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

Cathepsin K regulates localization and secretion of Tartrate-Resistant Acid Phosphatase (TRAP) in TRAP-overexpressing MDA-MB-231 breast cancer cells

Background

Tartrate–resistant acid phosphatase (TRAP/ ACP5) belongs to the binuclear metallophosphatase family and is present in two isoforms. The primary translation product is an uncleaved TRAP 5a isoform with low phosphatase activity. TRAP 5a can be post-translationally processed to a cleaved TRAP 5b isoform with high phosphatase activity by e.g. cysteine proteinases, such as Cathepsin K (CtsK). The relevance of the phosphatase activity of TRAP 5b has been demonstrated for proliferation, migration and invasion of cancer cells. TRAP-overexpressing MDA-MB-231 breast cancer cells displayed higher levels of TRAP 5a and efficient processing of TRAP 5a to TRAP 5b protein, but no changes in levels of CtsK when compared to mock-transfected cells. In TRAP-overexpressing cells colocalization of TRAP 5a and proCtsK was augmented, providing a plausible mechanism for generation of TRAP 5b. CtsK expression has been associated with cancer progression and has been pharmacologically targeted in several clinical studies.

Results

In the current study, CtsK inhibition with MK-0822/Odanacatib did not abrogate the formation of TRAP 5b, but reversibly increased the intracellular levels of a N-terminal fragment of TRAP 5b and reduced secretion of TRAP 5a reversibly. However, MK-0822 treatment neither altered intracellular TRAP activity nor TRAP-dependent cell migration, suggesting involvement of additional proteases in proteolytic processing of TRAP 5a. Notwithstanding, CtsK was shown to be colocalized with TRAP and to be involved in the regulation of secretion of TRAP 5a in a breast cancer cell line, while it still was not essential for processing of TRAP 5a to TRAP 5b isoform.

Conclusion

In cancer cells multiple proteases are involved in cleaving TRAP 5a to high-activity phosphatase TRAP 5b. However, CtsK-inhibiting treatment was able to reduce secretion TRAP 5a from TRAP-overexpressing cancer cells.

Bone Alkaline Phosphatase and Tartrate-Resistant Acid Phosphatase: Potential Co-regulators of Bone Mineralization

Phosphorylated osteopontin (OPN) inhibits hydroxyapatite crystal formation and growth, and bone alkaline phosphatase (BALP) promotes extracellular mineralization via the release of inorganic phosphate from the mineralization inhibitor inorganic pyrophosphate (PPi). Tartrate-resistant acid phosphatase (TRAP), produced by osteoclasts, osteoblasts, and osteocytes, exhibits potent phosphatase activity towards OPN; however, its potential capacity as a regulator of mineralization has not previously been addressed. We compared the efficiency of BALP and TRAP towards the endogenous substrates for BALP, i.e., PPi and pyridoxal 5'-phosphate (PLP), and their impact on mineralization in vitro via dephosphorylation of bovine milk OPN. TRAP showed higher phosphatase activity towards phosphorylated OPN and PPi compared to BALP, whereas the activity of TRAP and BALP towards PLP was comparable. Bovine milk OPN could be completely dephosphorylated by TRAP, liberating all its 28 phosphates, whereas BALP dephosphorylated at most 10 phosphates. OPN, dephosphorylated by either BALP or TRAP, showed a partially or completely attenuated phosphorylation-dependent inhibitory capacity, respectively, compared to native OPN on the formation of mineralized nodules. Thus, there are phosphorylations in OPN important for inhibition of mineralization that are removed by TRAP but not by BALP. In conclusion, our data indicate that both BALP and TRAP can alleviate the inhibitory effect of OPN on mineralization, suggesting a potential role for TRAP in skeletal mineralization. Further studies are warranted to explore the possible physiological relevance of TRAP in bone mineralization.

