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.

C␣-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 calciumbinding 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 Nterminal 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 photocrosslinking 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.

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.posttranslational modification ͉ oxygenase ͉ enzyme mechanism S ulfatases catalyze the hydrolysis of sulfate esters such as glycosaminoglycans, sulfolipids, and steroid sulfates in eukaryotic cells. The key catalytic residue in sulfatases is a unique formylglycine (FGly), which is generated from a cysteine precursor (Fig. 1a) and functions as a nucleophilic aldehyde hydrate in the initial addition reaction of sulfate ester hydrolysis (1). Inactivity of individual sulfatases in humans may lead to severe diseases such as mucopolysaccharidoses, metachromatic leukodystrophy, X-linked ichthyosis, and chondrodysplasia punctata. However, a severe reduction or complete lack of all sulfatase activities, termed multiple sulfatase deficiency, originates from mutations in the FGlygenerating enzyme (FGE) (2, 3).FGE is localized in the endoplasmic reticulum (ER) and modifies the unfolded form of newly synthesized sulfatases (4, 5). The generation of FGly from a cysteine residue is a multistep redox process that involves disulfide bond formation and requires a reducing agent (6) and molecular oxygen (J. Peng, B. Schmidt, A. Preusser-Kunze, M. Mariappan, K. von Figura, and T. Dierks, personal communication) but does not require any cofactors or metal ions. Peptides that contain the minimal motif C-[TSAC]-PSR with flanking sequences according to human sulfatases are FGE substrates and are converted to the...

pFGE is the paralog of the formylglycine-generating enzyme (FGE), which catalyzes the oxidation of a specific cysteine to C␣-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 threedimensional 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 sulfatasederived 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.

The catalytic residue in sulfatases is a unique formylglycine that is post-translationally generated by oxidation of a cysteine or serine precursor. Molecular oxygen oxidizes the cysteine precursor in eukaryotic sulfatases, a reaction that is catalysed by the formylglycine-generating enzyme FGE. Previously, FGE was crystallized in complex with a chloride ion which, based on its similar polarizability and hydrophobicity, indicates the site of molecular oxygen binding. Here, two structures of FGE in complex with bromide and iodide were determined in order to further delineate the volume and stereochemical restraints of the oxygen-binding site for potential reaction intermediates. Anomalous difference density maps unambiguously assigned the nature of the halide ions. Unexpectedly, data collected at a wavelength of 1.54 A from the iodide-containing crystal and data collected at a wavelength of 0.8 A from a bromide-containing crystal were sufficient for SIRAS phasing.