Crystal structure of human estrogenic 17 beta-hydroxysteroid dehydrogenase complexed with 17 beta-estradiol

Molecular determinants of steroid recognition and catalysis in aldo-keto reductases. Lessons from 3alpha-hydroxysteroid dehydrogenase

Hydroxysteroid Dehydrogenases (HSDs) regulate the occupancy of steroid hormone receptors by converting active steroid hormones into their cognate inactive metabolites. HSDs belong to either the Short-chain Dehydrogenase/Reductases (SDRs) or the Aldo-Keto Reductases (AKRs). The AKRs include virtually all mammalian 3alpha-HSDs, Type 5 17beta-HSD, ovarian 20alpha-HSDs as well as the steroid 5beta-reductases. Selective inhibitors of 3alpha-HSD isoforms could control occupancy of the androgen and GABA(A) receptors, while broader based AKR inhibitors targeting 3alpha-HSD, 20alpha-HSD and prostaglandin F2alpha synthase could maintain pregnancy. We have determined three X-ray crystal structures of rat liver 3alpha-HSD, a representative AKR. These structures are of the apoenzyme (E), the binary-complex (E.NADP-), and the ternary complex (E.NADP+.testosterone). These structures are being used with site-directed mutagenesis to define the molecular determinants of steroid recognition and catalysis as a first step in rational inhibitor design. A conserved catalytic tetrad (Tyr55, Lys84, His117 and Asp50) participates in a 'proton-relay' in which Tyr55 acts as general acid/base catalyst. Its bifunctionality relies on contributions from His117 and Lys84 which alter the pKb and pKa, respectively of this residue. Point mutation of the tetrad results in different enzymatic activities. H117E mutants display 5beta-reductase activity while Y55F and Y55S mutants retain quinone reductase activity. Our results suggest that different transition states are involved in these reaction mechanisms. The ternary complex structure shows that the mature steroid binding pocket is comprised of ten residues recruited from five loops, and that there is significant movement of a C-terminal loop on binding ligand. Mutagenesis of pocket tryptophans shows that steroid substrates and classes of nonsteroidal inhibitors exhibit different binding modes which may reflect ligand-induced loop movement. Exploitation of these findings using steroidal and nonsteroidal mechanism based inactivators may lead to selective and broad based AKR inhibitors.

3D-structure of human estrogenic 17beta-HSD1: binding with various steroids

Human estrogenic dehydrogenase (17beta-HSD1) catalyses the last step in the biosynthesis of the active estrogens that stimulate the proliferation of breast cancer cells. While the primary substrate for the enzyme is estrone, the enzyme has some activity for the non-estrogenic substrates. To better understand the structure function relationships of 17beta-HSD1 and to provide a better ground for the design of inhibitors, we have determined the crystal structures of 17beta-HSD1 in complex with different steroids. The structure of the complex of estradiol with the enzyme determined previously (Azzi et al., Nature Structural Biology 3, 665-668) showed that the narrow active site was highly complementary to the substrate. The substrate specificity is due to a combination of hydrogen bonding and hydrophobic interactions between the steroid and the enzyme binding pocket. We have now determined structures of 17beta-HSD1 in complex with dihydrotestosterone and 20alpha-OH-progesterone. In the case of the C19 androgen, several residues within the enzyme active site make some small adjustments to accommodate the increased bulk of the substrate. In addition, the C19 steroids bind in a slightly different position from estradiol with shifts in positions of up to 1.4 A. The altered binding position avoids unfavorable steric interactions between Leu 149 and the C19 methyl group (Han et al., unpublished). The known kinetic parameters for these substrates can be rationalized in light of the structures presented. These results give evidence for the structural basis of steroid recognition by 17beta-HSD1 and throw light on the design of new inhibitors for this pivotal steroid enzyme.

