Sulfatase activities towards the regulation of cell metabolism and signaling in mammals

In higher vertebrates, sulfatases belong to a conserved family of enzymes that are involved in the regulation of cell metabolism and in developmental cell signaling. They cleave the sulfate from sulfate esters contained in hormones, proteins, and complex macromolecules. A highly conserved cysteine in their active site is post-translationally converted into formylglycine by the formylglycine-generating enzyme encoded by SUMF1 (sulfatase modifying factor 1). This post-translational modification activates all sulfatases. Sulfatases are extensively glycosylated proteins and some of them follow trafficking pathways through cells, being secreted and taken up by distant cells. Many proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates, which are sulfatase substrates. Indeed, sulfatases operate as decoding factors for a large amount of biological information contained in the structures of the sulfated sugar chains that are covalently linked to proteins and lipids. Modifications to these sulfate groups have pivotal roles in modulating specific signaling pathways and cell metabolism in mammals.

Human sulfatases: a structural perspective to catalysis

The sulfatase family of enzymes catalyzes hydrolysis of sulfate ester bonds of a wide variety of substrates. Seventeen genes have been identified in this class of sulfatases, many of which are associated with genetic disorders leading to reduction or loss of function of the corresponding enzymes. Amino acid sequence homology suggests that the enzymes have similar overall folds, mechanisms of action, and bivalent metal ion-binding sites. A catalytic cysteine residue, strictly conserved in prokaryotic and eukaryotic sulfatases, is post-translationally modified 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 the enzyme's sulfatase activity. Crystal structures of three human sulfatases, arylsulfatases A and B(ARSA and ARSB), and estrone/dehydroepiandrosterone sulfatase or steroid sulfatase (STS), also known as arylsulfatase C, have been determined. While ARSA and ARSB are water-soluble enzymes, STS has a hydrophobic domain and is an integral membrane protein of the endoplasmic reticulum. In this article, we compare and contrast sulfatase structures and revisit the proposed catalytic mechanism in light of available structural and functional data. Examination of the STS active site reveals substrate-specific interactions previously identified as the estrogen-recognition motif. Because of the proximity of the catalytic cleft of STS to the membrane surface, the lipid bilayer has a critical role in the constitution of the active site, unlike other sulfatases.

Measuring the activities of the Sulfs: two novel heparin/heparan sulfate endosulfatases

Sulfatases hydrolyze sulfate esters on a variety of molecules including glycosaminoglycans, sulfoglycolipids, and cytosolic steroids. These enzymes are found in a wide range of organisms with their basic enzymatic mechanisms broadly conserved. In mammals, many of the sulfatases localize in the lysosome and exhibit enzymatic activity on a small aryl substrate such as 4-methylumbelliferyl sulfate (4-MUS). They are known as arylsulfatases. Sulf-1 and Sulf-2 have been cloned and identified as sulfatases that release sulfate groups on the C-6 position of GlcNAc residue from an internal subdomain in intact heparin. Hence, these enzymes are endosulfatases. The Sulfs are secreted in an active form into conditioned medium of transfected Chinese hamster ovary (CHO) cells. In this chapter, arylsulfatase and endoglucosamine-6-sulfatase assays for the Sulfs are described. A solid-phase binding assay is also detailed, which allows investigation of the ability of the Sulfs to modulate the interaction of heparin-binding proteins with immobilized heparin. The example illustrated is vascular endothelial growth factor (VEGF). This assay is projected to be very useful in the investigation of the biological functions of the Sulfs.

