The AphA enzyme of Escherichia coli, a molecular class B periplasmic phosphatase that belongs to the DDDD superfamily of phosphohydrolases, was purified and subjected to biochemical characterization. Kinetic analysis with several substrates revealed that the enzyme essentially behaves as a broad-spectrum nucleotidase highly active on 3V-and 5V-mononucleotides and monodeoxynucleotides, but not active on cyclic nucleotides, or nucleotides di-and triphosphate. Mononucleotides are degraded to nucleosides, and AphA apparently does not exhibit any nucleotide phosphomutase activity. However, it can transphosphorylate nucleosides in the presence of phosphate donors. Kinetic properties of AphA are consistent with structural data, and suggest a role for the hydrophobic pocket present in the active site crevice, made by residues Phe 56, Leu71, Trp77 and Tyr193, in conferring preferential substrate specificity by accommodating compounds with aromatic rings. AphA was inhibited by several chelating agents, including EDTA, EGTA, 1,10-phenanthroline and dipicolinic acid, with EDTA being apparently the most powerful inhibitor. D

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The Escherichia coli gene aphA codes for a periplasmic acid phosphatase called AphA, belonging to class B bacterial phosphatases, which is part of the DDDD superfamily of phosphohydrolases. After our first report about its crystal structure, we have started a series of crystallographic studies aimed at understanding of the catalytic mechanism of the enzyme. Here, we report three crystal structures of the AphA enzyme in complex with the hydrolysis products of nucleoside monophosphate substrates and a fourth with a proposed intermediate analogue that appears to be covalently bound to the enzyme. Comparison with the native enzyme structure and with the available X-ray structures of different phosphatases provides clues about the enzyme chemistry and allows us to propose a catalytic mechanism for AphA, and to discuss it with respect to the mechanism of other bacterial and human phosphatases.

The class B non-specific acid phosphatase AphA from Escherichia coli has been expressed in E. coli and purified following a new protocol. ESI mass spectroscopy shows that the purified enzyme solution contains two polypeptides with molecular weights differing by 185 Da corresponding to two different cleavage sites of the signal peptide from the AphA E. coli precursor. Despite the solution heterogeneity, X-ray quality crystals have been obtained. However, the crystals have a tendency to give polymorphs and to lose long-range order with time while maintaining an intact crystal habit. Crystals have been grown in space groups I222 and C2 with three different unit cells and different asymmetric unit contents. Diffraction data to 1.6 A resolution have been collected with synchrotron radiation at ESRF and DESY.

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