Structure of the ternary complex of human 17beta-hydroxysteroid dehydrogenase type 1 with 3-hydroxyestra-1,3,5,7-tetraen-17-one (equilin) and NADP+

Excess 17beta-estradiol (E2), the most potent of human estrogens, is known to act as a stimulus for the growth of breast tumors. Human estrogenic 17beta-hydroxysteroid dehydrogenase type 1 (17beta-HSD1), which catalyzes the reduction of inactive estrone (E1) to the active 17beta-estradiol in breast tissues, is a key enzyme responsible for elevated levels of E2 in breast tumor tissues. We present here the structure of the ternary complex of 17beta-HSD1 with the cofactor NADP+ and 3-hydroxyestra-1,3,5,7-tetraen-17-one (equilin), an equine estrogen used in estrogen replacement therapy. The ternary complex has been crystallized with a homodimer, the active form of the enzyme, in the asymmetric unit. Structural and kinetic data presented here show that the 17beta-HSD1-catalyzed reduction of E1 to E2 in vitro is specifically inhibited by equilin. The crystal structure determined at 3.0-A resolution reveals that the equilin molecule is bound at the active site in a mode similar to the binding of substrate. The orientation of the 17-keto group with respect to the nicotinamide ring of NADP+ and catalytic residues Tyr-155 and Ser-142 is different from that of E2 in the 17beta-HSD1-E2 complex. The ligand and substrate-entry loop densities are well defined in one subunit. The substrate-entry loop adopts a closed conformation in this subunit. The result demonstrates that binding of equilin at the active site of 17beta-HSD1 is the basis for inhibition of E1-to-E2 reduction by this equine estrogen in vitro. One possible outcome of estrogen replacement therapy in vivo could be reduction of E2 levels in breast tissues and hence the reduced risk of estrogen-dependent breast cancer.

A novel 17beta-hydroxysteroid dehydrogenase in the fungus Cochliobolus lunatus: new insights into the evolution of steroid-hormone signalling

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) from the filamentous fungus Cochliobolus lunatus (17beta-HSDcl) catalyses the reduction of steroids and of several o- and p-quinones. After purification of the enzyme, its partial amino acid sequence was determined. A PCR fragment amplified with primers derived from peptide sequences was generated for screening the Coch. lunatus cDNA library. Three independent full-length cDNA clones were isolated and sequenced, revealing an 810-bp open reading frame encoding a 270-amino-acid protein. After expression in Escherichia coli and purification to homogeneity, the enzyme was found to be active towards androstenedione and menadione, and was able to form dimers of Mr 60000. The amino acid sequence of the novel 17beta-HSD demonstrated high homology with fungal carbonyl reductases, such as versicolorin reductase from Emericella nidulans (Aspergillus nidulans; VerA) and Asp. parasiticus (Ver1), polyhydroxynaphthalene reductase from Magnaporthe grisea, the product of the Brn1 gene from Coch. heterostrophus and a reductase from Colletotrichum lagenarium, which are all members of the short-chain dehydrogenase/reductase superfamily. 17beta-HSDcl is the first discovered fungal 17beta-hydroxysteroid dehydrogenase belonging to this family. The primary structure of this enzyme may therefore help to elucidate the evolutionary history of steroid dehydrogenases.

GDP-4-keto-6-deoxy-D-mannose epimerase/reductase from Escherichia coli, a key enzyme in the biosynthesis of GDP-L-fucose, displays the structural characteristics of the RED protein homology superfamily

BACKGROUND

The process of guanosine 5'-diphosphate L-fucose (GDP-L-fucose) biosynthesis is conserved throughout evolution from prokaryotes to man. In animals, GDP-L-fucose is the substrate of fucosyltransferases that participate in the biosynthesis and remodeling of glycoconjugates, including ABH blood group and Lewis-system antigens. The 'de novo' pathway of GDP-L-fucose biosynthesis from GDP-D-mannose involves a GDP-D-mannose 4,6 dehydratase (GMD) and a GDP-4-keto-6-deoxy-D-mannose epimerase/reductase (GMER). Neither of the catalytic mechanisms nor the three-dimensional structures of the two enzymes has been elucidated yet. The severe leukocyte adhesion deficiency (LAD) type II genetic syndrome is known to result from deficiencies in this de novo pathway.

RESULTS

The crystal structures of apo- and holo-GMER have been determined at 2.1 A and 2.2 A resolution, respectively. Each subunit of the homodimeric (2 x 34 kDa) enzyme is composed of two domains. The N-terminal domain, a six-stranded Rossmann fold, binds NADP+; the C-terminal domain (about 100 residues) displays an alpha/beta topology. NADP+ interacts with residues Arg12 and Arg36 at the adenylic ribose phosphate; moreover, a protein loop based on the Gly-X-X-Gly-X-X-Gly motif (where X is any amino acid) stabilizes binding of the coenzyme diphosphate bridge. The nicotinamide and the connected ribose ring are located close to residues Ser107, Tyr136 and Lys140, the putative GMER active-site center.