The small chemical enzyme inhibitor 5-phenylnicotinic acid/CD13 inhibits cell migration and invasion of tartrate-resistant acid phosphatase/ACP5-overexpressing MDA-MB-231 breast cancer cells

Tartrate-resistant acid phosphatase (TRAP/ACP5/uteroferrin/purple acid phosphatase/PP5) has received considerable attention as a newly discovered proinvasion metastasis driver associated with different malignancies. This renders TRAP an interesting target for novel anti-cancer therapy approaches. TRAP exists as two isoforms, 5a and 5b, where the 5a isoform represents an enzymatically less active monomeric precursor to the more enzymatically active 5b isoform generated by proteolytic excision of a repressive loop domain. Recently, three novel lead compounds were identified by fragment-based screening and demonstrated to be efficient TRAP enzyme inhibitors in vitro. We conclude that one of the three compounds i.e. 5-phenylnicotinic acid (CD13) was efficient as a TRAP inhibitor with Kic values in the low micromolar range towards the TRAP 5b isoform, but was not able to inhibit the TRAP 5a isoform. Structure-based docking revealed similar interactions of CD13 with the active site in both TRAP isoforms. In stably TRAP-overexpressing MDA-MB-231 breast cancer cells, CD13 inhibited intracellular TRAP activity and showed no cytotoxicity at 200 µM. Furthermore, CD13 selectively blocked the TRAP 5b isoform compared to the TRAP 5a in cultured cells, indicating the usefulness of CD13 for assessing the different biological functions of the two TRAP isoforms 5a and 5b in cell systems. Moreover, inhibition of cell migration and invasion of stably TRAP-overexpressing MDA-MB-231 by CD13 was observed. These data establish a proof of principle that a small chemical inhibitor of the TRAP enzyme can block TRAP-dependent functions in cancer cells.

Catalytic mechanisms of metallohydrolases containing two metal ions

At least one-third of enzymes contain metal ions as cofactors necessary for a diverse range of catalytic activities. In the case of polymetallic enzymes (i.e., two or more metal ions involved in catalysis), the presence of two (or more) closely spaced metal ions gives an additional advantage in terms of (i) charge delocalisation, (ii) smaller activation barriers, (iii) the ability to bind larger substrates, (iv) enhanced electrostatic activation of substrates, and (v) decreased transition-state energies. Among this group of proteins, enzymes that catalyze the hydrolysis of ester and amide bonds form a very prominent family, the metallohydrolases. These enzymes are involved in a multitude of biological functions, and an increasing number of them gain attention for translational research in medicine and biotechnology. Their functional versatility and catalytic proficiency are largely due to the presence of metal ions in their active sites. In this chapter, we thus discuss and compare the reaction mechanisms of several closely related enzymes with a view to highlighting the functional diversity bestowed upon them by their metal ion cofactors.

Tartrate-resistant acid phosphatase (TRAP) co-localizes with receptor activator of NF-KB ligand (RANKL) and osteoprotegerin (OPG) in lysosomal-associated membrane protein 1 (LAMP1)-positive vesicles in rat osteoblasts and osteocytes

Tartrate-resistant acid phosphatase (TRAP) is well known as an osteoclast marker; however, a recent study from our group demonstrated enhanced number of TRAP + osteocytes as well as enhanced levels of TRAP located to intracellular vesicles in osteoblasts and osteocytes in experimental osteoporosis in rats. Such vesicles were especially abundant in osteoblasts and osteocytes in cancellous bone as well as close to bone surface and intracortical remodeling sites. To further investigate TRAP in osteoblasts and osteocytes, long bones from young, growing rats were examined. Immunofluorescence confocal microscopy displayed co-localization of TRAP with receptor activator of NF-KB ligand (RANKL) and osteoprotegerin (OPG) in hypertrophic chondrocytes and diaphyseal osteocytes with Pearson's correlation coefficient ≥0.8. Transmission electron microscopy showed co-localization of TRAP and RANKL in lysosomal-associated membrane protein 1 (LAMP1) + vesicles in osteoblasts and osteocytes supporting the results obtained by confocal microscopy. Recent in vitro data have demonstrated OPG as a traffic regulator for RANKL to LAMP1 + secretory lysosomes in osteoblasts and osteocytes, which seem to serve as temporary storage compartments for RANKL. Our in situ observations indicate that TRAP is located to RANKL-/OPG-positive secretory lysosomes in osteoblasts and osteocytes, which may have implications for osteocyte regulation of osteoclastogenesis.

