Pb 2+ binding to lentil lectin and its influence on the protein aggregation

Binding of Pb 2+ 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 Pb 2+ 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 Pb 2+ 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 Pb 2+ . Pb 2+ 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 + Pb 2+ } (45 ± 10 nm) as derived based on AFM and TEM. In presence of Pb 2+ , the larger particles break down to smaller ones because of the coordination ability of Pb 2+ 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 Pb 2+ into smaller ones.