PDC-109, the major bovine seminal plasma protein, binds to sperm plasma membrane and modulates capacitation in the presence of heparin. In view of this, the PDC-109/heparin interaction has been investigated employing E L. various biophysical approaches. Isothermal titration calorimetric studies yielded the association constant and changes in enthalpy and entropy for the interaction at 25 degrees C (pH 7.4) as 1.92 (+/- 0.2) x 10(5) M(-1), 18.6 (+/- 1.6) kcal M(-1), and 86.5 (+/- 5.1) cal M(-1) K(-1), respectively, whereas differential scanning calorimetric studies indicated that heparin binding results in a Temperature (C) significant increase in the thermal stability of PDC-109. The affinity decreases with increase in pH and ionic strength, consistent with the involvement of electrostatic forces in this interaction. Circular dichroism spectroscopic studies indicated that PDC-109 retains its conformational features even up to 70-75 degrees C in the presence of heparin, whereas the native protein unfolds it about 55 degrees C. Atomic force microscopic studies demonstrated that large oligomeric structures are formed upon binding of PDC-109 to heparin, indicating an increase in the local density of the protein, which may be relevant to the ability of heparin to potentiate PDC-109 induced sperm capacitation.