Mechanism of steroid binding to serum proteins

Association and dissociation rate constants of steroid complexes with progesterone-binding globulin (PBG) and with corticosteroid-binding globulin have been determined, utilizing the fluorescence quenching phenomenon observed on steroid binding to protein. Stopped-flow techniques were used in most cases. The dissociation rates of the complexes with steroid-binding proteins of serum are much greater than those of steroid-receptor complexes, in accordance with the biological functions of these two types of proteins. Association of steroids with PBG is accompanied by conformational changes in both components of the complexes. Chemical modification of tryptophan, lysine, and tyrosine in PBG results in inactivation of the binding site; complex formation with progesterone protects against this inactivation. A comparison of the affinity constants of PBG complexes with steroids of different structures leads to a conceptual image of the binding site and to localization of the various forces of interaction over the binding site area.