Structure of the ternary complex of human 17 beta-hydroxysteroid dehydrogenase type 1 with 3-hydroxyestra-1,3,5,7-tetraen-17-one (equilin) and NADP(+)

Excess 17 beta-estradiol (E-2), the most potent of human estrogens, is know n to act as a stimulus for the growth of breast tumors. Human estrogenic 17 beta-hydroxysteroid dehydrogenase type 1 (17 beta-HSD1), which catalyzes t he reduction of inactive estrone (E-1) to the active 17 beta-estradiol in b reast tissues, is a key enzyme responsible for elevated levels of E-2 in br east tumor tissues, We present here the structure of the ternary complex of 17 beta-HSD1 with the cofactor NADP(+) and 3-hydroxyestra-1,3,5,7-tetraen- 17-one (equilin), an equine estrogen used in estrogen replacement therapy. The ternary complex has been crystallized with a homodimer, the active form of the enzyme, in the asymmetric unit. Structural and kinetic data present ed here show that the 17 beta-HSD1-catalyzed reduction of E-1 to E-2 in vit ro is specifically inhibited by equilin, The crystal structure determined a t 3.0-Angstrom resolution reveals that the equilin molecule is bound at the active site in a mode similar to the binding of substrate, The orientation of the 17-keto group with respect to the nicotinamide ring of NADP(+) and catalytic residues Tyr-155 and Ser-142 is different from that of E-2 in the 17 beta-HSD1-E-2 complex. The ligand and substrate-entry loop densities ar e well defined in one subunit, The substrate-entry loop adopts a closed con formation in this subunit. The result demonstrates that binding of equilin at the active site of 17 beta-HSD1 is the basis for inhibition of E-1-to-E- 2 reduction by this equine estrogen in vitro. One possible outcome of estro gen replacement therapy in vivo could be reduction of E2 levels in breast t issues and hence the reduced risk of estrogen-dependent breast cancer.