Estrogen Sulfatase

Estrogen sulfatase is a microsomal enzyme and is ubiquitously distributed in several mammalian tissues, among which the liver, placenta, and endocrine tissues exhibit relatively high activity. Because the major circulating precursors of estrogen are estrone 3-sulfate and dehydroepiandrosterone 3-sulfate, estrogen sulfatase plays an important role not only in their incorporation and metabolism, but also in the controls of estrogen activity by regulating the binding potential of estrogen as to its receptor through sulfoconjugation and desulfation reactions. Accordingly, an increase in sulfoconjugation through transfection of the sulfotransferase gene or inhibition of estrogen sulfatase by specific inhibitors has been successfully applied to abolish the estrogen activity in estrogen-dependent breast cancer- and uterine endometrial adenocarcinoma-derived cells. Inhibitors of estrogen sulfatase are expected to be developed as new drugs for estrogen-dependent cancer therapy, particularly in postmenopausal women.

Highly enantioselective stereo-inverting sec-alkylsulfatase activity of hyperthermophilic Archaea

rac-sec-Alkyl sulfate esters 1a-8a were resolved in low to excellent enantioselectivities with E-values up to >200 using whole cells of aerobically-grown hyperthermophilic sulfur-metabolizers, such as Sulfolobus solfataricus DSM 1617, Sulfolobus shibatae DSM 5389 and, most notably, Sulfolobus acidocaldarius DSM 639. Significantly enhanced selectivities were obtained using cells grown on sucrose-enriched Brock-medium. The stereochemical course of this biohydrolysis was shown to proceed with strict inversion of configuration, thus the preferred (R)-enantiomers were converted into the corresponding (S)-sec-alcohols to furnish a homochiral product mixture.

Three‐Dimensional Structures of Sulfatases

The sulfatase family of enzymes catalyzes the hydrolysis of sulfate ester bonds of a wide variety of substrates. Nine human sulfatase proteins and their genes have been identified, many of which are associated with genetic disorders leading to reduction or loss of function of the corresponding enzyme. A catalytic cysteine residue, strictly conserved in prokaryotic and eukaryotic sulfatases, is modified posttranslationally into a formylglycine. Hydroxylation of the formylglycine residue by a water molecule forming the activated hydroxylformylglycine (a formylglycine hydrate or a gem-diol) is a necessary step for sulfatase activity of the enzyme. Crystal structures of three human sulfatases, arylsulfatases A and B (ARSA and ARSB) and C, also known as steroid sulfatase or estrone/dehydroepiandrosterone sulfatase (ES), have been determined. In addition, the crystal structure of a homologous bacterial arylsulfatase from Pseudomonas aeruginosa (PAS) is also available. While ARSA, ARSB, and PAS are water-soluble enzymes, ES has a hydrophobic domain and is presumed to be bound to the endoplasmic reticulum membrane. This chapter compares and contrasts four sulfatase structures and revisits the proposed catalytic mechanism in light of available structural and functional data. Examination of the ES active site reveals substrate-specific interactions previously identified in another steroidogenic enzyme. Possible influence of the lipid bilayer in substrate capture and recognition by ES is described. Finally, mapping the genetic mutations into the ES structure provides an explanation for the loss of enzyme function in X-linked ichthyosis.

Highly Enantioselective sec-Alkyl Sulfatase Activity of Sulfolobus acidocaldarius DSM 639

[reaction: see text] rac-sec-Alkyl sulfate esters 1a-4a were resolved in high enantioselectivities with E-values up to >200 using whole cells of aerobically grown Sulfolobus acidocaldarius DSM 639. The stereochemical course of this biohydrolysis was shown to proceed with strict inversion of configuration; thus, the preferred (R)-enantiomers were converted into the corresponding (S)-sec-alcohols to furnish a homochiral product mixture.

