Alkaline phosphatases are non-specific phosphomonoesterases that catalyze the hydrolysis reaction via a phosphoseryl intermediate to produce inorganic phosphate and the corresponding alcohol, optimally at high pH. Alkaline phosphatase exists as a dimer, each monomer binding 2 zinc atoms and one magnesium atom, which are essential for enzymatic activity. Although SPAP is a subclass of alkaline phosphatase, SPAP has many differences from other APs: 1) the catalytic residue is a threonine instead of serine, 2) there is no binding pocket for the third metal ion, and 3) the arginine residue forming bidentate hydrogen bonding is deleted in SPAP. A lysine and an asparagine residue, recruited together for the first time into the active site, bind the substrate phosphoryl group in a manner not observed before in any other AP.
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