Molecular mechanisms of estrogen recognition and 17-keto reduction by human 17 beta-hydroxysteroid dehydrogenase

The reduction of inactive estrone (El) to the active estrogen 17 beta -estr adiol (E2) is catalyzed by type 1 17 beta -hydroxysteroid dehydrogenase (17 HSD1). Crystallographic studies, modeling and activity measurement of mutan ts and chimeric enzymes have led to the understanding of its mechanism of a ction and the molecular basis for the estrogenic specificity. An electrophi lic attack on the C17-keto oxygen by the Tyr 155 hydroxyl is proposed for i nitiation of the transition state. The active site is a hydrophobic pocket with catalytic residues at one end and the recognition machinery on the oth er. Tyr 155, Lys 159 and Ser 142 are essential for the activity. The presen ce of certain other amino acids near the substrate recognition end of the a ctive site including His 152 and Pro 187 is critical to the shape complemen tarity of estrogenic ligands. His 221 and Glu 282 form hydrogen bonds with 3-hydroxyl of the aromatic A-ring of the ligand. This mechanism of recognit ion of El by 17HSD1 is similar to that of E2 by estrogen receptor a. In a t ernary complex with NADP(+) and equilin, an equine estrogen with C7 = C8 do uble bond, the orientation of C17 = O of equilin relative to the C4-hydride is more acute than the near normal approach of the hydride for the substra te. In the ape-enzyme structure, a substrate-entry loop (residues 186-201) is in an open conformation. The loop is closed in this complex and Phe 192 and Met 193 make contacts with the ligand. Residues of the entry loop could be partially responsible for the estrogenic specificity. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.