3-DIMENSIONAL STRUCTURE OF RAT-LIVER 3-ALPHA-HYDROXYSTEROID DIHYDRODIOL DEHYDROGENASE - A MEMBER OF THE ALDO-KETO REDUCTASE SUPERFAMILY

The 3.0-angstrom-resolution x-rav structure of rat liver 3alpha-hydrox ysteroid dehydrogenase/dihydrodiol dehydrogenase (3alpha-HSD, EC 1.1.1 .50) was determined by molecular replacement using human placental ald ose reductase as the search model. The protein folds into an alpha/bet a or triose-phosphate isomerase barrel and lacks a canonical Rossmann fold for binding pyridine nucleotide. The structure contains a concent ration of hydrophobic amino acids that lie in a cavity near the top of the barrel and that are presumed to be involved in binding hydrophobi c substrates (steroids, prostaglandins, and polycyclic aromatic hydroc arbons) and inhibitors (nonsteroidal antiinflammatory drugs). At the d istal end of this cavity lie three residues in close proximity that ha ve been implicated in catalysis by site-directed mutagenesis-Tyr-55, A sp-50, and Lys-84. Tyr-55 is postulated to act as the general acid. 3a lpha-HSD shares significant sequence identity with other HSDs that bel ong to the aldo-keto reductase superfamily and these may show similar architecture. Other members of this family include prostaglandin F syn thase and rho-crystallin. By contrast, 3alpha-HSD shares no sequence i dentity with HSDs that are members of the short-chain alcohol dehydrog enase family but does contain the Tyr-Xaa-Xaa-Xaa-Lys consensus sequen ce implicated in catalysis in this family. In the 3alpha-HSD structure these residues are on the periphery of the barrel and are unlikely to participate in catalysis.