The effect of the stoichiometric ratio of zinc towards the fibrillation of Bovine Serum Albumin (BSA): A mechanistic insight.

Fibrillation of proteins is a major cause of various neurodegenerative diseases and its exact mechanism of formation is yet unclear instead of extensive research. However, the role of metal ions influencing fibrillation of proteins is gaining more attention recently. Herein, we have investigated the role of various concentrations of the transition metal, Zn(II), on the fibrillation of Bovine Serum Albumin (BSA) at the physiological pH 7.4. Several biophysical and simulation techniques were employed in order to analyze the same. Thioflavin T intensity and residual protein investigations revealed that fibrillation of BSA was significantly decelerated and accelerated at 1:3 and 1:4 ratios of BSA-Zn(II), respectively; while it was found to be independent at other ratios (1:1 and 1:2). Fourier transform infrared spectroscopy analysis revealed that the transition of BSA from α-helical conformation to the β-sheet rich structure is greatly resisted at 1:3 ratio, however, the same is promoted at 1:4 ratio. Similarly, dynamic light scattering and field emission transmission electron microscopy analyses further confirmed the above observations. Furthermore, Isothermal Titration Calorimetry revealed the interaction of Zn(II) towards four binding sites of BSA with preferential affinities. Molecular dynamics studies predicted that at 1:3 ratio, the C- and N-terminal zones of BSA were least flexible owing to more stable conformation. Moreover, the solvent accessible surface area and structural analyses showed increase in hydrophilicity and more conserved secondary structure, respectively at 1:3 ratio. We propose that BSA fibrillation is indeed dependent on particular Zn(II) concentration, the temperature of the microenvironment of BSA, the number of binding sites exposed due to unfolding and the conformation after metal binding.