Effects of thyroxine binding on the stability, conformation, and fluorescence properties of thyroxine-binding globulin.

The effects of thyroxine (T4) on several molecular properties of human thyroxine-binding globulin (TBG) have been evaluated. Changes in the sedimentation constant and relaxation time indicate that TBG becomes more symmetric and compact when T4 is bound. This modification in structure is associated with an increase in the stability of TBG to denaturation by either acid or guanidinium chloride. T4 binding also produces changes in the emission and excitation spectra of TBG, reflecting different environments of the four tryptophanyl residues. T4 preferentially quenches residues in a less polar environment. In addition, it alters the effect of the collisional quencher, acrylamide, so as to indicate a shift in the environment of some of the exposed tryptophanyl residues.