Sulfatases: Structure, Mechanism, Biological Activity, Inhibition, and Synthetic Utility

Sulfatases, which cleave sulfate esters in biological systems, play a key role in regulating the sulfation states that determine the function of many physiological molecules. Sulfatase substrates range from small cytosolic steroids, such as estrogen sulfate, to complex cell-surface carbohydrates, such as the glycosaminoglycans. The transformation of these molecules has been linked with important cellular functions, including hormone regulation, cellular degradation, and modulation of signaling pathways. Sulfatases have also been implicated in the onset of various pathophysiological conditions, including hormone-dependent cancers, lysosomal storage disorders, developmental abnormalities, and bacterial pathogenesis. These findings have increased interest in sulfatases and in targeting them for therapeutic endeavors. Although numerous sulfatases have been identified, the wide scope of their biological activity is only beginning to emerge. Herein, accounts of the diversity and growing biological relevance of sulfatases are provided along with an overview of the current understanding of sulfatase structure, mechanism, and inhibition.

Crystal structure of the alkylsulfatase AtsK: insights into the catalytic mechanism of the Fe(II) alpha-ketoglutarate-dependent dioxygenase superfamily

The alkylsulfatase AtsK from Pseudomonas putida S-313 belongs to the widespread and versatile non-heme iron(II) alpha-ketoglutarate-dependent dioxygenase superfamily and catalyzes the oxygenolytic cleavage of a variety of different alkyl sulfate esters to the corresponding aldehyde and sulfate. The enzyme is only expressed under sulfur starvation conditions, providing a selective advantage for bacterial growth in soils and rhizosphere. Here we describe the crystal structure of AtsK in the apo form and in three complexes: with the cosubstrate alpha-ketoglutarate, with alpha-ketoglutarate and iron, and finally with alpha-ketoglutarate, iron, and an alkyl sulfate ester used as substrate in catalytic studies. The overall fold of the enzyme is closely related to that of the taurine/alpha-ketoglutarate dioxygenase TauD and is similar to the fold observed for other members of the enzyme superfamily. From comparison of these structures with the crystal structure of AtsK and its complexes, we propose a general mechanism for the catalytic cycle of the alpha-ketoglutarate-dependent dioxygenase superfamily.

Post-translational Formylglycine Modification of Bacterial Sulfatases by the Radical S-Adenosylmethionine Protein AtsB

C(alpha)-Formylglycine (FGly) is the catalytic residue of sulfatases. FGly is generated by post-translational modification of a cysteine (prokaryotes and eukaryotes) or serine (prokaryotes) located in a conserved (C/S)XPXR motif. AtsB of Klebsiella pneumoniae is directly involved in FGly generation from serine. AtsB is predicted to belong to the newly discovered radical S-adenosylmethionine (SAM) superfamily. By in vivo and in vitro studies we show that SAM is the critical co-factor for formation of a functional AtsB.SAM.sulfatase complex and for FGly formation by AtsB. The SAM-binding site of AtsB involves (83)GGE(85) and possibly also a juxtaposed FeS center coordinated by Cys(39) and Cys(42), as indicated by alanine scanning mutagenesis. Mutation of these and other conserved cysteines as well as treatment with metal chelators fully impaired FGly formation, indicating that all three predicted FeS centers are crucial for AtsB function. It is concluded that AtsB oxidizes serine to FGly by a radical mechanism that is initiated through reductive cleavage of SAM, thereby generating the highly oxidizing deoxyadenosyl radical, which abstracts a hydrogen from the serine-C(beta)H(2)-OH side chain.

Identification and determination of phase II nabumetone metabolites by high-performance liquid chromatography with photodiode array and mass spectrometric detection

