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 × 10⁴ 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 (ΔH⁰) and entropy of binding (ΔS⁰), 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.