Sulfatases

Sulfatases (EC 3.1.6.-) belong to the esterase class that catalyze the hydrolysis of sulfate esters, and play a key role in regulating the sulfation states that determine the function of many physiological molecules. The substrates of sulfatase range from small cytosolic steroids, such as estrogen sulfate, to complex cell-surface carbohydrates, such as the glycosaminoglycans. Together with sulfotransferases, sulfatases form the major catalytic machinery for the synthesis and breakage of sulfate esters. The reactions catalyzed by sulfatases have been linked with important cellular functions, including hormone regulation, cellular degradation, and modulation of signaling pathways. In addition, sulfatases have also been implicated in the onset of various pathophysiological conditions, including hormone dependent cancers, lysosomal storage disorders, developmental abnormalities, and bacterial pathogenesis.

Distribution

Sulfatases are found in lower and higher organisms. Lower eukaryotic, and bacterial sulfatases are soluble and have been found to reside in cytoplasmic, periplasmic, and extracellular regions. In higher organisms they are found in intracellular and extracellular spaces. Eukaryotes sulfatases are targeted for the secretory pathway and are extensively glycosylated before being transported to the extracellular matrix (ECM) or to subcellular locations, such as the endoplasmatic reticulum (ER), Golgi complex, and lysosome. Lysosomal sulfatases cleave a range of sulfated carbohydrates including sulfated glycosaminoglycans and glycolipids. Steroid sulfatase is distributed in a wide range of tissues throughout the body, enabling sulfated steroids synthesized in the adrenals and gonads to be desulfated following distribution through the circulation system.

Structure

The structures of four sulfatases have been solved to date by X-ray crystal-structure analysis: HARSA (human aryl sulfatase A), HARSB (human aryl sulfatase B), and HARSC (aryl sulfatase C) from humans, as well as PARS (Pseudomonas aeruginosa aryl sulfatase). The structures are strikingly similar, revealing a nearly spherical globular monomer with mixed α/β topology, which is divided into two domains. ARSC additionally contains a unique transmembrane domain from which the soluble domain “sprouts”, giving the enzyme a “mushroom like” morphology. The larger, N-terminal domain consists of a helices surrounding a large mixed β sheet, which consists of 10 strands in the HARSA, HARSB, and PARS structures and 11 strands in HARSC. The smaller, C-terminal domain contains a four stranded antiparallel β sheet tightly packed against a long, solvent-exposed C-terminal α helix. As is typical for the α/β family of enzymes, the active-site cavity is nestled at the C-terminal end of the large β sheet, with the FGly residue located at the bottom of a narrow cleft lined with charged amino acids.

The active site of the sulfatase is comprised of 10 highly interconnected polar residues and a divalent metal cation. In the case of HARSA, a metal-binding region is formed by the generic residues AspA, AspB, AspC, and AsnA, which is conservatively replaced by a glutamine residue in ARSC. In HARSB, HARSC, and PARS these metal-binding residues, along with FGly and an oxygen atom of the sulfate (or water in its absence), coordinate a Ca²⁺ cation heptavalently; in HARSA an Mg²⁺ ion is coordinated octahedrally.

Mechanism

• Addition–hydrolysis (AH) mechanism

The mechanism begins with a nucleophilic attack by a sulfate group oxygen atom at the electrophilic aldehyde group of FGly to form a sulfate diester. The alcohol conjugate is then released through the action of a nucleophile, such as an activated water molecule (akin to the AP mechanism).

• Transesterification–elimination (TE) mechanism

In the first step, one of the geminal hydroxy groups of the FGly hydrate (FGH) acts as a nucleophile. An SN2 attack at the sulfur atom of the sulfate hydrolyzes the conjugate alcohol and creates a transient FGS. The second geminal hydroxy group then reacts to eliminate sulfate and re-form the aldehyde. Finally, the catalytic cycle is completed by hydration of the aldehyde to reform the FGH.

Figure 1. Proposed mechanistic schemes for the hydrolysis of sulfate esters by the active-site aldehyde FGly. a) Addition–hydrolysis (AH) mechanism. b) Transesterification–elimination (TE) mechanism.
(Sarah R. et al. 2010)

Biological Functions

Human sulfatases have been found in several subcellular locations, where they play key roles in important biological processes, including the synthesis of hormones in the ER, the degradation of glycosaminoglycans and glycolipids in the lysosome, and the modulation of developmental-cell signaling in the ECM. Several other human sulfatases have been identified, but their biological functions are unknown. Potentially these enzymes might be involved in the desulfonation of other important biomolecules, such as sialyl Lewis^X-6S (sLe^X-6S) and tyrosine O-sulfate. The sulfatases of invertebrates and lower eukaryotes are largely unexplored. Several genes have been cloned, and in some cases exhibit activities, including a role in the development of sea urchin embryos, the inactivation of endogenous plant toxins, and involvement in the sulfur metabolism in algae and fungi.

High levels of ARS, alkyl sulfatase, and glycosulfatase activity have been identified in many different types of bacteria. However, only a handful of bacterial sulfatase genes have been cloned and characterized. These enzymes typically function as scavengers, removing sulfate groups from exogenous substrates to provide sulfur and carbon sources for their hosts. Recent studies have demonstrated further functions of bacterial sulfatases in osmoprotection and pathogenic processes.

Reference

1. Sarah R. Hanson, Michael D. Best Dr, Chi-Huey Wong Prof. Sulfatases: Structure, Mechanism, Biological Activity, Inhibition, and Synthetic Utility [J]. Angewandte Chemie International Edition, 2010, 43(27):3526-3548.