Three dimensional structure of rat liver 3alpha-hydroxysteroid dehydrogenase

The three-dimensional structure of rat liver 3$\alpha$-hydroxysteroid dehydrogenase (EC 1.1.1.50) is sought in order to better understand the structure-function relationships of the enzyme. As an hydroxysteroid dehydrogenase, the enzyme plays a pivotal role in the regulation of steroid hormone action by catalyzing the degradation of steroid hormones. Specifically, it is involved in the inactivation of circulating androgens, progestins, and glucocorticoids, and also potently inhibited by non-steroidal anti-inflammatory drugs. The structure, in conjunction with site-directed mutagenesis will provide a molecular understanding of how the mechanism of catalysis, cofactor-binding, and steroid recognition relate to steroid hormone inactivation. The x-ray crystal structure was solved by molecular replacement using human placental aldose reductase as the search model, to 3.0 A resolution. The protein folds into an $\alpha/\beta$ barrel and lacks a Rossmann fold for binding pyridine nucleotide, where instead the cofactor straddles the lip of the barrel. The structure contains a concentration of hydrophobic amino acids that lie in a cavity near the top of the barrel and that are presumed to be involved in binding hydrophobic substrates. At the distal end of the cavity lie three residues in close proximity that have been implicated in catalysis by site-directed mutagenesis-Tyr55, Lys84, and Asp50; where Tyr55 is postulated to act as the general acid. 3$\alpha$-HSD shares significant sequence identity with other HSDs that belong to the aldo-keto reductase superfamily and these may show similar architecture. In contrast, 3$\alpha$-HSD shares no sequence identity with HSDs that are members of the short-chain alcohol dehydrogenase family but does contain the Tyr-X-X-X-Lys consensus sequence implicated in catalysis in this family. These residues, however are on the periphery of the barrel and are unlikely to participate in the catalysis. Modeling of secosteroid mechanism-based inactivators into the putative active site is in accordance with biochemical studies revealing backwards binding and catalysis of partial steroids