Thermodynamics of phosphorylcholine and lysophosphatidylcholine binding to the major protein of bovine seminal plasma, PDC-109

Abstract

PDC-109 binds to sperm plasma membranes by specific interaction with choline phospholipids and induces cholesterol efflux, a necessary event before capacitation – and subsequent fertilization – can occur. The binding of phosphorylcholine (PrC) and lysophosphatidylcholine (Lyso-PC) with PDC-109 was investigated by monitoring the ligand-induced changes in the absorption spectrum of PDC-109. At 20 °C, the association constants ( K a), for PrC and Lyso-PC were obtained as 81.4 M −1 and 2.02 × 10 4 M −1, respectively, indicating that the binding of Lyso-PC to PDC-109 is 250-fold stronger than that of PrC. From the temperature dependence of the K a values, enthalpy of binding (Δ H 0) and entropy of binding (Δ S 0), were obtained as −79.7 and −237.1 J mol −1 K −1 for PrC and −73.0 kJ mol −1 and −167.3 J mol −1 K −1 for Lyso-PC, respectively. These results demonstrate that although the binding of these two ligands is driven by enthalpic forces, smaller negative entropy of binding associated with Lyso-PC results in its significantly stronger binding.