Homology model of thyroxine binding globulin and elucidation of the thyroid hormone binding site

The tertiary structure of thyroxine binding globulin (TBG) has been modelled on the basis of its close homology to α₁-antitrypsin, the archetype of the serine protease inhibitor (serpin) superfamily. Energy minimization was applied to the model to refine the structure further. The putative thyroid hormone binding region suggested in previous labelling studies was found to exist within a ß-barrel structure of complementary dimensions to the thyroid hormones. The model also revealed that the binding cleft provides the hydrophobic environment and specific ionic interaction sites deemed important for thyroid hormone binding. The model is in good agreement with evidence derived from previously reported T₃ and T₄ binding, stability and isoelectric focussing studies of TBG and TBG variants. Finally, T₄ analogue and drug binding studies have enabled us to postulate the orientation and manner of hormone binding to TBG. This may prove to be of assistance in the development of potent and specific, non-thyroidal ligands and also aid in the understanding of physiological thyroid hormone binding interactions.