Chromatographic analyses play an important role in the identification and determination of phase I and phase II drug metabolites. While the chemical standards of phase I metabolites are usually available from commercial sources or by various synthetic, degradation or isolation methods, the phase II drug metabolites have usually more complicated structures, their standards are in general inaccessible and their identification and determination require a comprehensive analytical approach involving the use of xenobiochemical methods and the employment of hyphenated analytical techniques. In this work, various high-performance liquid chromatography (HPLC) methods were employed in the evaluation of xenobiochemical experiments leading to the identification and determination of phase II nabumetone metabolites. Optimal conditions for the quantitative enzymatic deconjugation of phase II metabolites were found for the samples of minipig bile, small intestine contents and urine. Comparative HPLC analyses of the samples of above-mentioned biomatrices and of the same biomatrices after their enzymatic treatment using beta-glucuronidase and arylsulfatase afforded the qualitative and quantitative information about phase II nabumetone metabolites. Hereby, three principal phase II nabumetone metabolites (ether glucuronides) were discovered in minipig's body fluids and their structures were confirmed using liquid chromatography (LC)-electrospray ionization mass spectrometric (MS) analyses.

Gender differences in the levels of bisphenol A metabolites in urine

The metabolism of bisphenol A (BPA), a suspected endocrine disruptor, should be considered for monitoring human exposure to BPA, because the conjugation with beta-D-glucuronide and sulfate reduces the estrogenic activity. In this study, BPA levels in 30 healthy Koreans (men, N=15, 42.6+/-2.4 years; women, N=15, 43.0+/-2.7 years) were analyzed from urine treated with/without beta-glucuronidase and/or sulfatase by an RP-HPLC with fluorescence detection. The total BPA concentrations including free BPA and the urinary conjugates were similar in men and women (2.82+/-0.73 and 2.76+/-0.54 ng ml(-1), respectively), but gender differences were found in the levels of urinary BPA conjugates. Men had significantly higher levels of BPA-glucuronide (2.34+/-0.85 ng ml(-1)) than women (1.00+/-0.34 ng ml(-1)), whereas women had higher levels of BPA-sulfate (1.20+/-0.32 ng ml(-1)) than men (0.49+/-0.27 ng ml(-1)).

Domain-specific modification of heparan sulfate by Qsulf1 modulates the binding of the bone morphogenetic protein antagonist Noggin

We have reported previously that Noggin is a heparin-binding protein and associates with the cell surface through heparan sulfate proteoglycans, where it remains functional for the binding of bone morphogenetic proteins (BMPs). Here we report that the binding of Noggin to the cell surface is highly selective for heparan sulfate and that specific structural features are required for the interaction. Noggin binds most efficiently to heparin sequences composed of 10 or more monosaccharides; N-, 6-O-, and 2-O-sulfates contribute to this interaction. In addition, we have shown that the developmentally regulated endosulfatase Qsulf1 selectively removes sulfate groups from the 6-O position of sugars within the most highly sulfated S domains of heparan sulfate, whereas 6-O-sulfates in the NA/NS domains are not substrates for the enzyme. The activity of Qsulf1 in cells in culture results in the release of Noggin from the cell surface and a restoration of BMP responsiveness to the cells. This shows that Noggin binds to the S domains of heparan sulfate and provides evidence that, in addition to modulating Wnt signaling in vivo by the release of heparan sulfate bound Wnt, Qsulf1 also modulates BMP signaling by the release of surface-bound Noggin.

The design, synthesis, and in vitro biochemical evaluation of a series of esters of 4-[(aminosulfonyl)oxy]benzoate as novel and highly potent inhibitors of estrone sulfatase

We report the initial results of our study into the use of a potential transition-state (TS) of the reaction catalysed by the enzyme estrone sulfatase (ES) in the design of a series of cyclic esters of 4-[(aminosulfonyl)oxy]benzoate as novel inhibitors of ES. The results of the study show that these compounds are some of the most potent inhibitors known todate, possessing greater inhibitory activity than the three standard compounds: 4-methylcoumarin-7-O-sulfamate (COUMATE); the tricyclic derivative of COUMATE, namely 667-COUMATE (which is in Phase I of clinical trials) and; the steroidal inhibitor estrone-3-O-sulfamate (EMATE).