CONCLUSIONS

The GMER fold is reminiscent of that observed for UDP-galactose epimerase (UGE) from Escherichia coli. Consideration of the enzyme fold and of its main structural features allows assignment of GMER to the reductase-epimerase-dehydrogenase (RED) enzyme homology superfamily, to which short-chain dehydrogenase/reductases (SDRs) also belong. The location of the NADP+ nicotinamide ring at an interdomain cleft is compatible with substrate binding in the C-terminal domain.

The refined crystal structure of Drosophila lebanonensis alcohol dehydrogenase at 1.9 A resolution

Drosophila alcohol dehydrogenase (DADH; EC 1.1.1.1) is a NAD(H)-dependent oxidoreductase belonging to the short-chain dehydrogenases/reductases (SDR) family. This homodimeric enzyme catalyzes the dehydrogenation of alcohols to their respective ketones or aldehydes in the fruit-fly Drosophila, both for metabolic assimilation and detoxification purposes. The crystal structure of the apo form of DADH, one of the first biochemically characterized member of the SDR family, was solved at 1.9 A resolution by Patterson methods. The initial model was improved by crystallographic refinement accompanied by electron density averaging, R-factor=20.5%, R-free=23.8%.DADH subunits show an alpha/beta single domain structure with a characteristic NAD(H) binding motif (Rossmann fold). The peptide chain of a subunit is folded into a central eight-stranded beta-sheet flanked on each side by three alpha-helices. The dimers have local 2-fold symmetry. Dimer association is dominated by a four-helix bundle motif as well as two C-terminal loops from each subunit, which represent a unique structural feature in SDR enzymes with known structure. Three structural features are characteristic for the active site architecture. (1) A deep cavity which is covered by a flexible loop (33 residues) and the C-terminal tail (11 residues) from the neighboring subunit. The hydrophobic surface of the cavity is likely to increase the specificity of this enzyme towards secondary aliphatic alcohols. (2) The residues of the catalytic triad (Ser138, Tyr151, Lys155) are known to be involved in enzymatic catalysis in the first line. The Tyr151 OH group is involved in an ionic bond with the Lys155 side-chain. Preliminary electrostatic calculations have provided evidence that the active form of Tyr151 is a tyrosinate ion at physiological pH. (3) Three well-ordered water molecules in hydrogen bond distance to side-chains of the catalytic triad may be significant for the proton release steps in DADH catalysis.A ternary structure-based sequence alignment with ten members of the SDR family with known three-dimensional structure has suggested to define a model consisting of four groups of residues, which relates the observed low degree of sequence identity to quite similar folding patterns and nearly identical distributions of residues involved in catalysis.

Dramatic differences in the binding of UDP-galactose and UDP-glucose to UDP-galactose 4-epimerase from Escherichia coli

UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose and UDP-glucose during normal galactose metabolism. Within recent years the enzyme from Escherichia coli has been studied extensively by both biochemical and X-ray crystallographic techniques. One of several key features in the catalytic mechanism of the enzyme involves the putative rotation of a 4'-ketopyranose intermediate within the active site region. The mode of binding of UDP-glucose to epimerase is well understood on the basis of previous high-resolution X-ray crystallographic investigations from this laboratory with an enzyme/NADH/UDP-glucose abortive complex. Attempts to prepare an enzyme/NADH/UDP-galactose abortive complex always failed, however, in that UDP-glucose rather than UDP-galactose was observed binding in the active site. In an effort to prepare an abortive complex with UDP-galactose, a site-directed mutant protein was constructed in which Ser 124 and Tyr 149, known to play critical roles in catalysis, were substituted with alanine and phenylalanine residues, respectively. With this double mutant it was possible to crystallize and solve the three-dimensional structures of reduced epimerase in the presence of UDP-glucose or UDP-galactose to high resolution. This study represents the first direct observation of UDP-galactose binding to epimerase and lends strong structural support for a catalytic mechanism in which there is free rotation of a 4'-ketopyranose intermediate within the active site cleft of the enzyme.