Probing the Origin of the Compromised Catalysis ofE.coliAlkaline Phosphatase in its Promiscuous Sulfatase Reaction

The catalytic promiscuity of E. coli alkaline phosphatase (AP) and many other enzymes provides a unique opportunity to dissect the origin of enzymatic rate enhancements via a comparative approach. Here, we use kinetic isotope effects (KIEs) to explore the origin of the 109-fold greater catalytic proficiency by AP for phosphate monoester hydrolysis relative to sulfate monoester hydrolysis. The primary 18O KIEs for the leaving group oxygen atoms in the AP-catalyzed hydrolysis of p-nitrophenyl phosphate (pNPP) and p-nitrophenylsulfate (pNPS) decrease relative to the values observed for nonenzymatic hydrolysis reactions. Prior linear free energy relationship results suggest that the transition states for AP-catalyzed reactions of phosphate and sulfate esters are "loose" and indistinguishable from that in solution, suggesting that the decreased primary KIEs do not reflect a change in the nature of the transition state but rather a strong interaction of the leaving group oxygen atom with an active site Zn2+ ion. Furthermore, the primary KIEs for the two reactions are identical within error, suggesting that the differential catalysis of these reactions cannot be attributed to differential stabilization of the leaving group. In contrast, AP perturbs the KIE for the nonbridging oxygen atoms in the reaction of pNPP but not pNPS, suggesting a differential interaction with the transferred group in the transition state. These and prior results are consistent with a strong electrostatic interaction between the active site bimetallo Zn2+ cluster and one of the nonbridging oxygen atoms on the transferred group. We suggest that the lower charge density of this oxygen atom on a transferred sulfuryl group accounts for a large fraction of the decreased stabilization of the transition state for its reaction relative to phosphoryl transfer.

Introducing genetically encoded aldehydes into proteins

Methods for introducing bioorthogonal functionalities into proteins have become central to protein engineering efforts. Here we describe a method for the site-specific introduction of aldehyde groups into recombinant proteins using the 6-amino-acid consensus sequence recognized by the formylglycine-generating enzyme. This genetically encoded 'aldehyde tag' is no larger than a His(6) tag and can be exploited for numerous protein labeling applications.

New enzymes for biotransformations: microbial alkyl sulfatases displaying stereo- and enantioselectivity

The majority of hydrolytic enzymes used in white biotechnology for the production of non-natural compounds--such as carboxyl ester hydrolases, lipases and proteases--show a certain preference for a given enantiomer. However, they are unable to alter the stereochemistry of the substrate during catalysis with respect to inversion or retention of configuration. The latter can be achieved by (alkyl) sulfatases, which can be employed for the enantio-convergent transformation of racemic sulfate esters into a single stereoisomeric secondary alcohol, with a theoretical yield of 100%. This is a major improvement over traditional kinetic resolution processes, which yield both enantiomers, each at 50%.

A New Type of Bacterial Sulfatase Reveals a Novel Maturation Pathway in Prokaryotes

Sulfatases are a highly conserved family of enzymes found in all three domains of life. To be active, sulfatases undergo a unique post-translational modification leading to the conversion of either a critical cysteine ("Cys-type" sulfatases) or a serine ("Ser-type" sulfatases) into a Calpha-formylglycine (FGly). This conversion depends on a strictly conserved sequence called "sulfatase signature" (C/S)XPXR. In a search for new enzymes from the human microbiota, we identified the first sulfatase from Firmicutes. Matrix-assisted laser desorption ionization time-of-flight analysis revealed that this enzyme undergoes conversion of its critical cysteine residue into FGly, even though it has a modified (C/S)XAXR sulfatase signature. Examination of the bacterial and archaeal genomes sequenced to date has identified many genes bearing this new motif, suggesting that the definition of the sulfatase signature should be expanded. Furthermore, we have also identified a new Cys-type sulfatase-maturating enzyme that catalyzes the conversion of cysteine into FGly, in anaerobic conditions, whereas the only enzyme reported so far to be able to catalyze this reaction is oxygen-dependent. The new enzyme belongs to the radical S-adenosyl-l-methionine enzyme superfamily and is related to the Ser-type sulfatase-maturating enzymes. This finding leads to the definition of a new enzyme family of sulfatase-maturating enzymes that we have named anSME (anaerobic sulfatase-maturating enzyme). This family includes enzymes able to maturate Cys-type as well as Ser-type sulfatases in anaerobic conditions. In conclusion, our results lead to a new scheme for the biochemistry of sulfatases maturation and suggest that the number of genes and bacterial species encoding sulfatase enzymes is currently underestimated.

