Molecular conformation of thyroid hormones: structure and binding interactions of thyroxine

In an accurately determined X-ray diffraction study of the thyroid hormone thyroxine (T4), the two independent conformations in the crystal lattice show significant differences in the outer phenyl ring geometry when compared with that of 3,5,3'-triiodothyronine (T3). The major differences between the T4 and T3 structures are a shortened C4'-O4' bond, contraction of the C3'-C4'-C5' angle and an increase in the C3' and C5' angles of T4. These changes can be correlated with the difference in acidity of the 4'-OH of T4 and T3 and help to explain binding affinity differences among thyroactive compounds. The hydrogen bond directionality observed in T4 and other thyroid structures offers an insight into the molecular details of the hormone-receptor site. The conformation of one T4 molecule is cisoid, that of the other transoid, the first such instance of different overall conformations to be found in the same crystal lattice. One T4 molecule has the side chain nearly coplanar with the inner ring, an unusual conformation among thyroid structures.