Overview of a rational approach to design type I 17beta-hydroxysteroid dehydrogenase inhibitors without estrogenic activity: chemical synthesis and biological evaluation

Hormone-sensitive diseases such as breast cancer are health problems of major importance in North America and Europe. Endocrine therapies using antiestrogens for the treatment and the prevention of breast cancer are presently under clinical trials. Antiestrogens are drugs that compete with estrogens for the estrogen receptor without activating the transcription of estrogen-sensitive genes. However, an optimal blockade of estrogen action could ideally be achieved by a dual-action compound that would antagonize the estrogen receptor and inhibit the biosynthesis of estradiol. Type I 17beta-hydroxysteroid dehydrogenase (17beta-HSD) was chosen as a key steroidogenic target enzyme to inhibit the formation of estradiol, which is the most potent estrogen. This article describes a rational approach that could lead to the development of compounds that exhibit both actions. The chemical syntheses of estradiol derivatives bearing a bromoalkyl and a bromoalkylamide side chain at the 16alpha-position are summarized. Two parameters were studied for biological evaluation of our synthetic inhibitors: (1) the inhibition of estrone reduction into estradiol by type I 17beta-HSD, and (2) the proliferative/antiproliferative cell assays performed on the estrogen-sensitive ZR-75-1 breast tumor cell line. First, the substitution of the 16alpha-position of estradiol by bromoalkyl side chain led to potent inhibitors of type I 17beta-HSD, but the estrogenic activity remained. Secondly, an alkylamide functionality at the 16alpha- or 7alpha-position of estradiol cannot abolish the estrogenic activity without affecting considerably the inhibitory potency on type I 17beta-HSD. In conclusion, the best dual-action inhibitor synthesized showed an IC50 of 13 +/- 1 microM for type I 17beta-HSD, while displaying antiestrogenic activity at 1.0 microM. Despite the fact that we did not obtain an ideal dual-action blocker, we have optimized several structural parameters providing important structure-activity relationship.

Unusual charge stabilization of NADP+ in 17beta-hydroxysteroid dehydrogenase

Type 1 17beta-hydroxysteroid dehydrogenase (17beta-HSD1), a member of the short chain dehydrogenase reductase (SDR) family, is responsible for the synthesis of 17beta-estradiol, the biologically active estrogen involved in the genesis and development of human breast cancers. Here, we report the crystal structures of the H221L 17beta-HSD1 mutant complexed to NADP+ and estradiol and the H221L mutant/NAD+ and a H221Q mutant/estradiol complexes. These structures provide a complete picture of the NADP+-enzyme interactions involving the flexible 191-199 loop (well ordered in the H221L mutant) and suggest that the hydrophobic residues Phe192-Met193 could facilitate hydride transfer. 17beta-HSD1 appears to be unique among the members of the SDR protein family in that one of the two basic residues involved in the charge compensation of the 2'-phosphate does not belong to the Rossmann-fold motif. The remarkable stabilization of the NADP+ 2'-phosphate by the enzyme also clearly establishes its preference for this cofactor relative to NAD+. Analysis of the catalytic properties of, and estradiol binding to, the two mutants suggests that the His221-steroid O3 hydrogen bond plays an important role in substrate specificity.

Natural potent androgens: lessons from human genetic models

Male pseudohermaphroditism due to 17 beta-hydroxysteroid dehydrogenase-3 (17 beta-HSD-3) deficiency and 5 alpha-reductase-2 (5 alpha-RD-2) deficiency provides natural human genetic models to elucidate androgen actions. To date, five 17 beta-HSD isozymes have been cloned that catalyse the oxidoreduction of androstenedione and testosterone and dihydrotestosterone (DHT), oestrone and oestradiol. Mutations in the isozyme 17 beta-HSD-3 gene are responsible for male pseudohermaphroditism due to 17 beta-HSD deficiency. The type 3 isozyme preferentially catalyses the reduction of androstenedione to testosterone and is primarily expressed in the testes. Fourteen mutations in the 17 beta-HSD-3 gene have been identified from different ethnic groups. Affected males with the 17 beta-HSD-3 gene defect have normal wolffian structures but ambiguous external genitalia at birth. Many are raised as girls but virilize at the time of puberty and adopt a male gender role. Some develop gynaecomastia at puberty, which appears to be related to the testosterone/oestradiol ratio. Two 5 alpha-reductase (5 alpha-RD) isozymes, types 1 and 2, have been identified, which convert testosterone to the more potent androgen DHT. Mutations in the 5 alpha-RD-2 gene cause male pseudohermaphroditism, and 31 mutations in the 5 alpha-RD-2 gene have been reported from various ethnic groups. Such individuals also have normal wolffian structure but ambiguous external genitalia at birth and are raised as girls. Virilization occurs at puberty, often with a gender role change. The prostate remains infantile and facial hair is decreased. Balding has not been reported.