The crystal structure of SdsA1, an alkylsulfatase from Pseudomonas aeruginosa, defines a third class of sulfatases

Pseudomonas aeruginosa is both a ubiquitous environmental bacterium and an opportunistic human pathogen. A remarkable metabolic versatility allows it to occupy a multitude of ecological niches, including wastewater treatment plants and such hostile environments as the human respiratory tract. P. aeruginosa is able to degrade and metabolize biocidic SDS, the detergent of most commercial personal hygiene products. We identify SdsA1 of P. aeruginosa as a secreted SDS hydrolase that allows the bacterium to use primary sulfates such as SDS as a sole carbon or sulfur source. Homologues of SdsA1 are found in many pathogenic and some nonpathogenic bacteria. The crystal structure of SdsA1 reveals three distinct domains. The N-terminal catalytic domain with a binuclear Zn2+ cluster is a distinct member of the metallo-beta-lactamase fold family, the central dimerization domain ensures resistance to high concentrations of SDS, whereas the C-terminal domain provides a hydrophobic groove, presumably to recruit long aliphatic substrates. Crystal structures of apo-SdsA1 and complexes with substrate analog and products indicate an enzymatic mechanism involving a water molecule indirectly activated by the Zn2+ cluster. The enzyme SdsA1 thus represents a previously undescribed class of sulfatases that allows P. aeruginosa to survive and thrive under otherwise bacteriocidal conditions.

Differential proteomic analysis of the Bacillus anthracis secretome: distinct plasmid and chromosome CO2-dependent cross talk mechanisms modulate extracellular proteolytic activities

The secretomes of a virulent Bacillus anthracis strain and of avirulent strains (cured of the virulence plasmids pXO1 and pXO2), cultured in rich and minimal media, were studied by a comparative proteomic approach. More than 400 protein spots, representing the products of 64 genes, were identified, and a unique pattern of protein relative abundance with respect to the presence of the virulence plasmids was revealed. In minimal medium under high CO(2) tension, conditions considered to simulate those encountered in the host, the presence of the plasmids leads to enhanced expression of 12 chromosome-carried genes (10 of which could not be detected in the absence of the plasmids) in addition to expression of 5 pXO1-encoded proteins. Furthermore, under these conditions, the presence of the pXO1 and pXO2 plasmids leads to the repression of 14 chromosomal genes. On the other hand, in minimal aerobic medium not supplemented with CO(2), the virulent and avirulent B. anthracis strains manifest very similar protein signatures, and most strikingly, two proteins (the metalloproteases InhA1 and NprB, orthologs of gene products attributed to the Bacillus cereus group PlcR regulon) represent over 90% of the total secretome. Interestingly, of the 64 identified gene products, at least 31 harbor features characteristic of virulence determinants (such as toxins, proteases, nucleotidases, sulfatases, transporters, and detoxification factors), 22 of which are differentially regulated in a plasmid-dependent manner. The nature and the expression patterns of proteins in the various secretomes suggest that distinct CO(2)-responsive chromosome- and plasmid-encoded regulatory factors modulate the secretion of potential novel virulence factors, most of which are associated with extracellular proteolytic activities.

Purification of a 75 kDa protein from the organelle matrix of human neutrophils and identification as N-acetylglucosamine-6-sulphatase

A 75 kDa protein was purified to homogeneity from granule extracts of normal human granulocytes using Sephadex G-75 chromatography, Mono-S cation exchange chromatography and chromatofocusing. The protein consisted of one chain with a molecular mass of 75 kDa, as determined by SDS/PAGE. Tryptic peptide analysis by MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS and sequence analysis by MS/MS identified the protein to be N-acetylglucosamine-6-sulphatase (EC 3.1.6.14). The identity of the protein was confirmed by demostrating enzymatic activity towards the substrate N-acetylglucosamine 6-sulphate. The enzyme was active over a broad pH range with an optimum of pH 7.0, and showed a K(m) value of 13.0 mM and a V(max) value of approximately 1.8 microM/min per mg. The enzyme also showed O-desulphation activity towards heparan sulphate-derived saccharides. Subcellular fractionation of neutrophil organelles showed the presence of enzymatic activity mainly in the same fractions as primary granules. Furthermore, PMA treatment of the neutrophils induced release of the enzyme, indicating its matrix protein nature. The presence of N-acetylglucosamine-6-sulphatase in human neutrophils implies that neutrophils may play a role in the modulation of cell surface molecules and extracellular matrix by O-desulphation.

