Ligand binding at the transthyretin dimer–dimer interface: structure of the transthyretin–T4Ac complex at 2.2 Å resolution

The crystal structure of the complex of human transthyretin (hTTR) with 3,3′,5,5′-tetraiodothyroacetic acid (T4Ac) has been determined to 2.2 Å resolution. The complex crystallizes in the orthorhombic space group P2₁2₁2, with unit-cell parameters a = 43.46, b = 85.85, c = 65.44 Å. The structure was refined to R = 17.3% and R_free = 21.9% for reflections without any σ-cutoff. T4Ac is bound in both the forward and the reverse mode in the two binding sites of hTTR. In the forward orientation, T4Ac binds in a position similar to that described for thyroxine (T4) in the orthorhombic hTTR–T4 complex. In this orientation, the iodine substituents of the phenolic ring are bound in the P3′/P2 halogen pockets. In the reverse orientation, which is the major binding mode of T4Ac, the ligand is bound deep in the TTR channel, with the carboxylic group bound in the P3′ pocket and forming simultaneous polar interactions with the residues constituting the two hormone-binding sites. Such interactions of a thyroxine-analogue ligand bound in the reverse mode have never been observed in TTR complexes previously.