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The active site of all sulfatases contain a conserved cysteine residue which undergoes posttranslational conversion to formylglycine (FGly) via oxidation at the beta-carbon.The FGly has an aldehyde as the functional group of the side chain. . In Multiple Sulfatase Deficiency this conversion is absent, yielding catalytically inactive sulfatases. In ASA the active site is located in a cavity lined with charged amino acids which coordinate the alignment of the Mg++  ion and the substrate.  Here's another view.

A close up of the Mg++ binding site with the coordinating amino acids shown in CPK colors 
reveals negatively charged oxygens in close proximity (~ 2.2 Angstroms) to the metal ion. The side chain of FGly also helps to coordinate the Mg++, completing the distorted octahedral.

This view of the active site displays amino acids involved in Mg++ coordination and those involved in the proposed catalytic mechanism of ASA (see figure below). In this proposed mechanism, FGly is hydrated. The side chains of Lys123, Ser150, His229 and Lys302 coordinate the sulfate group of the substrate upon its entry  into the active site (I). Mg++ activates one of the hydroxyl groups of FGly for nucleophilic attack of the sulfur, forming a covalent intermediate (II). The side chain of His125 H-bonds and deprotonates the second hydroxyl of FGly.  HSO4-is then eliminated from the intermediate and the FGly aldehyde is reformed (III).

 
 
    The above proposed mechanism for the hydrolysis of RO-SO3- by ASA is comparable to, and modeled on, the hydrolysis of RO-PO3 -- by Alkaline Phosphatase (AP). The assumptions of this model are based on the structural similarity of  the two active sites and functional similarity in that ox liver ASA demonstrates phosphodiesterase activity.
    ASA and AP also share a high degree of structural homology, even though sequence similarity is low.  The main feature of AP is a beta-pleated sheet surrounded by alpha-helices. AP occurs as a homodimer, like ASA, but does not form an octamer. It carries two Zn++ and one Mg++ ion.    Compare this to the previously discussed structure of  ASA. 
 

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