Ga3+ as a functional substitute for Fe3+: preparation and characterization of the Ga3+Fe2+ and Ga3+Zn2+ forms of bovine spleen purple acid phosphatase

Iron-sulphur protein catalysed [4+2] cycloadditions in natural product biosynthesis

To the best of our knowledge, enzymes that catalyse intramolecular Diels-Alder ([4+2] cycloaddition) reactions are frequently reported in natural product biosynthesis; however, no native enzymes utilising Lewis acid catalysis have been reported. Verticilactam is a representative member of polycyclic macrolactams, presumably produced by spontaneous cycloaddition. We report that the intramolecular [4+2] cycloadditions can be significantly accelerated by ferredoxins (Fds), a class of small iron-sulphur (Fe-S) proteins. Through iron atom substitution by Lewis acidic gallium (Ga) iron and computational calculations, we confirm that the ubiquitous Fe-S cluster efficiently functions as Lewis acid to accelerate the tandem [4+2] cycloaddition and Michael addition reactions by lowering free energy barriers. Our work highlights Nature's ingenious strategy to generate complex molecule structures using the ubiquitous Fe-S protein. Furthermore, our study sheds light on the future design of Fd as a versatile Lewis acid catalyst for [4+2] cycloaddition reactions.

Evaluation of immunotoxicity of sodium metavanadate following drinking water exposure in female B6C3F1/N mice in a 28-day study

Sodium metavanadate (NaVO3 ) is a pentavalent vanadium compound used in the metal industry and dietary supplements; human exposure occurs through inhalation of fumes and dust and ingestion of NaVO3 -containing products. The objective of this study was to assess the potential immunotoxicity of NaVO3 . Female B6C3F1/N mice were exposed to 0-500 ppm NaVO3 in drinking water for 28 days and evaluated for effects on immune cell populations and innate, cellular-mediated, and humoral-mediated immunity. There was a decreasing trend in body weight (BW) and BW gain in NaVO3 exposed mice, with a decrease (p ≤ 0.05) in BW gain at ≥250 ppm, relative to control. Conversely, increasing trends in spleen weights and an increase (p ≤ 0.05) in the spleen:BW ratio at ≥250 ppm NaVO3 were observed. NaVO3 exposure altered antibody production against sheep red blood cells (SRBC). Antibody forming cells (AFC)/106 spleen cells exhibited a decreasing trend, with a decrease (p ≤ 0.05) at 500 ppm NaVO3 , concurrent with an increase in percent B cells. NaVO3 had no effect on the serum anti-SRBC IgM antibody titers or anti-keyhole limpet hemocyanin antibody production. Exposure to NaVO3 decreased the percentage of natural killer cells at all dose levels (p ≤ 0.05), with no effect on the lytic activity. NaVO3 altered T-cell populations at 500 ppm but had no effect on T-cell proliferative responses or the lytic activity of cytotoxic T cells. Collectively, these data indicate that NaVO3 exposure can adversely affect the immune system by inducing alterations in humoral-mediated immunity, specifically the AFC response, with no effect on cell-mediated or innate immunity.

Nano sized gallium oxide surface features for enhanced antimicrobial and osteo-integrative responses

Gallium oxide has known beneficial osteo-integrative properties. This may have importance for improving the osteointegration of orthopedic implants. At high concentrations gallium is cytotoxic. Therefore, integration of gallium into implant devices must be carefully controlled to limit its concentration and release. A strategy based on surface doping of gallium although challenging seems an appropriate approach to limit dose amounts to minimize cytotoxicity and maximize osteointegration benefits. In this work we develop a novel form of patterned surface doping via a block copolymer-based surface chemistry that enables very low gallium content but enhanced osteointegration as proven by comprehensive bioassays. Polystyrene-b-poly 4vinyl pyridine (PS-b-P4VP) BCP (block copolymer) films were produced on surfaces. Selective infiltration of the BCP pattern with a gallium salt precursor solution and subsequent UV-ozone treatment produced a surface pattern of gallium oxide nanodots as evidenced by atomic force and scanning electron microscopy. A comprehensive study of the bioactivity was carried out, including antimicrobial and sterility testing, gallium ion release kinetics and the interaction with human marrow mesenchymal stomal cells and mononuclear cells. Comparing the data from osteogenesis media assay tests with osteoclastogenesis tests demonstrated the potential for the gallium oxide nanodot doping to improve osteointegration properties of a surface.

