Structural insights into PFAS-β-lactoglobulin binding mechanism mediating PFAS toxicity

The strong, polar-covalent nature of C-F bonds contributes to the forever nature of per- and polyfluoroalkyl (PFAS) substances. PFAS are toxic to humans. Here, we have examined the ability of the small, globular, milk protein β-lactoglobulin to bind PFAS. The protein transports hydrophobic and amphiphilic compounds, including retinol and fatty acids, for vision and brain development; therefore, underscoring its interactions with PFAS is significant. The crystal structures of β-lactoglobulin complexed with PFOA (Perfluorooctanoic acid) at 2.0 (Å), PFOS (Perfluorooctanesulfonic acid) at 2.5 (Å), and PFDA (Perfluorodecanoic acid) at 2.0 (Å) reveal high affinity of the compounds for the central calyx of β-lactoglobulin, which is the canonical retinol and fatty acid binding site. Analyses of the data indicate significant hydrophobic interactions stabilizing the binding of the PFAS hydrophobic "tails" within the calyx and interactions between Lys60 and Lys69 and PFAS polar head groups. Comparative structural analysis revealed the presence of an open conformation of the EF loop containing the Glu89 latch residue in the complexed structures vis-a-vis the apo-form. Molecular dynamics (MD) simulations revealed high stability of the PFAS binding and attainment of energy minima in all complexes. The average binding energy of PFDA in β-lactoglobulin calyx was -25 kcal/mol, which was higher than PFOS (-21 kcal/mol) and PFOA (-23 kcal/mol) due to increased van der Waals interactions of the longer hydrophobic chain of PFDA with β-lactoglobulin. This work advances a mechanism by which β-lactoglobulin can recruit PFAS and act as a transporter for the "forever" chemical, potentially mediating its neurotoxicity.