Homology model for the ligand-binding domain of the human estrogen receptor

We have modeled the ligand-binding domain (LBD) of the human estrogen receptor protein (hER) by homology to the known crystal structure of the LBD of the alpha isoform of human retinoate-X receptor (hRX). Alignment of hER with members of the nuclear receptor superfamily defined probable secondary structures which we used to constrain backbone torsion angles and hydrogen bonds. From published studies we identified key interactions between hER and estradiol to use to dock the hormone in its ligand-binding pocket. Since the hRX crystal structure corresponds to the unliganded form of the LBD, we adopted the "mousetrap" mechanism proposed by Renaud et al to predict the structure of the E2-bound hER. Refinement by molecular dynamics and energy minimization gave a model which matches well the known facts about the estradiol phamacophore. It also provides a possible explanation for how hER discriminates between estradiol and testosterone.

Regulation of sex steroid formation by interleukin-4 and interleukin-6 in breast cancer cells

Sex steroids play a predominant role in the development and differentiation of normal mammary gland as well as in the regulation of hormone-sensitive breast cancer growth. There is evidence suggesting that local intracrine formation of sex steroids from inactive precursors secreted by the adrenals namely, dehydroepiandrosterone (DHEA) and 4-androstenedione (4-dione) play an important role in the regulation of growth and function of peripheral target tissues, including the breast. Moreover, human breast carcinomas are often infiltrated by stromal/immune cells secreting a wide spectra of cytokines. These might in turn regulate the activity of both immune and neoplastic cells. The present study was designed to examine the action of cytokines on 17beta-hydroxysteroid dehydrogenase (17beta-HSD) and 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) activities in human breast cancer cells. The various types of human 17beta-HSD (five types) and 3beta-HSD (two types), because of their tissue- and cell-specific expression and substrate specificity, provide each cell with necessary mechanisms to control the level of intracellular active androgens and estrogens. We first investigated the effect of exposure to IL-4 and IL-6 on reductive and oxidative 17beta-HSD activities in both intact ZR-75-1 and T-47D human breast cancer cells. In ZR-75-1 cells, a 6 d exposure to IL-4 and IL-6 decreased E2-induced cell proliferation, the half maximal inhibitory effect being exerted at 88 and 26 pM, respectively. In parallel, incubation with IL-4 and IL-6 increased oxidative 17beta-HSD activity by 4.4- and 1.9-fold, respectively, this potent activity being observed at EC50 values of 22.8 and 11.3 pM, respectively. Simultaneously, reductive 17beta-HSD activity leading to E2 formation was decreased by 70 and 40% by IL-4 and IL-6, respectively. Moreover, IL-4 and IL-6 exerted the same regulatory effects on 17beta-HSD activities when testosterone and 4-dione were used as substrates, thus strongly suggesting the expression of the type 2 17beta-HSD ZR-75-1 cells. In contrast, in T-47D cells, IL-4 increased the formation of E2, whereas IL-6 exerts no effect on this parameter. However, we found that T-47D cells failed to convert testosterone efficiently into 4-DIONE, thus suggesting that there is little or no expression of type 2 17beta-HSD in this cell line. The present findings demonstrate that the potent regulatory effects of IL-4 and IL-6 on 17beta-HSD activities depend on the cell-specific gene expression of various types of 17beta-HSD enzymes. We have also studied the effect of cytokines on the regulation of the 3beta-HSD expression in both ZR-75-1 and T-47D human breast cancer cells. Under basal culture conditions, there is no 3beta-HSD activity detectable in these cells. However, exposure to IL-4 caused a rapid and potent induction of 3beta-HSD activity, whereas IL-6 failed to induce 3beta-HSD expression. Our data thus demonstrate that cytokines may play a crucial role in sex steroid biosynthesis from inactive adrenal precursors in human breast cancer cells.

