Hydration effects on the fibrillation process of a globular protein: the case of human serum albumin

Abstract

In this work, we have studied the fibrillation process of human serum albumin (HSA) under different solution conditions. In particular, aggregation kinetics, fibril morphology, and composition structural changes were investigated under varying experimental conditions such as pH (2.0 and 7.4), temperature (at 25 and 65 °C), and solvent polarity (ethanol/water mixtures, 10–90% v/v). The characterization was carried out by means of static and dynamic light scattering (SLS and DLS), ThT fluorescence, circular dichroism (CD) and Fourier Transform Infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). The aggregation process and the α-helix to β-sheet transitions were found to be favored by temperature and physiological pH. Also, pH was observed to influence both the fibrillation pathway and aggregation kinetics, changing from a classical fibrillation process with a lag phase under acidic conditions to a downhill polymerization process at physiological pH in the presence of the alcohol. Regarding protein structural composition, at room temperature and physiological pH ethanol was found to promote an α-helix to β-sheet conformational transition at intermediate alcohol concentrations, whereas at low and high ethanol contents α-helix prevailed as the predominant structure. Under acidic conditions, ethanol promotes an important fibrillation at high cosolvent concentrations due to screening of electric charges and a decrease in solvent polarity. On the other hand, important differences in the morphology of the resulting fibrils and aggregates are observed depending on the solution conditions. In particular, the formation of classical amyloid-like fibrils at physiological pH and high temperature is observed, in contrast to the usual curly morphology displayed by HSA fibrils under most of the solution conditions. Although the high temperature and pH are the main parameters influencing the protein structure destabilization and subsequent aggregation upon incubation, ethanol helps to regulate the hydrogen bonding, the attractive hydrophobic interactions, and the protein accessible surface area, thus, modifying the packing constraints and the resulting aggregate morphologies.