Molecular interactions of progesterone analogues with rabbit uterine cytoplasmic receptor

Binding energies of progesterone analogues with single modifications were calculated from their affinities for the cytosolic receptor of rabbit uteri. The effects of individual substituents were analyzed in terms of hydrogen bonds, van der Waals' forces, and hydrophobic interactions. Binding to the receptor is attributed to hydrogen bonds involving the ketones at carbons 3 and 20, and van der Waals' interactions at carbons 2, 4, 7, 9, 12, 18, and 19 at which positions the separation of the steroid from the receptor appears to be about 0.1 nm or less. Greater separation occurs at carbons 6, 11, 14, 15, 16, and 21. The receptor probably has a hydrogen acceptor approximating the 11 beta position of the bound steroid. The enthalpy of binding of the progesterone molecule is about 26 kcal/mol but on the basis that the two hydrogen bonds contribute about 6 kcal/mol and each of the van der Waals' attractions about 1 kcal/mol, the sum of the individual bonds totals only about 20 kcal/mol. The difference of 6 kcal/mol is attributed to intrareceptor bonds that are established after a change in receptor conformation is initiated by progesterone binding. This change in conformation fixes the steroid in its protein niche and retards dissociation. We speculate that this alteration in conformation is related to "activation" and possibly other functions of the complex.