Structure of human estrogenic 17β-hydroxysteroid dehydrogenase at 2.20 å resolution

Background: The principal human estrogen, 17β-estradiol, is a potent stimulator of certain endocrine-dependent forms of breast cancer. Because human estrogenic 17β-hydroxysteroid dehydrogenase (type I 17β-HSD) catalyzes the last step in the biosynthesis of 17β-estradiol from the less potent estrogen, estrone, it is an attractive target for the design of inhibitors of estrogen production and tumor growth. This human enzyme shares less than 15% sequence identity with a bacterial 3α,20β-HSD, for which the three-dimensional structure is known. The amino acid sequence of 17β-HSD also differs from that of bacterial 3α,20β-HSD by two insertions (of 11 and 14 residues) and 52 additional residues at the C terminus. Results The 2.20 å resolution structure of type I 17β-HSD, the first mammalian steroidogenic enzyme studied by X-ray crystallographic techniques, reveals a fold characteristic of the short-chain dehydrogenases. The active site contains a Tyr-X-X-X-Lys sequence (where X is any amino acid) and a serine residue, features that are conserved in short-chain steroid dehydrogenases. The structure also contains three α-helices and a helix-turn-helix motif, not observed in short-chain dehydrogenase structures reported previously. No cofactor density could be located. Conclusion The helices present in 17β-HSD that were not in the two previous short-chain dehydrogenase structures are located at one end of the substrate-binding cleft away from the catalytic triad. These helices restrict access to the active site and appear to influence substrate specificity. Modeling the position of estradiol in the active site suggests that a histidine side chain may play a critical role in substrate recognition. One or more of these helices may also be involved in the reported association of the enzyme with membranes. A model for steroid and cofactor binding as well as for the estrone to estradiol transition state is proposed. The structure of the active site provides a rational basis for designing more specific inhibitors of this breast cancer associate enzyme.