Picloram binds to the h1 and h4 helices of HSA domain IIIA at drug binding site 2

Picloram (PC) is a systemic herbicide that controls herbaceous weeds and woody plants. HSA, the most abundant protein in human physiology, binds to all exogenic and endogenic ligands. PC is a stable molecule (t_1/2∼157-513 days) and a potential threat to human health via the food chain. HSA and PC binding study has been done to decipher the location and thermodynamics of binding. It has been studied with prediction tools like autodocking and MD simulation and then confirmed with fluorescence spectroscopy. HSA fluorescence was quenched by PC at pH 7.4 (N state), pH 3.5 (F state), and pH 7.4 with 4.5 M urea (I state) at temperatures 283 K, 297 K, and 303 K. The location of binding was found to be interdomain between II and III which overlaps with drug binding site 2. The binding was spontaneous, and entropy-driven that show a noticeable increase in binding with the increase in temperature. No secondary structure change at the native state has been observed due to binding. The binding results are important to understand the physiological assimilation of PC. In silico predictions and the results of spectroscopic studies unambiguously indicate the locus and nature of the binding.