Modulation of osteoclastic migration by metabolism of 25OH-vitamin D3

We have reported the metabolism of 25(OH) vitamin D3 (25D) into active 1α,25(OH)2 vitamin D3 (1,25D) by osteoclasts derived from human peripheral blood mononuclear cells (PBMC), RAW 264.7cells or giant cell tumor of bone (GCT), which appears to optimize osteoclast differentiation but inhibit their activity. In this study, to elucidate the mechanism by which 25D reduces osteoclast resorption, we further examined the effect of 25D on osteoclast function by using GCT-derived osteoclasts. 25D treated cells on dentine slices resulted in decreased resorption volume and depth in 3D image analysis. Tartrate-resistant acid phosphatase (TRAP) has been reported to enhance the dephosphorylation of substrate binding proteins, resulting in reduced osteoclast attachment. Therefore, we next investigated the effect of 25D on cell migration. Treatment of GCT cells with 25D augmented cell migration, as determined by live cell imaging. These observations suggest that 25D metabolism by osteoclasts reduces their resorptive capacity, in part by modifying their surface adhesion and migration properties. This article is part of a Special Issue entitled "Vitamin D Workshop".

Predicting flexible loop regions that interact with ligands: the challenge of accurate scoring

Flexible loop regions play a critical role in the biological function of many proteins and have been shown to be involved in ligand binding. In the context of structure-based drug design, using or predicting an incorrect loop configuration can be detrimental to the study if the loop is capable of interacting with the ligand. Three protein systems, each with at least one flexible loop region in close proximity to the known binding site, were selected for loop prediction using the CorLps program; a six residue loop region from phosphoribosylglycinamide formyltransferase (GART), two nine residue loop regions from cytochrome P450 (CYP) 119, and an 11 residue loop region from enolase were selected for loop prediction. The results of this study indicate that the statistically based DFIRE scoring function implemented in the CorLps program did not accurately rank native-like predicted loop configurations in any protein system. In an attempt to improve the ranking of the native-like predicted loop configurations, the MM/GBSA and the optimized MM/GBSA-dsr scoring functions were used to re-rank the predicted loops with and without bound ligand. In general, single snapshot MM/GBSA scoring provided the best ranking of native-like loop configurations. Based on the scoring function analyses presented, the optimal ranking of native-like loop configurations is still a difficult challenge and the choice of the "best" scoring function appears to be system dependent.

New computational method for prediction of interacting protein loop regions

Flexible loop regions of proteins play a crucial role in many biological functions such as protein-ligand recognition, enzymatic catalysis, and protein-protein association. To date, most computational methods that predict the conformational states of loops only focus on individual loop regions. However, loop regions are often spatially in close proximity to one another and their mutual interactions stabilize their conformations. We have developed a new method, titled CorLps, capable of simultaneously predicting such interacting loop regions. First, an ensemble of individual loop conformations is generated for each loop region. The members of the individual ensembles are combined and are accepted or rejected based on a steric clash filter. After a subsequent side-chain optimization step, the resulting conformations of the interacting loops are ranked by the statistical scoring function DFIRE that originated from protein structure prediction. Our results show that predicting interacting loops with CorLps is superior to sequential prediction of the two interacting loop regions, and our method is comparable in accuracy to single loop predictions. Furthermore, improved predictive accuracy of the top-ranked solution is achieved for 12-residue length loop regions by diversifying the initial pool of individual loop conformations using a quality threshold clustering algorithm.

