title: Thermodynamics of steroid binding to the human glucocorticoid receptor.
abstract: The thermodynamics of the interaction of glucocorticoids with their receptor were studied in cytosol from human lymphoblastoid cells. The rate and affinity constants of dexamethasone and cortisol between 0 degree and 25 degrees C were calculated by curve-fitting from time-course and equilibrium kinetics. The data were consistent with a simple reversible bimolecular interaction. Arrhenius and Van't Hoff plots were curvilinear for both steroids. At equilibrium, the solution for the equation delta G = delta H - T X delta S (eqn. 1) was (in kJ X mol-1) -47 = 36 - 83 (dexamethasone) and -42 = -9 - 33 (cortisol) at 0 degree C. Enthalpy and entropy changes decreased quasi-linearly with temperature such that, at 25 degrees C, the respective values were -50 = -75 + 25 and -43 = -48 + 5. Thus, for both steroids, the interaction was entropy-driven at low temperature and became entirely enthalpy-driven at 20 degrees C. Thermodynamic values for the transition state were calculated from the rate constants. For the forward reaction, eqn. (1) gave 45 = 84 - 39 (dexamethasone) and 46 = 60 - 14 (cortisol) at 0 degree C, and 44 = 24 + 20 (dexamethasone) and 46 = 28 + 18 (cortisol) at 25 degrees C. These data fit quite well with a two-step model [Ross & Subramanian (1981) Biochemistry 20, 3096-3102] proposed for ligand-protein interactions, which involves a partial immobilization of the reacting species governed by hydrophobic forces, followed by stabilization of the complex by short-range interactions. On the basis of this model, an analysis of the transition-state thermodynamics led to the conclusion that no more than half of the steroid molecular area is engaged in the binding process.
mesh_terms: D002460:Cell Line; D002474:Cell-Free System; D003600:Cytosol / Q000378:metabolism; D003907:Dexamethasone / Q000378:metabolism*; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism*; D007700:Kinetics; D008214:Lymphocytes / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000378:metabolism*; D011987:Receptors, Steroid / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D011965:Receptors, Glucocorticoid; D011987:Receptors, Steroid; D003907:Dexamethasone; D006854:Hydrocortisone