The Ruthenium Nitrosyl Moiety in Clusters: Trinuclear Linear μ-Hydroxido Magnesium(II)-Diruthenium(II), μ3-Oxido Trinuclear Diiron(III)-Ruthenium(II), and Tetranuclear μ4-Oxido Trigallium(III)-Ruthenium(II) Complexes

The ruthenium nitrosyl moiety, {RuNO}6, is important as a potential releasing agent of nitric oxide and is of inherent interest in coordination chemistry. Typically, {RuNO}6 is found in mononuclear complexes. Herein we describe the synthesis and characterization of several multimetal cluster complexes that contain this unit. Specifically, the heterotrinuclear μ3-oxido clusters [Fe2RuCl4(μ3-O)(μ-OMe)(μ-pz)2(NO)(Hpz)2] (6) and [Fe2RuCl3(μ3-O)(μ-OMe)(μ-pz)3(MeOH)(NO)(Hpz)][Fe2RuCl3(μ3-O)(μ-OMe)(μ-pz)3(DMF)(NO)(Hpz)] (7·MeOH·2H2O) and the heterotetranuclear μ4-oxido complex [Ga3RuCl3(μ4-O)(μ-OMe)3(μ-pz)4(NO)] (8) were prepared from trans-[Ru(OH)(NO)(Hpz)4]Cl2 (5), which itself was prepared via acidic hydrolysis of the linear heterotrinuclear complex {[Ru(μ-OH)(μ-pz)2(pz)(NO)(Hpz)]2Mg} (4). Complex 4 was synthesized from the mononuclear Ru complexes (H2pz)[trans-RuCl4(Hpz)2] (1), trans-[RuCl2(Hpz)4]Cl (2), and trans-[RuCl2(Hpz)4] (3). The new compounds 4-8 were all characterized by elemental analysis, ESI mass spectrometry, IR, UV-vis, and 1H NMR spectroscopy, and single-crystal X-ray diffraction, with complexes 6 and 7 being characterized also by temperature-dependent magnetic susceptibility measurements and Mössbauer spectroscopy. Magnetometry indicated a strong antiferromagnetic interaction between paramagnetic centers in 6 and 7. The ability of 4 and 6-8 to form linkage isomers and release NO upon irradiation in the solid state was investigated by IR spectroscopy. A theoretical investigation of the electronic structure of 6 by DFT and ab initio CASSCF/NEVPT2 calculations indicated a redox-noninnocent behavior of the NO ancillary ligand in 6, which was also manifested in TD-DFT calculations of its electronic absorption spectrum. The electronic structure of 6 was also studied by an X-ray charge density analysis.

A perspective essay on the use of Ga3+ as a proxy for Fe3+ in bioinorganic model studies and its successful use for therapeutic purposes

The use of Ga³⁺ as a structural mimic for Fe³⁺ in model bioinorganic investigations is usually based on a common assumption that Ga³⁺ and Fe³⁺ should form bioligand complexes of similar stabilities due to their similar charge/radius ratio (z/r). However, the literature survey presented here is contrary to this notion, showing that under laboratory conditions often Ga³⁺ forms weaker bioligand complexes than Fe³⁺in aqueous medium. We hypothesize that this is because Ga³⁺ is more aquaphilic than Fe³⁺ as suggested by their relative heats of hydration (ΔH_hyd). The successful use of Ga³⁺ as a therapeutic agent is also briefly reviewed, showing this success often stems from the redox inertness as well as different pharmacokinetics of Ga³⁺ than Fe³⁺, but similar metabolic pathways as Fe³⁺ in human serum.

