Interaction of S-carboxymethylated uteroglobin with progesterone

S-Carboxymethylated uteroglobin, a small progesterone-binding globular protein, was studied by means of high-field proton magnetic resonance spectroscopy. Conformational changes induced by the steroid have been observed and indicate a well-defined rearrangement of the structure of the protein. The unusual stoichiometry of one progesterone to two uteroglobin dimers is confirmed by 1H NMR. His-8 plays a central role in the mechanism of interaction of uteroglobin with progesterone. The pH dependence of affinity constant for the complexation of the steroid parallels the titration of this histidine. Although it is not a part of the active site, it influences a crucial conformational transition of the protein through the charge carried by its imidazole ring. Formation of a tetramer of uteroglobin subunits able to bind progesterone is critically compared to a kinetic scheme involving only dimers.