Purple acid phosphatase in the walls of tobacco cells

Purple acid phosphatase isolated from the walls of tobacco cells appears to be a 220kDa homotetramer composed of 60kDa subunits, which is purple in color and which contains iron as its only metal ion. Although the phosphatase did not require dithiothreitol for activity and was not inhibited by phenylarsine oxide, the enzyme showed a higher catalytic efficiency (k(cat)/K(m)) for phosphotyrosine-containing peptides than for other substrates including p-nitrophenyl-phosphate and ATP. The phosphatase formed as a 120kDa dimer in the cytoplasm and as a 220kDa tetramer in the walls, where Brefeldin A blocked its secretion during wall regeneration. According to our double-immunofluorescence labeling results, the enzyme might be translocated through the Golgi apparatus to the walls at the interphase and to the cell plate during cytokinesis.

Crystal structures of a purple acid phosphatase, representing different steps of this enzyme's catalytic cycle

BACKGROUND

Purple acid phosphatases belong to the family of binuclear metallohydrolases and are involved in a multitude of biological functions, ranging from bacterial killing and bone metabolism in animals to phosphate uptake in plants. Due to its role in bone resorption purple acid phosphatase has evolved into a promising target for the development of anti-osteoporotic chemotherapeutics. The design of specific and potent inhibitors for this enzyme is aided by detailed knowledge of its reaction mechanism. However, despite considerable effort in the last 10 years various aspects of the basic molecular mechanism of action are still not fully understood.

RESULTS

Red kidney bean purple acid phosphatase is a heterovalent enzyme with an Fe(III)Zn(II) center in the active site. Two new structures with bound sulfate (2.4 A) and fluoride (2.2 A) provide insight into the pre-catalytic phase of its reaction cycle and phosphorolysis. The sulfate-bound structure illustrates the significance of an extensive hydrogen bonding network in the second coordination sphere in initial substrate binding and orientation prior to hydrolysis. Importantly, both metal ions are five-coordinate in this structure, with only one nucleophilic mu-hydroxide present in the metal-bridging position. The fluoride-bound structure provides visual support for an activation mechanism for this mu-hydroxide whereby substrate binding induces a shift of this bridging ligand towards the divalent metal ion, thus increasing its nucleophilicity.

CONCLUSION

In combination with kinetic, crystallographic and spectroscopic data these structures of red kidney bean purple acid phosphatase facilitate the proposal of a comprehensive eight-step model for the catalytic mechanism of purple acid phosphatases in general.

Development of a novel fragments absorbed immunocapture enzyme assay system for tartrate-resistant acid phosphatase 5b

BACKGROUND

Osteoclastic activity is mainly assessed by measuring urinary markers. To correct for differences in renal clearance, the levels of urinary markers are usually corrected by the urine creatinine concentration. Therefore, alternative serum markers to evaluate osteoclastic activity are required. We developed a novel system for the determination of serum tartrate-resistant acid phosphatase 5b (TRACP5b) activity to evaluate osteoclastic activity.

METHODS

Two unique monoclonal antibodies were generated and the specificity was tested using a surface enhanced laser desorption/ionization time-of-flight mass spectroscopy (SELDI TOF-MS). A novel fragments absorbed immunocapture enzymatic assay (FAICEA) method was developed using 2 monoclonal antibodies.

RESULTS

FAICEA gave a sensitivity 0.1 U/l, linearity of 0.1-28 U/l, recovery 92-103%, inter-assay CV 2.95% and intra-assay CV 2.15%. Unlike other TRACP5b assay systems, FAICEA avoided interference from TRACP 5a.

CONCLUSIONS

According to the FAICEA, postmenopausal women had higher TRACP5b concentrations than younger women. The results show that TRACP5b is a novel bone resorption marker in serum.

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.