QSulf1 remodels the 6-O sulfation states of cell surface heparan sulfate proteoglycans to promote Wnt signaling

The 6-O sulfation states of cell surface heparan sulfate proteoglycans (HSPGs) are dynamically regulated to control the growth and specification of embryonic progenitor lineages. However, mechanisms for regulation of HSPG sulfation have been unknown. Here, we report on the biochemical and Wnt signaling activities of QSulf1, a novel cell surface sulfatase. Biochemical studies establish that QSulf1 is a heparan sulfate (HS) 6-O endosulfatase with preference, in particular, toward trisulfated IdoA2S-GlcNS6S disaccharide units within HS chains. In cells, QSulf1 can function cell autonomously to remodel the sulfation of cell surface HS and promote Wnt signaling when localized either on the cell surface or in the Golgi apparatus. QSulf1 6-O desulfation reduces XWnt binding to heparin and HS chains of Glypican1, whereas heparin binds with high affinity to XWnt8 and inhibits Wnt signaling. CHO cells mutant for HS biosynthesis are defective in Wnt-dependent Frizzled receptor activation, establishing that HS is required for Frizzled receptor function. Together, these findings suggest a two-state "catch or present" model for QSulf1 regulation of Wnt signaling in which QSulf1 removes 6-O sulfates from HS chains to promote the formation of low affinity HS-Wnt complexes that can functionally interact with Frizzled receptors to initiate Wnt signal transduction.

Structure of human estrone sulfatase suggests functional roles of membrane association

Estrone sulfatase (ES; 562 amino acids), one of the key enzymes responsible for maintaining high levels of estrogens in breast tumor cells, is associated with the membrane of the endoplasmic reticulum (ER). The structure of ES, purified from the microsomal fraction of human placentas, has been determined at 2.60-A resolution by x-ray crystallography. This structure shows a domain consisting of two antiparallel alpha-helices that protrude from the roughly spherical molecule, thereby giving the molecule a "mushroom-like" shape. These highly hydrophobic helices, each about 40 A long, are capable of traversing the membrane, thus presumably anchoring the functional domain on the membrane surface facing the ER lumen. The location of the transmembrane domain is such that the opening to the active site, buried deep in a cavity of the "gill" of the "mushroom," rests near the membrane surface, thereby suggesting a role of the lipid bilayer in catalysis. This simple architecture could be a prototype utilized by the ER membrane in dictating the form and the function of ER-resident enzymes.

The heparin/heparan sulfate 2-O-sulfatase from Flavobacterium heparinum. A structural and biochemical study of the enzyme active site and saccharide substrate specificity

In the previous paper (Myette, J. R., Shriver, Z., Claycamp, C., McLean, M. W., Venkataraman, G., and Sasisekharan, R. (2003) J. Biol. Chem. 278, 12157-12166), we described the molecular cloning, recombinant expression, and preliminary biochemical characterization of the heparin/heparan sulfate 2-O-sulfatase from Flavobacterium heparinum. In this paper, we extend our structure-function investigation of the 2-O-sulfatase. First, we have constructed a homology-based structural model of the enzyme active site, using as a framework the available crystallographic data for three highly related arylsulfatases. In this model, we have identified important structural parameters within the enzyme active site relevant to enzyme function, especially as they relate to its substrate specificity. By docking various disaccharide substrates, we identified potential structural determinants present within these substrates that would complement this unique active site architecture. These determinants included the position and number of sulfates present on the glucosamine, oligosaccharide chain length, the presence of a Delta4,5-unsaturated double bond, and the exolytic versus endolytic potential of the enzyme. The predictions made from our model provided a structural basis of substrate specificity originally interpreted from the biochemical and kinetic data. Our modeling approach was further complemented experimentally using peptide mapping in tandem with mass spectrometry and site-directed mutagenesis to physically demonstrate the presence of a covalently modified cysteine (formylglycine) within the active site. This combinatorial approach of structure modeling and biochemical studies provides insight into the molecular basis of enzyme function.