Molecular cloning and initial characterization of three novel human sulfatases

Sulfatases constitute a group of enzymes capable of hydrolyzing the sulphate ester bond of a variety of biological compounds. To date, thirteen members of this family have been cloned and characterized as part of the human genome. In this work, the identification, molecular cloning and initial characterization of three new members of this human gene family is reported. Two map in chromosome 5 (5q15 and 5q32), whereas the third one maps in chromosome 4 (4q26). Two of them are closely related and are coded in only two exons, what is a unique genomic feature among the known sulfatases. The three new members were cloned from different DNA sources, and the predicted protein sizes range from 536 aa to 596 aa. Interestingly, initial characterization of two of them showed that their expression pattern was mainly restricted to embryonic tissues and some cancer cell lines.

Fate of 3,3‘-Diindolylmethane in Cultured MCF-7 Human Breast Cancer Cells

3,3'-Diindolylmethane (DIM) is a major in vivo product of the cancer preventative agent indole-3-carbinol that is found in vegetables of the genus Brassica. Here, we report on the metabolic fate of radiolabeled DIM in MCF-7 cells. DIM was slowly metabolized to several sulfate conjugates of oxidized DIM products that were primarily detected in the medium. The radioactivity detected in cells was predominantly unmodified DIM (81-93%) at all time intervals up to 72 h treatment. Co-treatment of MCF-7 cells with quercetin slowed the rate that oxidized DIM products accumulated in the medium, while indole[3,2-b]carbazole (ICZ) co-treatment accelerated their production. ICZ is an inducer of P450 1A2, while quercetin is a specific inhibitor of this isoform, suggesting that P450 1A2 is primarily responsible for the oxidation of DIM, probably through 2,3-epoxidation similar to 3-methylindole. Sulfate conjugates of oxidized DIM metabolites were cleaved by sulfatase digestion and identified by LC/MS as 3-(1H-indole-3-ylmethyl)-2-oxindole (2-ox-DIM), bis(1H-indol-3-yl)methanol (3-methylenehydroxy-DIM), 3-[hydroxy-(1H-indol-3-yl)-methyl]-1,3-dihydro-2-oxindole (3-methylenehydroxy-2-ox-DIM), and 3-hydroxy-3-(1H-indole-3-ylmethyl)-2-oxindole (3-hydroxy-2-ox-DIM). Derivatives of 2-ox-DIM represented greater than 30% of the radioactivity in the sulfatase-digested medium. Although oxindole formation was the primary metabolic pathway in MCF-7 cells, synthetic 2-ox-DIM was inactive in a 4-ERE-luciferase reporter assay and, therefore, probably not responsible for the estrogenic activity previously observed for DIM. Unmodified DIM rapidly accumulated in the nuclear membranes representing approximately 35-40% of the radioactivity after 0.5-2 h treatment. Uptake of radiolabeled DIM appeared to be a passive partitioning into the nuclear membranes and was not dependent upon the cell cytosol. The nuclear uptake of DIM was not saturable and could not be blocked by pretreatment with unlabeled DIM (100 microM). Further, treatments in serum-free medium increased the uptake of radiolabeled DIM by the MCF-7 cells. These findings show that the uptake of DIM by membranes significantly increases its localized concentration, which may contribute to its biological activities.

An alpha-formylglycine building block for fmoc-based solid-phase peptide synthesis

[reaction: see text] Nearly all known sulfatases share a common active site modification that is required for their activity: conversion of cysteine to alpha-formylglycine. We report the synthesis of an alpha-formylglycine building block suitable for Fmoc-based solid-phase peptide synthesis. The building block was incorporated into a synthetic peptide derived from the active site of a Mycobacterium tuberculosis sulfatase.

