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 ( 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.