The Divalent Metal Ion in the Active Site of Uteroferrin Modulates Substrate Binding and Catalysis

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Journal of the American Chemical Society

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The purple acid phosphatases (PAP) are binuclear metallohydrolases that catalyze the hydrolysis of a broad range of phosphomonoester substrates. The mode of substrate binding during catalysis and the identity of the nucleophile is subject to debate. Here, we used native Fe3+−Fe2+ pig PAP (uteroferrin; Uf) and its Fe3+−Mn2+ derivative to investigate the effect of metal ion substitution on the mechanism of catalysis. Replacement of the Fe2+ by Mn2+ lowers the reactivity of Uf. However, using stopped-flow measurements it could be shown that this replacement facilitates approximately a ten-fold faster reaction between both substrate and inorganic phosphate with the chromophoric Fe3+ site. These data also indicate that in both metal forms of Uf, phenyl phosphate hydrolysis occurs faster than formation of a μ-1,3 phosphate complex. The slower rate of interaction between substrate and the Fe3+ site relative to catalysis suggests that the substrate is hydrolyzed while coordinated only to the divalent metal ion. The likely nucleophile is a water molecule in the second coordination sphere, activated by a hydroxide terminally coordinated to Fe3+. The faster rates of interaction with the Fe3+ site in the Fe3+−Mn2+ derivative than the native Fe3+−Fe2+ form are likely mediated via a hydrogen bond network connecting the first and second coordination spheres, and illustrate how the selection of metal ions may be important in fine-tuning the function of this enzyme.

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