Mechanistic roles of tyrosine 149 and serine 124 in UDP-galactose 4-epimerase from Escherichia coli

Synthesis and overexpression of a gene encoding Escherichia coli UDP-galactose 4-epimerase and engineered to facilitate cassette mutagenesis are described. General acid-base catalysis at the active site of this epimerase has been studied by kinetic and spectroscopic analysis of the wild-type enzyme and its specifically mutated forms Y149F, S124A, S124V, and S124T. The X-ray crystal structure of Y149F as its abortive complex with UDP-glucose is structurally similar to that of the corresponding wild-type complex, except for the absence of the phenolic oxygen of Tyr 149. The major effects of mutations are expressed in the values of kcat and kcat/Km. The least active mutant is Y149F, for which the value of kcat is 0.010% of that of the wild-type epimerase. The activity of S124A is also very low, with a kcat value that is 0.035% of that of the native enzyme. The values of Km for Y149F and S124A are 12 and 21% of that of the wild-type enzyme, respectively. The value of kcat for S124T is about 30% of that of the wild-type enzyme, and the value of Km is similar to that of the native enzyme. The reactivities of the mutants in UMP-dependent reductive inactivation by glucose are similarly affected, with kobs being decreased by 6560-, 370-, and 3.4-fold for Y149F, S124A, and S124T, respectively. The second-order rate constants for reductive inactivation by NaBH3CN, which does not require general base catalysis, are similar to that for the native enzyme in the cases of S124A, S124T, and S124V. However, Y149F reacts with NaBH3CN 12-20-fold faster than the wild-type enzyme at pH 8.5 and 7.0, respectively. The increased rate for Y149F is attributed to the weakened charge-transfer interaction between Phe 149 and NAD+, which is present with Tyr 149 in the wild-type enzyme. The charge-transfer band is present in the serine mutants, and its intensity at 320 nm is pH-dependent. The pH dependencies of A320 showed that the pKa values for Tyr 149 are 6.08 for the wild-type epimerase, 6.71 for S124A, 6.86 for S124V, and 6.28 for S124T. The low pKa value for Tyr 149 is attributed mainly to the positive electrostatic field created by NAD+ and Lys 153 (4.5 kcal mol-1) and partly to hydrogen bonding with Ser 124 (1 kcal mol-1). The pKa of Tyr 149 is the same as the kinetic pKa for the Bronsted base that facilitates hydride transfer to NAD+. We concluded that Tyr 149 provides the driving force for general acid-base catalysis, with Ser 124 playing an important role in mediating proton transfer.

Steroid recognition and regulation of hormone action: crystal structure of testosterone and NADP+ bound to 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase

BACKGROUND

Mammalian 3 alpha-hydroxysteroid dehydrogenases (3 alpha-HSDs) modulate the activities of steroid hormones by reversibly reducing their C3 ketone groups. In steroid target tissues, 3 alpha-HSDs act on 5 alpha-dihydrotestosterone, a potent male sex hormone (androgen) implicated in benign prostate hyperplasia and prostate cancer. Rat liver 3 alpha-HSD belongs to the aldo-keto reductase (AKR) superfamily and provides a model for mammalian 3 alpha-, 17 beta- and 20 alpha-HSDs, which share > 65% sequence identity. The determination of the structure of 3 alpha-HSD in complex with NADP+ and testosterone (a competitive inhibitor) will help to further our understanding of steroid recognition and hormone regulation by mammalian HSDs.

RESULTS

We have determined the 2.5 A resolution crystal structure of recombinant rat liver 3 alpha-HSD complexed with NADP+ and testosterone. The structure provides the first picture of an HSD ternary complex in the AKR superfamily, and is the only structure to date of testosterone bound to a protein. It reveals that the C3 ketone in testosterone, corresponding to the reactive group in a substrate, is poised above the nicotinamide ring which is involved in hydride transfer. In addition, the C3 ketone forms hydrogen bonds with two active-site residues implicated in catalysis (Tyr55 and His117).

