Characterization of the substrate binding site in rat liver 3 alpha-hydroxysteroid dihydrodiol dehydrogenase

Rat liver 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD), a member of the aldoketoreductase superfamily, inactivates circulating steroid hormones using NAD(P)H as cofactor, Despite determination of the 3 alpha-HSD . NADP(+) binary complex structure, the functional elements that dictate the binding of steroids remain unclear (Bennett, M.J., Schlegel, B.P., Jez, J.M., Penning, T.M., and Lewis, M. (1996) Biochemistry 35, 10702-10711), Two tryptophans (Trp(86) and Trp(227)) near the active site may have roles in substrate binding, and their fluorescence may be quenched upon binding of NADPH, Trp(86) is located within an apolar cleft, while Trp(227) is found on an opposing loop near the active site, A third tryptophan, Trp(148), is on the periphery of the structure, To investigate the roles of these tryptophans in protein fluorescence and ligand binding, we generated three mutant enzymes (W86Y, W148Y, and W227Y) by site-directed mutagenesis, Spectroscopic measurements on these proteins showed that Trp(148) contributed the most to the enzyme fluorescence spectra, with Trp(227) adding the least, Trp(86) was identified as the tryptophan quenched by bound NADPH through an energy transfer mechanism, The W86Y mutant altered binding of cofactor (a 3-fold increase in K-d for NADPH) and steroid (a 7-fold increase in K-d for testosterone). This mutation also dramatically decreased the catalytic efficiency observed with one-, two-, and three-ring substrates and decreased the binding affinity for nonsteroidal anti-inflammatory drugs but had little effect on the binding of aldose reductase inhibitors, Interestingly, mutation of Trp(227) significantly impaired steroid binding (a 22-fold increase in k(d) for testosterone), but did not alter binding of cofactor, smaller substrates, or inhibitors, Kinetically, the W148Y mutant was similar to wild-type enzyme, Our results demonstrate that Trp(86) and the apolar cleft is part of the substrate binding pocket, In addition, we propose a role for Trp(227) and its associated loop in binding steroids, but not small substrates or inhibitors, most likely through interaction with the C- and D-rings of the steroid. This work provides the first evidence that tryptophans on opposite sides of the apolar cleft are part of the steroid binding pocket and suggests how the enzymemay discriminate between nonsteroidal anti-inflammatory drugs and aldose reductase Inhibitors like zopolrestat, A model of how androstanedione binds in the apolar cleft is developed, These data provide further evidence that loop structures in members of the aldoketoreductase superfamily are critical determinants of ligand binding.