Structure of the ternary complex of human 17β-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 known to act as a stimulus for the growth of breast tumors. Human estrogenic 17 beta-hydroxysteroid dehydrogenase type 1 (17 beta-HSD1), which catalyzes the reduction of inactive estrone (E-1) to the active 17 beta-estradiol in breast tissues, is a key enzyme responsible for elevated levels of E-2 in breast 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 presented here show that the 17 beta-HSD1-catalyzed reduction of E-1 to E-2 in vitro is specifically inhibited by equilin, The crystal structure determined at 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 are well defined in one subunit, The substrate-entry loop adopts a closed conformation 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 estrogen replacement therapy in vivo could be reduction of E2 levels in breast tissues and hence the reduced risk of estrogen-dependent breast cancer.