Purification of the crude solution from Helix pomatia for use as beta-glucuronidase and aryl sulfatase in phytoestrogen assays

Phytoestrogens occur in a variety of foods and are thought to offer a protective effect against a number of complex diseases. Due to the diversity of phytoestrogen conjugates formed in the human body, most assays include an enzymatic hydrolysis step prior to analysis. beta-Glucuronidase from Helix pomatia, which also contains sulfatase activity, is popular for this task but contains appreciable levels of some phytoestrogens and related compounds, which could affect accurate quantification at low concentrations. Use of solid phase extraction on a polymeric resin has been found to remove the majority of these compounds from the enzyme, without affecting the enzyme activity for almost all of the analytes tested.

A general binding mechanism for all human sulfatases by the formylglycine-generating enzyme

The formylglycine (FGly)-generating enzyme (FGE) uses molecular oxygen to oxidize a conserved cysteine residue in all eukaryotic sulfatases to the catalytically active FGly. Sulfatases degrade and remodel sulfate esters, and inactivity of FGE results in multiple sulfatase deficiency, a fatal disease. The previously determined FGE crystal structure revealed two crucial cysteine residues in the active site, one of which was thought to be implicated in substrate binding. The other cysteine residue partakes in a novel oxygenase mechanism that does not rely on any cofactors. Here, we present crystal structures of the individual FGE cysteine mutants and employ chemical probing of wild-type FGE, which defined the cysteines to differ strongly in their reactivity. This striking difference in reactivity is explained by the distinct roles of these cysteine residues in the catalytic mechanism. Hitherto, an enzyme-substrate complex as an essential cornerstone for the structural evaluation of the FGly formation mechanism has remained elusive. We also present two FGE-substrate complexes with pentamer and heptamer peptides that mimic sulfatases. The peptides isolate a small cavity that is a likely binding site for molecular oxygen and could host reactive oxygen intermediates during cysteine oxidation. Importantly, these FGE-peptide complexes directly unveil the molecular bases of FGE substrate binding and specificity. Because of the conserved nature of FGE sequences in other organisms, this binding mechanism is of general validity. Furthermore, several disease-causing mutations in both FGE and sulfatases are explained by this binding mechanism.

Substrate specificity and domain functions of extracellular heparan sulfate 6-O-endosulfatases, QSulf1 and QSulf2

The extracellular sulfatases (Sulfs) are an evolutionally conserved family of heparan sulfate (HS)-specific 6-O-endosulfatases. These enzymes remodel the 6-O-sulfation of cell surface HS chains to promote Wnt signaling and inhibit growth factor signaling for embryonic tissue patterning and control of tumor growth. In this study we demonstrate that the avian HS endosulfatases, QSulf1 and QSulf2, exhibit the same substrate specificity toward a subset of trisulfated disaccharides internal to HS chains. Further, we show that both QSulfs associate exclusively with cell membrane and are enzymatically active on the cell surface to desulfate both cell surface and cell matrix HS. Mutagenesis studies reveal that conserved amino acid regions in the hydrophilic domains of QSulf1 and QSulf2 have multiple functions, to anchor Sulf to the cell surface, bind to HS substrates, and to mediate HS 6-O-endosulfatase enzymatic activity. Results of our current studies establish the hydrophilic domain (HD) of Sulf enzymes as an essential multifunctional domain for their unique endosulfatase activities and also demonstrate the extracellular activity of Sulfs for desulfation of cell surface and cell matrix HS in the control of extracellular signaling for embryonic development and tumor progression.

Sulfatases and human disease

Sulfatases are a highly conserved family of proteins that cleave sulfate esters from a wide range of substrates. The importance of sulfatases in human metabolism is underscored by the presence of at least eight human monogenic diseases caused by the deficiency of individual sulfatases. Sulfatase activity requires a unique posttranslational modification, which is impaired in patients with multiple sulfatase deficiency (MSD) due to a mutation of the sulfatase modifying factor 1 (SUMF1). Here we review current knowledge and future perspectives on the evolution of the sulfatase gene family, on the role of these enzymes in human metabolism, and on new developments in the therapy of sulfatase deficiencies.

