fast aggregation of Aβ42, and the fast phase was extremely accelerated by Zn2+, indicating that accelerated aggregation mainly occurred in the fast phase. The reactions occurring in this phase were closely related to the association of Zn2+ and Aβ42. Moreover, Zn2+ largely broadened the pH range of Aβ42 fast aggregation from pH 5.2-6.2 without Zn2+ to pH 5.2-7.8 in the presence of Zn2+. Besides, the promoting effect of Zn2+ on Aβ42 fast aggregation peaked at pH 6.8-7.8, which includes the pH values of the cerebrospinal fluid (pH 7.3) and hippocampus (pH 7.15-7.35). The findings demonstrate the significant effect of Zn2+ on Aβ aggregation and provide new insight into its mechanisms.