Effects of Cu2+ and pH on the binding of alizarin red S to bovine serum albumin based on the analysis of protein conformation

Abstract

Characterizing the binding properties of bovine serum albumin (BSA) for alizarin red S (ARS) is necessary for the understanding of its physiological functions and developing a delivery vehicle for a range of therapeutic and diagnostic reagents. Especially, the investigations of the effects of surrounding circumstances including metal ions, pH, and protein conformational change on the interactions are very important to understand the relationship between the protein conformation and its function. Therefore, effects of Cu²⁺ and pH on the reaction between BSA and ARS were investigated using spectral and molecular docking techniques based on the conformational analysis of protein. The results show that ARS has a high affinity to the subdomain IIA of BSA through hydrogen bonds between ARS and residues ARG(193), ARG(255), LYS(220), ARG(197), and ALA(289) of BSA. In the presence of Cu²⁺, it was firstly coordinated with ARS to form the ARS–Cu complex, which then bound to the site I of BSA with the similar binding process between BSA and ARS. On the basis of the binding parameters, the presence of Cu²⁺ can not affect the binding mode of ARS to the site I of BSA, however, it can improve the binding affinity of BSA for ARS. The interactions of BSA with ARS and ARS–Cu were further explored in the pH range 4.0–7.0. The binding constants (K_b) and binding number (m) at pH 4.0 are lower than those at pH 7.0 due to the unfolding of the BSA conformation at low pH. Consequently, the unfolded hydrophobic cavity of the BSA native conformation decreases the binding affinities of BSA for ARS.