The heparin/heparan sulfate 2-O-sulfatase from Flavobacterium heparinum. Molecular cloning, recombinant expression, and biochemical characterization

Heparan sulfate glycosaminoglycans are structurally complex polysaccharides critically engaged in a wide range of cell and tissue functions. Any structure-based approach to study their respective biological functions is facilitated by the use of select heparan sulfate glycosaminoglycan-degrading enzymes with unique substrate specificities. We recently reported of one such enzyme, the Delta4,5-glycuronidase cloned from Flavobacterium heparinum and recombinantly expressed in Escherichia coli (Myette, J. R., Shriver, Z., Kiziltepe, T., McLean, M. W., Venkataraman, G., and Sasisekharan, R. (2002) Biochemistry 41, 7424-7434). In this study, we likewise report the molecular cloning of the 2-O-sulfatase from the same bacterium and its recombinant expression as a soluble, highly active enzyme. At the protein level, the flavobacterial 2-O-sulfatase possesses considerable sequence homology to other members of a large sulfatase family, especially within its amino terminus, where the highly conserved sulfatase domain is located. Within this domain, we have identified by sequence homology the critical active site cysteine predicted to be chemically modified as a formylglycine in vivo. We also present a characterization of the biochemical properties of the enzyme as it relates to optimal in vitro reaction conditions and a kinetic description of its substrate specificity. In particular, we demonstrate that in addition to the fact that the enzyme exclusively hydrolyzes the sulfate at the 2-O-position of the uronic acid, it also exhibits a kinetic preference for highly sulfated glucosamines within each disaccharide unit, especially those possessing a 6-O-sulfate. The sulfatase also displays a clear kinetic preference for disaccharides with beta1-->4 linkages but is able, nevertheless, to hydrolyze unsaturated, 2-O-sulfated chondroitin disaccharides. Finally, we describe the substrate-product relationship of the 2-O-sulfatase to the Delta4,5-glycuronidase and the analytical value of using both of these enzymes in tandem for elucidating heparin/heparan sulfate composition.

Identification of formylglycine in sulfatases by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

C(alpha)-Formylglycine, the catalytic amino acid residue in the active site of sulfatases, is generated by post-translational modification of a cysteine or serine residue. We describe a highly sensitive procedure for the detection of C(alpha)-formylglycine-containing peptides in tryptic digests of sulfatase proteins. The protocol is based on the formation of hydrazone derivatives of C(alpha)-formylglycine-containing peptides when using dinitrophenylhydrazine as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The hydrazone derivatives desorb and ionize with high efficiency and can be detected in the sub-femtomole range. The presence of C(alpha)-formylglycine is indicated by a mass increment of 180.13 u, corresponding to the hydrazone moiety, and also by a unique C-terminal fragment ion, characteristic of sulfatases, that becomes prominent in MALDI post-source decay mass spectra of the hydrazone derivatives.

Cloning and characterization of two extracellular heparin-degrading endosulfatases in mice and humans

Here we report the cloning of a full-length cDNA encoding the human ortholog (HSulf-1) of the developmentally regulated putative sulfatases QSulf-1 (Dhoot, G. K., Gustafsson, M. K., Ai, X., Sun, W., Standiford, D. M., and Emerson, C. P., Jr. (2001) Science 293, 1663-1666) and RSulfFP1 (Ohto, T., Uchida, H., Yamazaki, H., Keino-Masu, K., Matsui, A., and Masu, M. (2002) Genes Cells 7, 173-185) as well as a cDNA encoding a closely related protein, designated HSulf-2. We have also obtained cDNAs for the mouse orthologs of both Sulfs. We demonstrate that the proteins encoded by both classes of cDNAs are endoproteolytically processed in the secretory pathway and are released into conditioned medium of transfected CHO cells. We demonstrate that the mammalian Sulfs exhibit arylsulfatase activity with a pH optimum in the neutral range; moreover, they can remove sulfate from the C-6 position of glucosamine within specific subregions of intact heparin. Taken together, our results establish that the mammalian Sulfs are extracellular endosulfatases with strong potential for modulating the interactions of heparan sulfate proteoglycans in the extracellular microenvironment.