New AlIIIZnII and AlIIICuII dinuclear complexes: Phosphatase-like activity and cytotoxicity

Herein, we report the synthesis and characterization of the first two Al^III(μ-OH)M^II (M = Zn (1) and Cu (2)) complexes with the unsymmetrical ligand H₂L{2-[[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl]-6-bis(pyridylmethyl)aminomethyl}-4-methylphenol. The complexes were characterized through elemental analysis, X-ray crystallography, IR spectroscopy, mass spectrometry and potentiometric titration. In addition, complex 2 was characterized by electronic spectroscopy. Kinetics studies on the hydrolysis of the model substrate bis(2,4-dinitrophenyl)phosphate by 1 and 2 show Michaelis-Menten behavior, with 1 being slightly more active (8.31%) than 2 (at pH 7.0). The antimicrobial effect of the compounds was studied using four bacterial strains (Staphylococcus aureus, Pseudomonas aeuruginosa, Shigella sonnei and Shigella dysenteriae) and for both complexes the inhibition of bacterial growth was superior to that caused by sulfapyridine, but inferior to that of tetracycline. The dark cytotoxicity and photocytotoxicity (under UV-A light) of the complexes in a chronic myelogenous leukemia cell line were investigated. Complexes 1 and 2 exhibited significant cytotoxic activity against K562 cells, which undergoes a 2-fold increase on applying 5 min of irradiation with UV-A light. Complex 2 was more effective and a good correlation between cytotoxicity and intracellular concentration was observed, the intracellular copper concentration required to inhibit 50% of cell growth being 3.5 × 10^-15 mol cell^-1.

Structural, Spectroscopic, and Theoretical Investigation of a T-Shaped [Fe3(μ3-O)] Cluster

The synthesis, X-ray crystal and electronic structures of [Fe₃(μ₃-O)(mpmae)₂(OAc)₂ Cl₃], 1, where mpmae-H = 2-(N-methyl-N-((pyridine-2-yl)methyl)amino)ethanol, are described. This cluster comprises three high-spin ferric ions and exhibits a T-shaped site topology. Variable-frequency electron paramagnetic resonance measurements performed on single crystals of 1 demonstrate a total spin S_T = 5/2 ground state, characterized by a small, negative, and nearly axial zero-field splitting tensor D = -0.49 cm^-1, E/D ≈ 0.055. Analysis of magnetic susceptibility, magnetization, and magneto-structural correlations further corroborate the presence of a sextet ground-spin state. The observed ground state originates from the strong anti-ferromagnetic interaction of two iron(III) spins, with J = 115(5) cm^-1, that, in turn, are only weakly coupled to the spin of the third site, with j = 7(1) cm^-1. These exchange interactions lead to a ground state with magnetic properties that are essentially entirely determined by the weakly coupled site. The contributions of the individual spins to the total ground state of the cluster were monitored using variable-field ⁵⁷Fe Mössbauer spectroscopy. Field-dependent spectra reveal that, while one of the iron sites exhibits a large negative internal field, typical of ferric ions, the other two sites exhibit small, but not null, negative and positive internal fields. A theoretical analysis reveals that these small internal fields originate from the mixing of the lowest S_T = 5/2 excited state into the ground state which, in turn, is induced by a minute structural distortion.

Orienting the heterocyclic periphery: a structural model for chloroquine's antimalarial activity

A fluorescent structurally characterized chloroquine–metalloporphyrin adduct has been prepared and characterized. This allows for new insights into antimalarial drug–heme interactions.

Structure elucidation and preliminary assessment of hydrolase activity of PqsE, the Pseudomonas quinolone signal (PQS) response protein

In bacteria, the transcription of virulence genes is usually controlled by a cell density-dependent process known as "quorum sensing" (QS). QS relies on small diffusible signaling molecules that cross the bacterial cell wall and activate target transcription factors after a threshold concentration has been reached. Besides two hierarchical QS circuits based on N-acylhomoserine lactones, the human opportunistic pathogen Pseudomonas aeruginosa integrates a signaling system that depends on 2-heptyl-3-hydroxy-4-quinolone, termed "Pseudomonas quinolone signal" (PQS). PQS is produced from genes encoded in the pqs operon, which in addition to the biosynthetic enzymes PqsA-D contains a fifth gene, pqsE, that is not required for production of PQS but whose disruption leads to loss of signal transduction in several but not all pqs operon-dependent processes. PqsE was hence termed "PQS response protein", but its exact mechanism of action is unknown. We have determined the crystal structure of recombinant PqsE and show that it possesses a metallo-beta-lactamase fold with an Fe(II)Fe(III) center in the active site. A copurified ligand was assigned as benzoate and may indicate that PqsE exerts its regulatory effect by converting a chorismate-derived molecule. Further, PqsE was found to slowly hydrolyze phosphodiesters including single- and double-stranded DNA as well as mRNA and also the thioester S-(4-nitrobenzoyl)mercaptoethane. Higher activity was observed after incubation with Co(2+) and, to lesser entent, Mn(2+), suggesting that the Fe(II)Fe(III) center of recombinant PqsE may be an artifact of heterologous expression. A crystal complex of the E182A mutant with bis-pNPP was obtained and suggests a catalytic mechanism for hydrolysis.

