Structure of the human 3 alpha-hydroxysteroid dehydrogenase type 3 in complex with testosterone and NADP at 1.25-A resolution

The first crystallographic structure of human type 3 3 alpha -hydroxysteroi d dehydrogenase (3 alpha -HSD3, AKR1C2), an enzyme playing a critical role in steroid hormone metabolism, has been determined in complex with testoste rone and NADP at 1.25-Angstrom resolution. The enzyme's 17 beta -HSD activi ty was studied in comparison with its 3 alpha -HSD activity. The enzyme cat alyzes the inactivation of dihydrotestosterone into 5 alpha -androstane-3 a lpha ,17 beta -diol (3 alpha -diol) as well as the transformation of andros tenedione into testosterone. Using our homogeneous and highly active enzyme preparation, we have obtained 150-fold higher 3 alpha -HSD specificity as compared with the former reports in the literature. Although the rat and th e human 3 alpha -HSDs share 81% sequence homology, our structure reveals si gnificantly different geometries of the active sites. Substitution of the S er(222) by a histidine in the human enzyme may compel the steroid to adopt a different binding to that previously described for the rat (Bennett, M. J ., Albert, R. H., Jez, J. M., Ma, H., Penning, T. M., and Lewis, M. (1997) Structure 5, 799-812). Furthermore, we showed that the affinity for the cof actor is higher in the human 3a-HSD3 than the rat enzyme due to the presenc e of additional hydrogen bonds on the adenine moiety and that the cofactor is present under its reduced form in the active site in our preparation.