Pb2+ binding to lentil lectin and its influence on the protein aggregation

Abstract

Binding of Pb²⁺ to lentil lectin (LL) was studied by spectroscopy, microscopy and thermodynamics and the species were modelled by molecular dynamics (MD). The effect of pH on Pb²⁺ binding was studied at pH = 5 in acetate buffer and pH = 7.2 in Tris buffer. Based on ITC, multiple binding sites were found for Pb²⁺ at pH = 7.2. No binding is noticed at pH = 5. The MD results showed the involvement of aspartate and glutamate with hemi-directed geometry for Pb²⁺. Pb²⁺ mediated β sheet to α-helix transition was noticed at pH = 7.2. At physiological pH, morphological changes were observed in the reduction of size of the particles of LL (160 ± 30 nm) to those in {LL + Pb²⁺} (45 ± 10 nm) as derived based on AFM and TEM. In presence of Pb²⁺, the larger particles break down to smaller ones because of the coordination ability of Pb²⁺ towards carboxylate and imidazole moieties. At pH = 5, the TEM shows much higher aggregation than that observed at pH = 7.2 for the protein alone, leading to linear aggregated species which were further broken down by Pb²⁺ into smaller ones.