The reaction mechanism of the Ga(III)Zn(II) derivative of uteroferrin and corresponding biomimetics

Purple acid phosphatase from pig uterine fluid (uteroferrin), a representative of the diverse family of binuclear metallohydrolases, requires a heterovalent Fe(III)Fe(II) center for catalytic activity. The active-site structure and reaction mechanism of this enzyme were probed with a combination of methods including metal ion replacement and biomimetic studies. Specifically, the asymmetric ligand 2-bis{[(2-pyridylmethyl)-aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl}-4-methylphenol and two symmetric analogues that contain the softer and harder sites of the asymmetric unit were employed to assess the site selectivity of the trivalent and divalent metal ions using (71)Ga NMR, mass spectrometry and X-ray crystallography. An exclusive preference of the harder site of the asymmetric ligand for the trivalent metal ion was observed. Comparison of the reactivities of the biomimetics with Ga(III)Zn(II) and Fe(III)Zn(II) centers indicates a higher turnover for the former, suggesting that the M(III)-bound hydroxide acts as the reaction-initiating nucleophile. Catalytically active Ga(III)Zn(II) and Fe(III)Zn(II) derivatives were also generated in the active site of uteroferrin. As in the case of the biomimetics, the Ga(III) derivative has increased reactivity, and a comparison of the pH dependence of the catalytic parameters of native uteroferrin and its metal ion derivatives supports a flexible mechanistic strategy whereby both the mu-(hydr)oxide and the terminal M(III)-bound hydroxide can act as nucleophiles, depending on the metal ion composition, the geometry of the second coordination sphere and the substrate.

Role for an essential tyrosine in peptide amidation

The catalytic core of the peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL) domain of peptidylglycine alpha-amidating monooxygenase was investigated with respect to its ability to function as a ureidoglycolate lyase and the identity and role of its bound metal ions. The purified PAL catalytic core (PALcc) contains molar equivalents of calcium and zinc along with substoichiometric amounts of iron and functions as a ureidoglycolate lyase. Limiting iron availability in the cells synthesizing PALcc reduces the specific activity of the enzyme produced. Concentrated samples of native PALcc have an absorption maximum at 560 nm, suggestive of a phenolate-Fe(III) charge transfer complex. An essential role for a Tyr residue was confirmed by elimination of PAL activity following site-directed mutagenesis. Purified PALcc in which the only conserved Tyr residue (Tyr(654)) was mutated to Phe was secreted normally, but was catalytically inactive and lacked bound iron and bound zinc. Our data demonstrate an essential role for Tyr(654) and suggest that it serves as an Fe(III) ligand in an essential iron-zinc bimetallic site.

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.

Substrate positioning by His92 is important in catalysis by purple acid phosphatase

Proteolysis of single polypeptide mammalian purple acid phosphatases (PAPs) results in the loss of an interaction between the loop residue Asp146 and the active site residues Asn91 and/or His92. While Asn91 is a ligand to the divalent metal of the mixed-valent di-iron center, the role of His92 in the catalytic mechanism is unknown. Site-directed mutagenesis of His92 was performed to examine the role of this residue in single polypeptide PAP. Conversion of His92 into Ala, which eliminates polar interactions of this residue with the active site, resulted in a 10-fold decrease in catalytic activity at the optimal pH. Conversely, conversion of this residue into Asn, which cannot function as either a proton donor or acceptor, but can provide hydrogen-bonding interactions, resulted in a three-fold increase in activity at the optimal pH. Both mutant enzymes had more acidic pH optima, with pK(es,1) values consistent with the involvement of an iron(III) hydroxide unit or a hydroxide in the second coordination sphere in catalysis. These results, together with EPR data, support a role of His92 in positioning either the nucleophile or the substrate, rather than directly in acid or base catalysis. The existence of an extensive hydrogen-bonding network that could fine-tune the position of His92 is consistent with this proposal.

