Molecular and Cellular Endocrinology 248 (2006) 38–46 Structural basis of the multispecificity demonstrated by 17-hydroxysteroid dehydrogenase types 1 and 5 S.-X. Lin ∗ , R. Shi, W. Qiu, A. Azzi, D.-W. Zhu, H. Al Dabbagh, M. Zhou Molecular Endocrinology Research Center at Laval University Hospital Research Center (CHUL, CHUQ), Laval University, Qu´ e., Canada G1V 4G2 Abstract 17-Hydroxysteroid dehydrogenases/ketosteroid reductases (17-HSDs/KSRs) catalyze the last step of sex steroid synthesis or the first step of their degradation, and are thus critical for many physiological processes. The multispecificity demonstrated by 17-HSDs is important for steroid metabolism in gonadal and peripheral tissues, and is a consequence of the architecture of their binding and catalytic sites. Structurally, most of the family members are short chain dehydrogenase-reductases (SDRs) except the type 5 enzyme, which is an aldo-keto reductase (AKR). 17-HSD type 1, a representative of the SDR family, has been studied extensively since the 1950s. However, its structure was not determined until the 1990s. It has always been considered as estrogen specific, in accord with the narrow binding tunnel that has been structurally determined and has been found to be complementary to estrogens. A recent study revealed that, in spite of the enzyme’s narrow binding tunnel, the pseudo- symmetry of C19 steroids leads to its alternative binding, resulting in the multispecificity of the enzyme. Expressed in ovary, breast and placenta, the enzyme catalyzes the formation of another estrogen A-diol from DHEA in addition to the biosynthesis of estradiol; it also inactivates the most active androgen DHT by both 17-hydroxysteroid oxidation and 3-ketosteroid reduction. Type 5 17-HSD (AKR1C3) differs significantly from the type 1 enzyme by possessing a spacious and flexible steroid-binding site. This is estimated to be about 960 or 470 ˚ A 3 in ternary complex with testosterone or 4-dione, respectively, whereas the binding site volume of 17-HSD1 is only about 340 ˚ A 3 . This characteristic of the 17-HSD5 binding site permits the docking of various steroids in different orientations, which encompasses a wider range of activities from 20-, 17- and 3-HSD/KSR to prostaglandin 11-ketoreductase. The in vitro activities of the enzyme are significantly lower than the type 1 enzyme. In the ternary complex with testosterone, the steroid C3-C17 position is quasi-reversed as compared to the complex with 4-dione. The multi-specificity contributes significantly to steroid metabolism in peripheral tissues, due to the high levels of 17-HSD5 mRNA in both breast and prostate tissues. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Pseudo symmetry of C-19 steroids; Alternative binding; Multi-specificity of enzymes; Flexibility of binding site; Short chain dehydrogenase-reductase; Aldo-keto reductase; 17-hydroxysteroid dehydrogenase types 1 and 5; AKR1C3 1. Introduction The 13 members of the 17-HSD family known to date (Peltoketo et al., 1999; Mindnich et al., 2004; Oppermann et al., 2003) share very low sequence identities reflecting a conver- gent evolution. The type 1 enzyme, representative of short chain dehydroge- nases/reductase in the 17-HSD family, has been studied exten- Abbreviations: HSD, hydroxysteroid dehydrogenase; DHT, Dihydrotestos- terone; DHEA, Dehydroepiandrosterone; E1: Estrone; E2, Estradiol; A-diol, Androst-5-ene-3,17-diol; A-dione, Androstanedione; 4-dione, 4- androstenedione; 3-diol, Androstane-3,17-diol; 3-diol: Androstane- 3,17-diol; ER-, Estrogen receptor-; SDR, Short chain dehydrogenase- reductase; AKR, Aldo-keto reductase; KSR, Ketosteroid reductase ∗ Corresponding author. Tel.: +1 418 654 2296; fax: +1 418 654 2761. E-mail address: sxlin@crchul.ulaval.ca (S.-X. Lin). sively since the early 1950s as an estrogenic enzyme (Fig. 1). Considerable effort has been devoted to it due to its critical roles in estrogen biosynthesis (Adams, 1985; Labrie et al., 1985; Poulin et al., 1989). Although crystals of the enzyme protein were obtained in the 1970s (Chin et al., 1976), the diffraction analysis of these crystals was not reported. Diffraction quality crystals were obtained later (Zhu et al., 1993), followed by the first apoenzyme structure (Ghosh et al., 1995). Various complex structures of the enzyme have since been reported (Azzi et al., 1996; Breton et al., 1996; Sawicki et al., 1999; Han et al., 2000; Qiu et al., 2002; Gangloff et al., 2002; Shi and Lin, 2004). The core structure of the type 1 enzyme is a seven-stranded, parallel -sheet surrounded by six parallel -helices with a typ- ical Rossmann fold. Two additional helix-turn-helix motifs con- stitute a “substrate-recognition domain” bordering the active site at the C-terminal, which were not found in previously reported short-chain dehydrogenase structures (Ghosh et al., 1995). The 0303-7207/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.mce.2005.11.035