Fibrillogenesis of amyloid beta-proteins (A beta) mediated by transition-metal ions such as Zn2+ in neuronal cells plays a causative role in Alzheimer's disease. Hence, it is highly desired to design multifunctional agents capable of inhibiting A beta aggregation and modulating metal-A beta species. In this study, we fabricated acidulated human serum albumin (A-HSA) as a multifunctional agent for binding Zn2+ and modulating Zn2+-mediated A beta fibrillogenesis and cytotoxicity. On average, 19.5 diglycolic anhydrides were modified onto the surface of human serum albumin (HSA). It was confirmed that A-HSA kept the stability and biocompatibility of native HSA: Moreover, it could inhibit A beta(42) fibrillogenesis and change the pathway of Zn2+-mediated A beta(42) aggregation, as demonstrated by extensive biophysical assays. In addition, upon incubation with A-HSA, the cytotoxicity presented by Zn2+-A beta(42) aggregates was significantly Mitigated in living cells. The results showed that A-HSA had much stronger inhibitory effect on Zn2+-mediated A beta(42) fibrillogenesis and cytotoxicity than equimolar HSA. Isothermal titration calorimetry and stopped-flow fluorescence measurements were then performed to investigate the working mechanism of A-HSA. The studies showed that the A-HSA surface, with more negative charges, not only had stronger affinity for Zn2+ but also might decrease the binding affinity of A beta(42) for Zn2+. Moreover, hydrophobic binding and electrostatic repulsion could work simultaneously on the bound A beta(42) on the A-HSA surface. As a result, A beta(42) conformations could be stretched, which avoided the formation of toxic Zn2+-A beta(42) aggregates. The research thus revealed that A-HSA is a multifunctional agent capable of altering the pathway of Zn2+-mediated A beta(42) aggregation and greatly mitigating the amyloid cytotoxicity.