Estrone 3-sulfate mimics, inhibitors of estrone sulfatase activity: homology model construction and docking studies

Steroid sulfatase (STS) is a new target for the endocrine therapy of breast cancer. To ascertain some of the requirements for inhibition of estrone sulfatase activity, a number of novel analogues of estrone 3-O-sulfate possessing sulfate surrogates were synthesized and evaluated as inhibitors of estrone sulfatase (STS) in comparison to a lead inhibitor, estrone-3-O-methylthiophosphonate (E1-3-MTP). Using a selective enzyme digestion, one of the diastereoisomers of this compound, (R(p))-E1-3-MTP, could be prepared and evaluated. From structure-activity studies, we show that chirality at the phosphorus atom, hydrophobicity, basicity, size, and charge all influence the ability of a compound to inhibit estrone sulfatase activity. Of these, hydrophobicity seems to be the most important since simple, active nonsteroidal inhibitors, based on 5,6,7,8-tetrahydronaphth-2-ol (THN), can be prepared, provided that they are lipophilic enough to partition into a nonpolar environment. Also, a negatively charged group is favorable for optimal binding, although it appears that the presence of a potentially cleavable group can compensate for lack of charge in certain cases. A homology model of STS has been constructed from the STS sequence, and molecular docking studies of inhibitors have been performed to broaden the understanding of enzyme/inhibitor interactions. This model clearly shows the positions of the key amino acid residues His136, His290, Lys134, and Lys368 in the putative catalytic region of the formylglycine at position 75, with residues Asp35, Asp36, Asp342, and Gln343 as ligands in the coordination sphere of the magnesium ion. Docking studies using the substrate and estrone-3-sulfate mimics that are active inhibitors indicate they are positioned in the area of proposed catalysis, confirming the predictive power of the model.

Characterization and expression of human bifunctional 3'-phosphoadenosine 5'-phosphosulphate synthase isoforms

Sulphonation, a fundamental process essential for normal growth and development, requires the sulphonate donor molecule 3'-phosphoadenosine 5'-phosphosulphate (PAPS), which is produced from ATP and inorganic sulphate by the bifunctional enzyme PAPS synthase. In humans, two genes encode isoenzymes that are 77% identical at the amino acid level, and alternative splicing creates two subtypes of PAPS synthase 2. The question as to whether distinctions in amino acid composition are reflected in differences in activity has been examined. The specific activity of the PAPS synthase 2 subtypes is 10- to 15-fold higher than that for PAPS synthase 1. The greater catalytic efficiency of the PAPS synthase 2 subtypes is demonstrated further by the 3- to 6-fold higher k(cat)/K(m) ratios for ATP and inorganic sulphate as compared with the ratios for PAPS synthase 1. In humans, PAPS synthase 1 is expressed ubiquitously, and is the dominant isoform in most tissues, whereas expression of the PAPS synthase 2 subtypes is variable and tissue-specific. It is noteworthy that, similar to other human tissues, PAPS synthase 1 also appears to be the dominant isoform expressed in cartilage. The latter finding initially created a conundrum, since there is a specific human dwarfing disorder that is known to be caused by a mutation in the PAPS synthase 2 gene. This apparent enigma would seem to be resolved by examination of cartilage from guinea-pigs as an animal model. Similar to humans, cartilage from mature animals predominantly expresses PAPS synthase 1. In contrast, expression of PAPS synthase 1 is relatively low in the cartilage of immature guinea-pigs, including the growth plate of long bones, whereas PAPS synthase 2 is the highly expressed isoenzyme.

Evidence for the mechanism of the irreversible inhibition of oestrone sulphatase (ES) by aminosulphonate based compounds

In our search for the mechanism of the enzyme oestrone sulphatase (ES) we have synthesised and evaluated a number of compounds that were predicted to possess some inhibitory activity. Some of these compounds were indeed found to be inhibitors of ES, whilst other compounds were not. From a consideration of the structure-activity relationship (SAR) of the inhibitors and non-inhibitors of this enzyme, we discovered a factor which we now believe is the main inhibitory moiety within the aminosulphonated inhibitors. We therefore report the results of our study into a series of phenyl and alkyl sulphamated compounds as inhibitors of ES. The results of the study show that the substituted phenyl sulphamates are potent inhibitors, whereas the alkyl compounds are, in general, non-inhibitors. Using the results of our SAR study, we postulate the probable mechanism for the irreversible and reversible inhibition of ES, and rationalise the role of the different physicochemical factors in the inhibition of this crucial enzyme.

