Phosphate forms an unusual tripodal complex with the Fe-Mn center of sweet potato purple acid phosphatase

Identification and molecular modeling of a novel, plant-like, human purple acid phosphatase

Purple acid phosphatases are a family of binuclear metallohydrolases that have been identified in plants, animals and fungi. Only one isoform of approximately 35 kDa has been isolated from animals, where it is associated with bone resorption and microbial killing through its phosphatase activity, and hydroxyl radical production, respectively. Using the sensitive PSI-BLAST search method, sequences representing new purple acid phosphatase-like proteins have been identified in mammals, insects and nematodes. These new putative isoforms are closely related to the approximately 55 kDa purple acid phosphatase characterized from plants. Secondary structure prediction of the new human isoform further confirms its similarity to a purple acid phosphatase from the red kidney bean. A structural model for the human enzyme was constructed based on the red kidney bean purple acid phosphatase structure. This model shows that the catalytic centre observed in other purple acid phosphatases is also present in this new isoform. These observations suggest that the sequences identified in this study represent a novel subfamily of plant-like purple acid phosphatases in animals and humans.

Recombinant purple acid phosphatase isoform 3 from sweet potato is an enzyme with a diiron metal center

Purple acid phosphatases (PAPs) from sweet potato (sp) have been classified on the basis of their primary structure and the dinuclear metal center into isoforms spPAP1 [Fe(III)-Zn(II)] and spPAP2 [Fe(III)-Mn(II)]; for spPAP3 only the cDNA is known. With the aim of unraveling the character of the dinuclear metal center we report here the characterization of this isoform at the protein level. We cloned spPAP3 cDNA in a baculovirus and overexpressed this enzyme in Sf9 insect cells. Preparation of recombinant spPAP3 in two steps afforded pure enzyme with yields of 4.5 mg.L(-1) culture medium. This enzyme is a dimeric, disulfide-linked PAP of 110 kDa, similar to known PAP isoforms from higher plants. Enzymatic studies and spectroscopic properties (max. absorption at 550-565 nm) indicated a diiron enzyme; quantitative and semiquantitative metal analysis using ICP-OES and TOF-SIMS, respectively, revealed the presence of only iron in purified spPAP3. Metal replacement in the second metal-binding site upon preparation of the semiapo-enzyme with Fe(II), Zn(II), or Mn(II) showed highest activities with Fe(II). The data show that recombinant spPAP3 has a diiron metal center. Site-directed mutagenesis was conducted to check catalytic efficiency at the atomic level. Tyr291 at the substrate-binding site in spPAP3 was mutated to His and Ala, the respective residues found in spPAP1 and spPAP2. Kinetic analysis showed that conversion of Tyr291 to His further optimized the performance of this protein as a diiron enzyme, whereas the Ala mutation weakened the catalytic efficiency regardless of the metal present in the second binding site.

Synthesis, Reactivities, and Magnetostructural Properties of FeIII, FeIII−O−FeIII, and ZnIIFeIII−O−FeIIIZnII Complexes of a Tetraiminodiphenolate Macrocycle

The mononuclear iron(III) complexes [Fe(LH2)(H2O)Cl](ClO4)2.2H2O (1) and [Fe(LH2)(H2O)2](ClO4)3.H2O (2) have been prepared by reacting [Pb(LH(2))](ClO4)2 with FeCl3.6H2O and Fe(ClO(4))(3).6H(2)O, respectively. Complex 2 upon treatment with 1 equiv of alkali produces the oxo-bridged dimer [{Fe(LH2)(H2O)}2(mu-O)](ClO4)4.2H2O (3). In these compounds, LH2 refers to the tetraiminodiphenol macrocycle in the zwitterionic form whose two uncoordinated imine nitrogens are protonated and hydrogen-bonded to the metal-bound phenolate oxygens. The aqua ligands of complexes 1-3 get exchanged in acetonitrile. Reaction equilibria involving binding and exchange of the terminal ligands (Cl-/H2O/CH3CN) in these complexes have been studied spectrophotometrically. The equilibrium constant for the aquation reaction (K(aq)) [1]2+ + H2O <==> [2]3+ + Cl- in acetonitrile is 8.65(5) M, and the binding constant (K(Cl)-) for the reaction [1]2+ + Cl- [1Cl]+ + CH3CN is 4.75(5) M. The pK(D) value for the dimerization reaction 2[2]3+ + 2OH- <==> [3]4+ + 3H(2)O in 1:1 acetonitrile-water is 9.38(10). Complexes 1-3 upon reaction with Zn(ClO4)(2).6H(2)O and sodium acetate (OAc), pivalate (OPiv), or bis(4-nitrophenyl)phosphate (BNPP) produce the heterobimetallic complexes [{FeLZn(mu-X)}2(mu-O)](ClO4)2, where X = OAc (4), OPiv (5), and BNPP (6). The pseudo-first-order rate constant (k(obs)) for the formation of 4 at 25 degrees C from either 1 or 3 with an excess of Zn(OAc)2.2H2O in 1:1 acetonitrile-water at pH 6.6 is found to be the same with k(obs) = 1.6(2) x 10(-4) s(-1). The X-ray crystal structures of 3, 4, and 6 have been determined, although the structure determination of 3 was severely affected because of heavy disordering. In 3, the Fe-O-Fe angle is 168.6(6) degrees, while it is exactly 180.0 degrees in 4 and 6. Cyclic and square-wave voltammetric (CV and SWV) measurements have been carried out for complexes 1-4 in acetonitrile. The variation of the solvent composition (acetonitrile-water) has a profound effect on the E(1/2) and DeltaE(p) values. The binding of an additional chloride ion to an iron(III) center in 1-3 is accompanied by a remarkable shift of E(1/2) to more negative values. The observation of quasi-reversible CV for complexes containing a Fe(III)-O-Fe(III) unit (3 and 4) indicates that in the electrochemical time scale unusual Fe(III)-O-Fe(II) is produced. The 1H NMR spectra of complexes 3-6 exhibit hyperfine-shifted signals in the range 0-90 ppm with similar features. The metal-hydrogen distances obtained from T(1) measurements are in good agreement with the crystallographic data. Variable-temperature (2-300 K) magnetic susceptibility measurements carried out for 3 and 4 indicate strong antiferromagnetic exchange interaction (H = -2JS1.S2) between the high-spin iron(III) centers in the Fe-O-Fe unit with J = -114 cm(-1) (3) and -107 cm(-1) (4).