Hydrolysis of Conjugated Metabolites of Buprenorphine II. The Quantitative Enzymatic Hydrolysis of Norbuprenorphine-3- -D-Glucuronide in Human Urine

In gas chromatographic-mass spectroscopic analysis of buprenorphine metabolites, the urine specimen must be first hydrolyzed to release buprenorphine and norbuprenorphine from their glucuronide conjugates. For evaluation of existing hydrolysis methods and to find out the optimal hydrolysis conditions, buprenorphine-3-beta-D-glucuronide (B3G) and norbuprenorphine-3-beta-D-glucuronide (NB3G) were synthesized. In a previous communication we reported on the optimum conditions for hydrolysis of B3G. Because we have previously shown that no hydrolysis was achieved under basic conditions and that acid hydrolysis resulted in extensive degradation, only enzymatic hydrolysis was examined for NB3G. This study therefore reports on the optimum enzymatic conditions for the hydrolysis of NB3G. Urine fortified with synthetic NB3G was hydrolyzed with beta-glucuronidases from different source species, including Helix pomatia, Escherichia coli, and Patella vulgata. Glusulase, a preparation containing both the beta-glucuronidase (H. pomatia) and sulfatase, was also tested. It was found that marked differences exist in the reactivity of these enzymes. Incubation with glucuronidases from H. pomatia or E. coli at 37 degrees C for 16 h resulted in quantitative hydrolysis of NB3G. At 60 degrees C, complete hydrolysis was achieved with 1000 units of H. pomatia in 4 h and after only 1 h with glusulase. On the other hand, incubation at 60 degrees C for 4 h with Patella vulgata resulted in only 14.7% hydrolysis.

De novocalcium/sulfur SAD phasing of the human formylglycine-generating enzyme using in-house data

Sulfatases are a family of enzymes essential for the degradation of sulfate esters. Formylglycine is the key catalytic residue in the active site of sulfatases and is generated from a cysteine residue by FGE, the formylglycine-generating enzyme. Inactivity of FGE owing to inherited mutations in the FGE gene results in multiple sulfatase deficiency (MSD), which leads to early death in infants. Human FGE was crystallized in the presence of traces of the protease elastase, which was absolutely essential for crystal growth, and the structure of FGE was determined by molecular replacement. Before this model was completed, the FGE structure was re-determined by SAD phasing using in-house data based on the anomalous signal of two calcium ions bound to the native enzyme and intrinsic S atoms. A 14-atom substructure was determined at 1.8 A resolution by SHELXD; SHELXE was used for density modification and phase extension to 1.54 A resolution. Automated model building with ARP/wARP and refinement with REFMAC5 yielded a virtually complete model without manual intervention. The minimal data requirements for successful phasing and the relative contributions of the Ca and S atoms are discussed and compared with the related FGE paralogue, pFGE. This work emphasizes the usefulness of de novo phasing using weak anomalous scatterers and in-house data.

Molecular basis for multiple sulfatase deficiency and mechanism for formylglycine generation of the human formylglycine-generating enzyme

Sulfatases are enzymes essential for degradation and remodeling of sulfate esters. Formylglycine (FGly), the key catalytic residue in the active site, is unique to sulfatases. In higher eukaryotes, FGly is generated from a cysteine precursor by the FGly-generating enzyme (FGE). Inactivity of FGE results in multiple sulfatase deficiency (MSD), a fatal autosomal recessive syndrome. Based on the crystal structure, we report that FGE is a single-domain monomer with a surprising paucity of secondary structure and adopts a unique fold. The effect of all 18 missense mutations found in MSD patients is explained by the FGE structure, providing a molecular basis of MSD. The catalytic mechanism of FGly generation was elucidated by six high-resolution structures of FGE in different redox environments. The structures allow formulation of a novel oxygenase mechanism whereby FGE utilizes molecular oxygen to generate FGly via a cysteine sulfenic acid intermediate.