CONCLUSIONS

The active-site arrangement observed in the 3 alpha-HSD ternary complex structure suggests that each positional-specific and stereospecific reaction catalyzed by an HSD requires a particular substrate orientation, the general features of which can be predicted. 3 alpha-HSDs are likely to bind substrates in a similar manner to the way in which testosterone is bound in the ternary complex, that is with the A ring of the steroid substrate in the active site and the beta face towards the nicotinamide ring to facilitate hydride transfer. In contrast, we predict that 17 beta-HSDs will bind substrates with the D ring of the steroid in the active site and with the alpha face towards the nicotinamide ring. The ability to bind substrates in only one or a few orientations could determine the positional-specificity and stereospecificity of each HSD. Residues lining the steroid-binding cavities are highly variable and may select these different orientations.

The estradiol pharmacophore: ligand structure-estrogen receptor binding affinity relationships and a model for the receptor binding site

The accumulated knowledge on the binding of estradiol (E2) and its analogs and the results of affinity-labeling studies have been reviewed and are used herein to derive a binding site model for the estrogen receptor (ER). Estradiol is nonpolar and hydrophobic, except at its molecular termini. Most of its skeletal flexibility resides in the B-ring, and it probably binds in a low-energy conformation. The phenolic OH group in the A-ring contributes about 1.9 kcal/mol to the binding free energy and probably acts primarily as a hydrogen bond donor. The 17 beta-hydroxyl group in the D-ring contributes approximately 0.6 kcal/mol to the binding and probably acts as a hydrogen bond acceptor, either directly or via a water molecule. There also seems to be a degree of flexibility in the region of the receptor that encompasses the D-ring. The aromatic ring contributes about 1.5 kcal/mol, probably through weak polar interactions with receptor residues that contact the beta-face of the steroid. The receptor seems to surround the ligand, so that all four rings contribute significantly to binding. Small hydrophobic substituents enhance binding affinity at positions 4, 12 beta, 14, and 16 alpha; whereas, larger hydrophobic substituents are tolerated at positions 7 alpha, 11 beta, and 17 alpha. In general, the ER is intolerant of polar substituents. Based on E2 analogs bearing affinity-labeling groups, cysteine residues might be present in the binding site in the area of C-4, C-17 alpha, and C-17 beta, and a lysine residue might be located near C-16. Models that represent the limits of deformability of the ligand binding site, the position of preformed pockets, and space occupied by the receptor are presented. The various elements in this model for the binding of steroidal estrogens by the estrogen receptor are consistent with evidence emerging from the crystal structures of related nuclear hormone receptor ligand complexes.

The key role of 17 beta-hydroxysteroid dehydrogenases in sex steroid biology

17 beta-Hydroxysteroid dehydrogenase (17 beta-HSD) controls the last step in the formation of all androgens and all estrogens. This crucial role of 17 beta-HSD is performed by at least five 17 beta-HSD isoenzymes having individual cell-specific expression, substrate specificity, regulation mechanisms, and reductive or oxidative catalytic activity. Both estrogenic and androgenic 17 beta-HSD activities were found in all 25 rhesus monkey and 15 human peripheral intracrine tissues examined. Type 1 17 beta-HSD is a protein of 327 amino acids catalyzing the formation of 17 beta-estradiol from estrone. Its x-ray structure was the first to be determined among mammalian steroidogenic enzymes. Initially crystallized with NAD, the crystal structure of type 1 17 beta-HSD has just been determined as a complex with 17 beta-estradiol, thereby illustrating the conformation of the substrate-binding site. Type 2 17 beta-HSD degrades 17 beta-estradiol into estrone and testosterone into androstenedione, and type 4 17 beta-HSD mainly degrades 17 beta-estradiol into estrone and androst-5-ene-3 beta, 17 beta-diol into dehydroepiandrosterone. Types 3 and 5 17 beta-HSD, on the other hand, catalyze the formation of testosterone from androstenedione in the testis and peripheral tissues, respectively. The various types of human 17 beta-HSD, because of their tissue-specific expression and substrate specificity, provide each peripheral cell with the necessary mechanisms to control the level of intracellular androgens and/or estrogens, a new area of hormonal control that we call intracrinology.