Distinct enzymic functional groups are required for the phosphomonoesterase and phosphodiesterase activities of Clostridium thermocellum polynucleotide kinase/phosphatase

The central phosphatase domain of Clostridium thermocellum polynucleotide kinase/phosphatase (CthPnkp) belongs to the dinuclear metallophosphoesterase superfamily. Prior mutational studies of CthPnkp identified 7 individual active site side chains (Asp-187, His-189, Asp-233, Asn-263, His-323, His-376, and Asp-392) required for Ni2+-dependent hydrolysis of p-nitrophenyl phosphate. Here we find that Mn2+-dependent phosphomonoesterase activity requires two additional residues, Arg-237 and His-264. We report that CthPnkp also converts bis-p-nitrophenyl phosphate to p-nitrophenol and inorganic phosphate via a processive two-step mechanism. The Ni2+-dependent phosphodiesterase activity of CthPnkp requires the same seven side chains as the Ni2+-dependent phosphomonoesterase. However, the Mn2+-dependent phosphodiesterase activity does not require His-189, Arg-237, or His-264, each of which is critical for the Mn2+-dependent phosphomonoesterase. Mutations H189A, H189D, and D392N transform the metal and substrate specificity of CthPnkp such that it becomes a Mn2+-dependent phosphodiesterase. The H189E change results in a Mn2+/Ni2+-dependent phosphodiesterase. Mutations H376N, H376D, and D392E convert the enzyme into a Mn2+-dependent phosphodiesterase-monoesterase. The phosphodiesterase activity is strongly stimulated compared with wild-type CthPnkp when His-189 is changed to Asp, Arg-237 is replaced by Ala or Gln, and His-264 is replaced by Ala, Asn, or Gln. Steady-state kinetic analysis of wild-type and mutated enzymes illuminates the structural features that affect substrate affinity and kcat. Our results highlight CthPnkp as an "undifferentiated" diesterase-monoesterase that can evolve toward narrower metal and substrate specificities via alterations of the active site milieu.

Mechanism of the phosphatase component of Clostridium thermocellum polynucleotide kinase-phosphatase

Polynucleotide kinase-phosphatase (Pnkp) from Clostridium thermocellum catalyzes ATP-dependent phosphorylation of 5'-OH termini of DNA or RNA polynucleotides and Ni(2+)/Mn(2+)-dependent dephosphorylation of 2',3' cyclic phosphate, 2'-phosphate, and 3'-phosphate ribonucleotides. CthPnkp is an 870-amino-acid polypeptide composed of three domains: an N-terminal module similar to bacteriophage T4 polynucleotide kinase, a central module that resembles the dinuclear metallo-phosphoesterase superfamily, and a C-terminal ligase-like adenylyltransferase domain. Here we conducted a mutational analysis of CthPnkp that identified 11 residues required for Ni(2+)-dependent phosphatase activity with 2'-AMP and 3'-AMP. Eight of the 11 CthPnkp side chains were also required for Ni(2+)-dependent hydrolysis of p-nitrophenyl phosphate. The ensemble of essential side chains includes the conserved counterparts (Asp187, His189, Asp233, Arg237, Asn263, His264, His323, His376, and Asp392 in CthPnkp) of all of the amino acids that form the dinuclear metal-binding site and the phosphate-binding site of bacteriophage lambda phosphatase. Three residues (Asp236, His264, and Arg237) required for activity with 2'-AMP or 3'-AMP were dispensable for Ni(2+)-dependent hydrolysis of p-nitrophenyl phosphate. Our findings, together with available structural information, provide fresh insights to the metallophosphoesterase mechanism, including the roles of His264 and Asp236 in proton donation to the leaving group. Deletion analysis defined an autonomous phosphatase domain, CthPnkp-(171-424).