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.

The Fe(III)Zn(II) form of recombinant human purple acid phosphatase is not activated by proteolysis

The kinetics and spectroscopic properties of the single polypeptide and proteolytically cleaved form of recombinant Fe(3+)Fe(2+) human purple acid phosphatase (recHPAP) exhibit significant differences, primarily due to a difference in pK(es,1) (the value of an acid dissociation constant of the ES complex). These differences are due to the presence or absence, respectively, of an interaction between an aspartate residue in an exposed loop of the protein and one or more active site residues. To further explore the origin of these differences, the ferrous ion of recHPAP has been replaced by zinc. Analysis of the reconstituted Fe(3+)Zn(2+)recHPAP reveals an unexpected catalytic activity versus pH profile, in that the optimal pH is 6.3, similar to that of the proteolytically cleaved form (6.5). Moreover, replacement of the ferrous ion by zinc increases the turnover number more than 10-fold; the pK(es) values are also shifted as expected for the change in the divalent metal ion. Although the EPR spectra of both single polypeptide and proteolytically cleaved Fe(3+)Zn(2+)-recHPAP are independent of pH over the range 4.5-6.2, the visible spectrum of Fe(3+)Zn(2+)-recHPAP is pH dependent. These results suggest that the properties and environment of the divalent metal are important in determining the catalytic properties of mammalian PAPs, and in particular that a solvent molecule coordinated to the divalent metal ion may play a critical role in the catalytic cycle of these enzymes.

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

Purple acid phosphatases (PAPs) are a family of binuclear metalloenzymes that catalyze the hydrolysis of phosphoric acid esters and anhydrides. A PAP in sweet potato has a unique, strongly antiferromagnetically coupled Fe(III)-Mn(II) center and is distinguished from other PAPs by its increased catalytic efficiency for a range of activated and unactivated phosphate esters, its strict requirement for Mn(II), and the presence of a mu-oxo bridge at pH 4.90. This enzyme displays maximum catalytic efficiency (k(cat)/K(m)) at pH 4.5, whereas its catalytic rate constant (k(cat)) is maximal at near-neutral pH, and, in contrast to other PAPs, its catalytic parameters are not dependent on the pK(a) of the leaving group. The crystal structure of the phosphate-bound Fe(III)-Mn(II) PAP has been determined to 2.5-A resolution (final R(free) value of 0.256). Structural comparisons of the active site of sweet potato, red kidney bean, and mammalian PAPs show several amino acid substitutions in the sweet potato enzyme that can account for its increased catalytic efficiency. The phosphate molecule binds in an unusual tripodal mode to the two metal ions, with two of the phosphate oxygen atoms binding to Fe(III) and Mn(II), a third oxygen atom bridging the two metal ions, and the fourth oxygen pointing toward the substrate binding pocket. This binding mode is unique among the known structures in this family but is reminiscent of phosphate binding to urease and of sulfate binding to lambda protein phosphatase. The structure and kinetics support the hypothesis that the bridging oxygen atom initiates hydrolysis.

