Temperature dependence of kinetic interactions between progesterone and the uterine cytoplasmic receptor

The temperature dependence of the rates of dissociation and association for progesterone-receptor interactions was measured over the temperature range of 0-20 degrees C. The dissociation process is biphasic indicating that either two forms of receptor are present or that the binding of progesterone to the receptor is a concatenated reaction. The enthalpy of activation for the dissociation of progesterone from the receptor is about 26-28 kcal/mol and the entropic energy of activation is about -5 kcal/mol. The enthalpy of activation for the association of these molecules is about 3 kcal/mol and the entropic energy of activation is about 6 kcal/mol. These data are consistent with a model of progesterone binding to the receptor that includes hydrogen bonds between each of the two ketone groups and hydrogen donors on the receptor protein and involves van der Waals' interactions, due to the close proximity of the receptor binding site to a large fraction of the progesterone surface.