TABLE 2.

Functional assignment of 6-O-sulfatase active site residues

Amino acids listed in this table were identified by inspection of the 6-O-sulfatase model proposed in Fig. 3. The critical active site cysteine is shown in boldface.

Amino acids
Sulfate ester hydrolysis
FGly-80	Cys-80 in the active enzyme is modified to FGly and the FGly is further hydrated to form an aldehyde hydrate comprised of Oγ−1 and Oγ−2 atoms
Arg-84, His-130	Stabilize the hydrated FGly by interaction with Oγ−2. His-130 is well positioned also for proton abstraction from Oγ−2 after the catalytic process for elimination of sulfate and regeneration of geminal diol
Lys-128, Lys187, Lys-387, Gln-306	Positioned to interact with oxygen atoms of the 6-O-sulfate group to enhance electron density withdrawal from sulfate, hence contributing to the electrophilicity increase of the sulfur center. Also, Lys-387 is well positioned to protonate the oxygen atom of the leaving substrate
Asp-374, Asp-40, Asp-41, Gln-375, Thr-300	Asp-374, Asp-40, Asp-41, and Gln-375 are well positioned to coordinate with a divalent metal ion such as Ca2+. Asp-374 also could donate proton and enhance nucleophilicity of Oγ−1. Thr-300 being weakly acidic could also participate instead of Gln-375 in metal ion coordination
Substrate positioning and specificity
Phe-189, Tyr-378	Positioned to stack with the pyranose ring of the Glc sugar
His-507, Asp-186, Tyr-378	His-507 is positioned to interact with equatorial 4-hydroxyl of the Glc sugar. Asp-86 is positioned to interact with the N-atom of the Glc-NS or GlcNAc sugars
Trp-307, Pro-188	Positioned to optimally interact with the acetyl group of the GlcNAc sugar