Synthesis and biochemical evaluation of novel and potent inhibitors of the enzyme oestrone sulphatase (ES)

In an effort to investigate the structural requirements for the inhibition of the enzyme oestrone sulphatase (ES), we have previously undertaken extensive structure-activity relationship studies. Using the data from molecular modelling and structure-activity relationship determination studies, we have designed a number of compounds based upon 4-sulphamated phenyl ketones. Here, we report the results of our study into a series of these compounds as potential inhibitors of ES. The results of the study show that these compounds are potent inhibitors the possessing greater inhibitory activity than 4-methylcoumarin-7-O-sulphamate derivative (COUMATE) (a potent non-steroidal inhibitor), but are weaker than oestrone-3-sulphamate (EMATE) and the recently reported 667- and 669-COUMATE, however, they provide good lead compounds in the search for potent inhibitors of ES. Furthermore, the compounds are observed to be irreversible inhibitors. From the consideration of the structure-activity relationship of these novel compounds, we have attempted to rationalise the significance of the log P factor in the inhibition of ES and suggest that a log P requirement of approximately 3.5 aids the inhibition through the rapid expulsion of the carbon backbone from the active site. We also propose that the same factor is responsible for the hydrolysis of oestrone sulphate reaction, appearing to be an irreversible process.

Isotope effects in the study of phosphoryl and sulfuryl transfer reactions

Phosphoryl and sulfuryl transfer reactions are essential biological processes. Multiple kinetic isotope effects have provided significant insights into the transition states of these reactions. The data are reviewed for the uncatalyzed reactions of phosphate and sulfate monoesters and for a number of enzymatic phosphoryl transfer reactions. Uncatalyzed phosphoryl and sulfuryl hydrolysis reactions are found to have very similar transition states. The phosphoryl transfer reaction catalyzed by protein-tyrosine phosphatases proceeds by a transition state very similar to that of the uncatalyzed reaction, but isotope effect data reveal an interesting interplay between the conserved arginine and enzyme dynamics involving general acid catalysis.

Determination and use of a transition state for the enzyme estrone sulfatase (ES) from a proposed reaction mechanism

Using the postulated mechanism for the enzyme estrone sulfatase (ES), we have determined a possible transition state for the reaction catalysed by ES as a representation of the active site. Using the derived structure, we have undertaken the molecular modelling of several steroidal and non-steroidal inhibitors in an attempt to rationalise the inhibitory activity of a number of potent inhibitors.

Purification, characterization and crystallization of human placental estrone/dehydroepiandrosterone sulfatase, a membrane-bound enzyme of the endoplasmic reticulum

Estrone (E1)/dehydroepiandrosterone (DHEA) sulfatase (ES/DHEAS) catalyzes the hydrolysis of E1 and DHEA-sulfates releasing unconjugated steroids. ES is a component of the three-enzyme system that has been implicated in intracrine biosynthesis of estradiol, hence, proliferation of hormone dependent breast tumors. ES is bound to the membrane of the endoplasmic reticulum, presumably through multiple transmembrane and other membrane anchoring segments. The highly hydrophobic nature of the enzyme has so far prevented its purification to homogeneity in quantities sufficient for crystallization. We report here the purification, biochemical characterization and crystallization of the full-length, active form of the enzyme from the membrane bound fraction of human placenta. Our results demonstrate that the key to successful purification and growth of diffraction quality crystals of this difficult membrane bound enzyme is the exploitation of optimal solubilization and detergent conditions to protect the structural and functional integrity of the molecule, thereby preventing nonspecific aggregation and other instabilities. This work paves the way for the first structural study of a membrane bound human sulfatase and subsequent rational design of inhibitors for use as anti-tumor agents.