Direct observation of multiple protonation states in recombinant human purple acid phosphatase

To date, most spectroscopic studies on mammalian purple acid phosphatases (PAPs) have been performed at a single pH, typically pH 5. The catalytic activity of these enzymes is, however, pH dependent, with optimal pH values of 5.5-6.2 (depending on the form). For example, the pH optimum of PAPs isolated as single polypeptides is around pH 5.5, which is substantially lower that of proteolytically cleaved PAPs (ca. pH 6.2). In addition, the catalytic activity of single polypeptide PAPs at their optimal pH values is four to fivefold lower than that of the proteolytically cleaved enzymes. In order to elucidate the chemical basis for the pH dependence of these enzymes, the spectroscopic properties of both the single polypeptide and proteolytically cleaved forms of recombinant human PAP (recHPAP) and their complexes with inhibitory anions have been examined over the pH range 4 to 8. The EPR spectra of both forms of recHPAP are pH dependent and show the presence of three species: an inactive low pH form (pH<pK( a,1)), an active form (pK( a,1)<pH<pK( a,2)), and an inactive high pH form (pH>pK( a,2)). The pK( a,1) values observed by EPR for the single polypeptide and proteolytically cleaved forms are similar to those previously observed in kinetics studies. The spectroscopic properties of the enzyme-phosphate complex (which should mimic the enzyme-substrate complex), the enzyme-fluoride complex, and the enzyme-fluoride-phosphate complex (which should mimic the ternary enzyme-substrate-hydroxide complex) were also examined. EPR spectra show that phosphate binds to the diiron center of the proteolytically cleaved form of the enzyme, but not to that of the single polypeptide form. EPR spectra also show that fluoride binds only to the low pH form of the enzymes, in which it presumably replaces a coordinated water molecule. The binding of fluoride and phosphate to form a ternary complex appears to be cooperative.

Human tartrate-resistant acid phosphatase becomes an effective ATPase upon proteolytic activation

Proteolytic cleavage in an exposed loop of human tartrate-resistant acid phosphatase (TRAcP) with trypsin leads to a significant increase in activity. At each pH value between 3.25 and 8.0 the cleaved enzyme is more active. Substrate specificity is also influenced by proteolysis. Only the cleaved form is able to hydrolyze unactivated substrates efficiently, and at pH >6 cleaved TRAcP acquires a marked preference for ATP. The cleaved enzyme also has altered sensitivity to inhibitors. Interestingly, the magnitude and mode of inhibition by fluoride depends not only on the proteolytic state but also pH. The combined kinetic data imply a role of the loop residue D158 in catalysis in the cleaved enzyme. Notably, at low pH this residue may act as a proton donor for the leaving group. In this respect the mechanism of cleaved TRAcP resembles that of sweet potato purple acid phosphatase.

A new heterobinuclear FeIIICuII complex with a single terminal FeIII–O(phenolate) bond. Relevance to purple acid phosphatases and nucleases

A novel heterobinuclear mixed valence complex [Fe(III)Cu(II)(BPBPMP)(OAc)(2)]ClO(4), 1, with the unsymmetrical N(5)O(2) donor ligand 2-bis[{(2-pyridylmethyl)aminomethyl}-6-{(2-hydroxybenzyl)(2-pyridylmethyl)}aminomethyl]-4-methylphenol (H(2)BPBPMP) has been synthesized and characterized. A combination of data from mass spectrometry, potentiometric titrations, X-ray absorption and electron paramagnetic resonance spectroscopy, as well as kinetics measurements indicates that in ethanol/water solutions an [Fe(III)-(mu)OH-Cu(II)OH(2)](+) species is generated which is the likely catalyst for 2,4-bis(dinitrophenyl)phosphate and DNA hydrolysis. Insofar as the data are consistent with the presence of an Fe(III)-bound hydroxide acting as a nucleophile during catalysis, 1 presents a suitable mimic for the hydrolytic enzyme purple acid phosphatase. Notably, 1 is significantly more reactive than its isostructural homologues with different metal composition (Fe(III)M(II), where M(II) is Zn(II), Mn(II), Ni(II), or Fe(II)). Of particular interest is the observation that cleavage of double-stranded plasmid DNA occurs even at very low concentrations of 1 (2.5 microM), under physiological conditions (optimum pH of 7.0), with a rate enhancement of 2.7 x 10(7) over the uncatalyzed reaction. Thus, 1 is one of the most effective model complexes to date, mimicking the function of nucleases.