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

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 (Ka), 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 Ka 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.