Effects of proteolysis and reduction on phosphatase and ROS-generating activity of human tartrate-resistant acid phosphatase

Osteoclasts and macrophages express high amounts of tartrate-resistant acid phosphatase (TRACP), an enzyme with unknown biological function. TRACP contains a disulfide bond, a protease-sensitive loop peptide, and a redox-active iron that can catalyze formation of reactive oxygen species (ROS). We studied the effects of proteolytic cleavage by trypsin, reduction of the disulfide bond by beta-mercaptoethanol, and reduction of the redox-active iron by ascorbate on the phosphatase and ROS-generating activity of baculovirus-generated recombinant human TRACP. Ascorbate alone and trypsin in combination with beta-mercaptoethanol increased k(cat)/K(m) of the phosphatase activity seven- to ninefold. The pH-optimum was changed from 5.4-5.6 to 6.2-6.4 by ascorbate and trypsin cleavage. Trypsin cleavage increased k(cat)/K(m) of the ROS-generating activity 2.5-fold without affecting the pH-optimum (7.0). These results suggest that the protease-sensitive loop peptide, redox-active iron, and disulfide bond are important regulatory sites in TRACP, and that the phosphatase and ROS-generating activity are performed with different reaction mechanisms.

Serum TRACP 5b is a useful marker for monitoring alendronate treatment: comparison with other markers of bone turnover

We studied clinical performance of serum TRACP 5b and other bone turnover markers, including S-CTX, U-DPD, S-PINP, S-BALP, and S-OC, for monitoring alendronate treatment. TRACP 5b had higher clinical sensitivity, area under the ROC curve, and signal-to-noise ratio than the other markers.

INTRODUCTION

The purpose of this study was to compare the clinical performance of serum TRACP 5b (S-TRACP5b) with that of other markers of bone turnover in the monitoring of alendronate treatment.

MATERIALS AND METHODS

This double-blinded study included 148 healthy postmenopausal women that were randomly assigned into two groups: one receiving 5 mg alendronate daily (n=75) and the other receiving placebo (n=73) for 12 months. All individuals in both groups received calcium and vitamin D daily. The bone resorption markers S-TRACP5b, serum C-terminal cross-linked telopeptides of type I collagen (S-CTX), and total urinary deoxypyridinoline (U-DPD), and the serum markers of bone formation procollagen I N-terminal propeptide (S-PINP), bone-specific alkaline phosphatase (S-BALP), and total osteocalcin (S-OC) were assessed at baseline and at 3, 6, and 12 months after initiation of treatment. Lumbar spine BMD (LBMD) was measured at baseline and 12 months.

RESULTS

Compared with the placebo group, LBMD increased, and all bone markers decreased significantly more in the alendronate group (p<0.001 for each parameter). The decrease of S-TRACP5b after first 3 months of alendronate treatment correlated significantly with the changes of all other markers except S-OC, the best correlation being with S-CTX (r=0.60, p<0.0001). The changes of LBMD at 12 months only correlated significantly with the changes of S-TRACP5b (r=-0.32, p=0.005) and S-CTX (r=-0.24, p=0.037) at 3 months. Based on clinical sensitivity, receiver operating characteristic (ROC) curves, and signal-to-noise ratio, S-TRACP5b, S-CTX, and S-PINP were the best markers for monitoring alendronate treatment. Clinical sensitivity, area under the ROC curve, and signal-to-noise ratio were higher for S-TRACP5b than for the other markers.

CONCLUSION

These results show that S-TRACP5b, S-CTX, and S-PINP are useful markers for monitoring alendronate treatment.

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.

