Molecular interactions between thyroid hormone analogs and the rat liver nuclear receptor. Partitioning of equilibrium binding free energy changes into substituent group interactions.

Abstract

Chromatin-localized non-histone proteins, which may mediate the actions of thyroid hormones, are present in rat liver nuclei.A study of the binding affinities of ~-3,6-[3'-'~~1]triiodothyronine (['261]T,) and a series of thyroid hormone analogs to the solubilized nuclear receptor has provided information about the structural and stereochemical nature and magnitude of interactions in the hormone receptor complex.The receptor binding affinities of 67 thyroid hormone anaIogs were determined in a competitive binding assay with I'*'IJTS (& = 1.2 & 0.4 X loB "l).These results were used to calculate the free energy of binding (AGO)for each analog in order to determine, by first order partitioning of free energies, the nature and magnitude of specific substituent interactions with the receptor.The binding of ['"IJT3 to the solubilized receptor is associated with a change in free energy.AGO = -12.4(-t 0.2) kcal/mol.The individual substituent group contributions are: 1) The 4"hydroxyl participates in a donor hydrogen bond oriented toward the 6' side of the outer ring and adds -1.2 (2 0.2) kcal/mol of binding free energy.2) The 3' substituent participates in direct hydrophobic and van der Waals bonding with a size limit at isopropyl.The 3' substituent also enhances the strength of the 4'-hydroxyl interaction.The contribution to the binding free energy of a 3"iodine in TI is -4.1 (& 0.4) kcal/mol.3) The optimal 3,6 substituents are iodine atoms which can each contribute an average of -3.4 ( 2 0.7) kcal/mol.T h i s value contains the interactive effect on orientation of the outer ring, as well as the direct contribution to binding by the 3,S-iodine atoms and the aromatic rings.4) The alanine side chain probably participates in an electrostatic attraction between the carboxylate anion and a positively charged amino acid residue in the receptor but due to the presence of the a-ammonio group adds a negligible -0.2 (& 0.8) kcal/mol.The thyroid hormones show a wide variety of biological