The bacterial receptor protein, transferrin-binding protein B, does not independently facilitate the release of metal ion from human transferrin

Pathogenic Gram-negative bacteria of the Pasteurellaceae and Neisseriaceae acquire iron for growth from host transferrin through the action of specific surface receptors. Iron is removed from transferrin by the receptor at the cell surface and is transported across the outer membrane to the periplasm. A periplasmic binding protein-dependent pathway subsequently transports iron into the cell. The transferrin receptor is composed of a largely surface-exposed lipoprotein, transferrin binding protein B, and a TonB-dependent integral outer membrane protein, transferrin binding protein A. To examine the role of transferrin binding protein B in the iron removal process, complexes of recombinant transferrin binding protein B and transferrin were prepared and compared with transferrin in metal-binding and -removal experiments. A polyhistidine-tagged form of recombinant transferrin binding protein B was able to purify a complex with transferrin that was largely monodisperse by dynamic light scattering analysis. Gallium was used instead of iron in the metal-binding studies, since it resulted in increased stability of recombinant transferrin binding protein B in the complex. Difference absorption spectra were used to monitor removal of gallium by nitrilotriacetic acid. Kinetic and equilibrium binding studies indicated that transferrin binds gallium more tightly in the presence of transferrin binding protein B. Thus, transferrin binding protein B does not facilitate metal ion removal and additional components are required for this process.

DNA hydrolytic cleavage by the diiron(III) complex Fe(2)(DTPB)(mu-O)(mu-Ac)Cl(BF(4))(2): comparison with other binuclear transition metal complexes

The binuclear structure of Fe(2)(DTPB)(mu-O)(mu-Ac)Cl(BF(4))(2) (DTPB = 1,1,4,7,7-penta (2'-benzimidazol-2-ylmethyl)-triazaheptane, Ac = acetate) was characterized by UV-visible absorption and infrared spectra and NMR and ESR. The binding interaction of DNA with the diiron complex was examined spectroscopically. Supercoiled and linear DNA hydrolytic cleavage by the diiron complex is supported by the evidence from anaerobic reactions, free radical quenching, high performance liquid chromatography experiments, and enzymatic manipulation such as T4 ligase ligation, 5'-(32)P end-labeling, and footprinting analysis. The estimation of rate for the supercoiled DNA double strand cleavage shows one of the largest known rate enhancement factors, approximately 10(10) against DNA. Moreover, the DNA hydrolysis chemistry needs no coreactant such as hydrogen peroxide. The poor sequence-specific DNA cleavage indicated by the restriction analysis of the pBR322 DNA linearized by the diiron complex might be due to the diiron complex bound to DNA by a coordination of its two ferric ions to the DNA phosphate oxygens, as suggested by spectral characterizations. The hydrolysis chemistry for a variety of binuclear metal complexes including Fe(2)(DTPB)(mu-O)(mu-Ac)Cl(BF(4))(2) is compared. It is established that the dominant factors for the DNA hydrolysis activities of the binuclear metal complexes are the mu-oxo bridge, labile and anionic ligands, and open coordination site(s). Concerning the hydrolytic mechanisms, the diiron complex Fe(2)(DTPB)(mu-O)(mu-Ac)Cl(BF(4))(2) might share many points in common with the native purple acid phosphatases.

The highly exposed loop region in mammalian purple acid phosphatase controls the catalytic activity

Recombinant human purple acid phosphatase (recHPAP) provides a convenient experimental system for assessing the relationship between molecular structure and enzymatic activity in mammalian purple acid phosphatases (PAPs). recHPAP is a monomeric protein with properties similar to those of uteroferrin (Uf) and other PAPs isolated as single polypeptide chains, but its properties differ significantly from those of bovine spleen PAP (BSPAP) and other PAPs isolated as proteolytically "clipped" forms. Incubation of recHPAP with trypsin results in proteolytic cleavage in an exposed region near the active site. The product is a tightly associated two-subunit protein whose collective spectroscopic and kinetics properties resemble those of BSPAP. These results demonstrate that the differences in spectroscopic and kinetics properties previously reported for mammalian PAPs are the result of proteolytic cleavage. Mass spectrometry shows that a three-residue segment, D-V-K, within the loop region is excised by trypsin. This finding suggests that important interactions between residues in the excised loop and one or more of the groups that participate in catalysis are lost or altered upon proteolytic cleavage. Analysis of available structural data indicates that the most important such interaction is that between Asp 146 in the exposed loop and active-site residues Asn 91 and His 92. Loss of this interaction should result in both an increase in the Lewis acidity of the Fe(II) ion and an increase in the nucleophilicity of the Fe(III)-bound hydroxide ion. Proteolytic cleavage thus constitutes a potential physiological mechanism for regulating the activity of PAP in vivo.