Proteolytic Excision of a Repressive Loop Domain in Tartrate-resistant Acid Phosphatase by Cathepsin K in Osteoclasts

Tartrate-resistant acid phosphatase (TRAP) is a metallophosphoesterase participating in osteoclast-mediated bone turnover. Activation of TRAP is associated with the redox state of the di-iron metal center as well as with limited proteolytic cleavage in an exposed loop domain. The cysteine proteinases cathepsin B, L, K, and S as well as the matrix metalloproteinase-2, -9, -13, and -14 are expressed by osteoclasts and/or other bone cells and have been implicated in the turnover of bone and cartilage. To identify proteases that could act as activators of TRAP in bone, we report here that cathepsins K and L, in contrast to the matrix metalloproteinases, efficiently cleaved and activated recombinant TRAP in vitro. Activation of TRAP by cathepsin K/L was because of increases in catalytic activity, substrate affinity, and sensitivity to reductants. Processing by cathepsin K occurred sequentially by an initial excision of the loop peptide Gly(143)-Gly(160) followed by the removal of a Val(161)-Ala(162) dipeptide at the N terminus of the C-terminal 16-kDa TRAP subunit. Cathepsin L initially released a shorter Gln(151)-Gly(160) peptide and completed processing at Ser(145) or Gly(143) at the C terminus of the N-terminal 23-kDa TRAP subunit and at Arg(163) at the N terminus of the C-terminal 16-kDa TRAP subunit. Mutation of Ser(145) to Ala partly mimicked the effect of proteolysis on catalytic activity, identifying Ser(145) as well as Asp(146) (Funhoff, E. G., Ljusberg, J., Wang, Y., Andersson, G., and Averill, B. A. (2001) Biochemistry 40, 11614-11622) as repressive amino acids of the loop region to maintain the TRAP enzyme in a catalytically latent state. The C-terminal sequence of TRAP isolated from rat bone was consistent with cathepsin K-mediated processing in vivo. Moreover, cathepsin K, but not cathepsin L, co-localized with TRAP in osteoclast-resorptive compartments, supporting a role for cathepsin K in the extracellular processing of monomeric TRAP in the resorption lacuna.

Structure–function relationships of purple acid phosphatase from red kidney beans based on heterologously expressed mutants

Purple acid phosphatases are binuclear metalloenzymes, which catalyze the conversion of orthophosphoric monoesters to alcohol and orthophosphate. The enzyme from red kidney beans is characterized with a Fe(III)-Zn(II) active center. So far, the reaction mechanisms postulated for PAPs assume the essentiality of two amino acids, residing near the bimetallic active site. Based on the amino acid sequence of kidney bean PAP (kbPAP), residues H296 and H202 are believed to be essential for catalytic function of the enzyme. In the present study, the role of residue H202 has been elucidated. Mutants H202A and H202R were prepared by site-directed mutagenesis and expressed in baculovirus-infected insect cells. Based on kinetic studies, residue H202 is assumed to play a role in stabilizing the transition state, particularly in charge compensation, steric positioning of the substrate, and facilitating the release of the product by protonating the substrate leaving groups. The study confirmed the essentiality and elucidates the functional role of H202 in the catalytic mechanism of kbPAP.

Crystal Structures of Recombinant Human Purple Acid Phosphatase With and Without an Inhibitory Conformation of the Repression Loop

The crystal structure of human purple acid phosphatase recombinantly expressed in Escherichia coli (rHPAP(Ec)) and Pichia pastoris (rHPAP(Pp)) has been determined in two different crystal forms, both at 2.2A resolution. In both cases, the enzyme crystallized in its oxidized (inactive) state, in which both Fe atoms in the dinuclear active site are Fe(III). The main difference between the two structures is the conformation of the enzyme "repression loop". Proteolytic cleavage of this loop in vivo or in vitro results in significant activation of the mammalian PAPs. In the crystals obtained from rHPAP(Ec), the carboxylate side-chain of Asp145 of this loop acts as a bidentate ligand that bridges the two metal atoms, in a manner analogous to a possible binding mode for a phosphate ester substrate in the enzyme-substrate complex. The carboxylate side-chain of Asp145 and the neighboring Phe146 side-chain thus block the active site, thereby inactivating the enzyme. In the crystal structure of rHPAP(Pp), the enzyme "repression loop" has an open conformation similar to that observed in other mammalian PAP structures. The present structures demonstrate that the repression loop exhibits significant conformational flexibility, and the observed alternate binding mode suggests a possible inhibitory role for this loop.