Expression, localization, structural, and functional characterization of pFGE, the paralog of the Calpha-formylglycine-generating enzyme

pFGE is the paralog of the formylglycine-generating enzyme (FGE), which catalyzes the oxidation of a specific cysteine to Calpha-formylglycine, the catalytic residue in the active site of sulfatases. The enzymatic activity of sulfatases depends on this posttranslational modification, and the genetic defect of FGE causes multiple sulfatase deficiency. The structural and functional properties of pFGE were analyzed. The comparison with FGE demonstrates that both share a tissue-specific expression pattern and the localization in the lumen of the endoplasmic reticulum. Both are retained in the endoplasmic reticulum by a saturable mechanism. Limited proteolytic cleavage at similar sites indicates that both also share a similar three-dimensional structure. pFGE, however, is lacking the formylglycine-generating activity of FGE. Although overexpression of FGE stimulates the generation of catalytically active sulfatases, overexpression of pFGE has an inhibitory effect. In vitro pFGE interacts with sulfatase-derived peptides but not with FGE. The inhibitory effect of pFGE on the generation of active sulfatases may therefore be caused by a competition of pFGE and FGE for newly synthesized sulfatase polypeptides.

Crystal structure of human pFGE, the paralog of the Calpha-formylglycine-generating enzyme

In eukaryotes, sulfate esters are degraded by sulfatases, which possess a unique Calpha-formylglycine residue in their active site. The defect in post-translational formation of the Calpha-formylglycine residue causes a severe lysosomal storage disorder in humans. Recently, FGE (formylglycine-generating enzyme) has been identified as the protein required for this specific modification. Using sequence comparisons, a protein homologous to FGE was found and denoted pFGE (paralog of FGE). pFGE binds a sulfatase-derived peptide bearing the FGE recognition motif, but it lacks formylglycine-generating activity. Both proteins belong to a large family of pro- and eukaryotic proteins containing the DUF323 domain, a formylglycine-generating enzyme domain of unknown three-dimensional structure. We have crystallized the glycosylated human pFGE and determined its crystal structure at a resolution of 1.86 A. The structure reveals a novel fold, which we denote the FGE fold and which therefore serves as a paradigm for the DUF323 domain. It is characterized by an asymmetric partitioning of secondary structure elements and is stabilized by two calcium cations. A deep cleft on the surface of pFGE most likely represents the sulfatase polypeptide binding site. The asymmetric unit of the pFGE crystal contains a homodimer. The putative peptide binding site is buried between the monomers, indicating a biological significance of the dimer. The structure suggests the capability of pFGE to form a heterodimer with FGE.

Molecular characterization of the human Calpha-formylglycine-generating enzyme

Calpha-formylglycine (FGly) is the catalytic residue in the active site of sulfatases. In eukaryotes, it is generated in the endoplasmic reticulum by post-translational modification of a conserved cysteine residue. The FGly-generating enzyme (FGE), performing this modification, is an endoplasmic reticulum-resident enzyme that upon overexpression is secreted. Recombinant FGE was purified from cells and secretions to homogeneity. Intracellular FGE contains a high mannose type N-glycan, which is processed to the complex type in secreted FGE. Secreted FGE shows partial N-terminal trimming up to residue 73 without loosing catalytic activity. FGE is a calcium-binding protein containing an N-terminal (residues 86-168) and a C-terminal (residues 178-374) protease-resistant domain. The latter is stabilized by three disulfide bridges arranged in a clamp-like manner, which links the third to the eighth, the fourth to the seventh, and the fifth to the sixth cysteine residue. The innermost cysteine pair is partially reduced. The first two cysteine residues are located in the sequence preceding the N-terminal protease-resistant domain. They can form intramolecular or intermolecular disulfide bonds, the latter stabilizing homodimers. The C-terminal domain comprises the substrate binding site, as evidenced by yeast two-hybrid interaction assays and photocross-linking of a substrate peptide to proline 182. Peptides derived from all known human sulfatases served as substrates for purified FGE indicating that FGE is sufficient to modify all sulfatases of the same species.