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.

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.

Porcine purple acid phosphatase: heterologous expression, characterization, and proteolytic analysis

Uteroferrin is an iron-binding glycoprotein, which is abundantly synthesized in porcine uterine glandular endometrium and believed to be involved in maternal/fetal iron transport. In the present study, uteroferrin has been cloned and functionally expressed using baculovirus-infected insect host cells Spodoptera frugiperda. The work also addresses the possible role of proteolytic cleavage to facilitate the release of uteroferrin-bound iron. The enzyme secreted in culture medium exhibits a molecular mass and catalytic properties similar to native porcine uteroferrin. The specific activity was estimated at 233 U/mg using p-nitrophenyl phosphate as substrate. Partial cleavage of the enzyme with trypsin resulted in a 1.7-fold enhancement in specific activity and a two-subunit polypeptide as observed in preparations of most mammalian purple acid phosphatases. Digestion with the aspartic protease pepsin resulted in a 2.5-fold enzyme inactivation correlated with the appearance of low molecular weight polypeptide fragments and the release of enzyme-bound iron.

Growth of Escherichia coli coexpressing phosphotriesterase and glycerophosphodiester phosphodiesterase, using paraoxon as the sole phosphorus source

Phosphotriesterases catalyze the hydrolytic detoxification of phosphotriester pesticides and chemical warfare nerve agents with various efficiencies. The directed evolution of phosphotriesterases to enhance the breakdown of poor substrates is desirable for the purposes of bioremediation. A limiting factor in the identification of phosphotriesterase mutants with increased activity is the ability to effectively screen large mutant libraries. To this end, we have investigated the possibility of coupling phosphotriesterase activity to cell growth by using methyl paraoxon as the sole phosphorus source. The catabolism of paraoxon to phosphate would occur via the stepwise enzymatic hydrolysis of paraoxon to dimethyl phosphate, methyl phosphate, and then phosphate. The Escherichia coli strain DH10B expressing the phosphotriesterase from Agrobacterium radiobacter P230 (OpdA) is unable to grow when paraoxon is used as the sole phosphorus source. Enterobacter aerogenes is an organism capable of growing when dimethyl phosphate is the sole phosphorus source. The enzyme responsible for hydrolyzing dimethyl phosphate has been previously characterized as a nonspecific phosphohydrolase. We isolated and characterized the genes encoding the phosphohydrolase operon. The operon was identified from a shotgun clone that enabled E. coli to grow when dimethyl phosphate is the sole phosphorus source. E. coli coexpressing the phosphohydrolase and OpdA grew when paraoxon was the sole phosphorus source. By constructing a short degradative pathway, we have enabled E. coli to use phosphotriesters as a sole source of phosphorus.

New insights into the mechanism of purple acid phosphatase through (1)H NMR spectroscopy of the recombinant human enzyme

Proton NMR spectra of FeIII-FeII recombinant single polypeptide human PAP (recHPAP) have been measured at, above, and below its pH optimum, as have the spectra of inhibited forms containing fluoride and phosphate, analogues of the substrates hydroxide and phosphate esters, respectively. The results demonstrate that binding of inhibitory anions to the dinuclear mixed-valent site of recHPAP is controlled by protonation of a ligand to the dinuclear center. Thus, the group that is responsible for pKa,1 in the enzymatic activity versus pH profile functions as a "gatekeeper", whose protonation state controls anion binding to the mixed-valent dinuclear site. The correlation between the pKa values observed in kinetics studies and for the spectroscopic changes strongly suggests that this group is the nucleophilic hydroxide that attacks the phosphate ester substrate.