Metal-Ion Mutagenesis: Conversion of a Purple Acid Phosphatase from Sweet Potato to a Neutral Phosphatase with the Formation of an Unprecedented Catalytically Competent MnIIMnII Active Site

Impaired glycosylation of GmPAP15a, a root-associated purple acid phosphatase, inhibits extracellular phytate-P utilization in soybean

Phosphorus (P) is an essential nutrient, but easily fixed in soils. Therefore, most of soil P exists in the form of inaccessible organic phosphorus (Po), particularly phytate-P. Root-associated purple acid phosphatases (PAPs) are considered to play a crucial role in phosphate (Pi) scavenging in soils. However, evidence for regulating root-associated PAPs in utilization of extracellular phytate-P remain largely unknown in plants at both transcriptional and posttranslational levels. In this study, a Pi-starvation responsive GmPAP15a was identified in soybean (Glycine max). Overexpressing GmPAP15a led to significant increases in root-associated phytase activities, as well as total P content when phytate-P was supplied as the sole P resource in soybean hairy roots. Meanwhile, mass spectrometry (MS) analysis showed GmPAP15a was glycosylated at Asn144 and Asn502 , and its glycan structures of N-linked oligosaccharide chains exhibited microheterogeneity. Moreover, two homologues of AtPHR1, GmPHR9 and GmPHR32 were found to activate GmPAP15a transcription through luciferase activity analysis. Taken together, it is strongly suggested that GmPAP15a plays a vital role in phytate-P utilization in soybean, which might be regulated at both transcriptional and glycosylation modification levels. Our results highlight the GmPHR9/GmPHR32-GmPAP15a signalling pathway might present, and control phytate-P utilization in soybean.

Biomimetics for purple acid phosphatases: A historical perspective

Biomimetics hold potential for varied applications in biotechnology and medicine but have also attracted particular interest as benchmarks for the functional study of their more complex biological counterparts, e.g. metalloenzymes. While many of the synthetic systems adequately mimic some structural and functional aspects of their biological counterparts the catalytic efficiencies displayed are mostly far inferior due to the smaller size and the associated lower complexity. Nonetheless they play an important role in bioinorganic chemistry. Numerous examples of biologically inspired and informed artificial catalysts have been reported, designed to mimic a plethora of chemical transformations, and relevant examples are highlighted in reviews and scientific reports. Herein, we discuss biomimetics of the metallohydrolase purple acid phosphatase (PAP), examples of which have been used to showcase synergistic research advances for both the biological and synthetic systems. In particular, we focus on the seminal contribution of our colleague Prof. Ademir Neves, and his group, pioneers in the design and optimization of suitable ligands that mimic the active site of PAP.

Purple acid phosphatases: roles in phosphate utilization and new emerging functions

Plants strive for phosphorus (P), which is an essential mineral for their life. Since P availability is limiting in most of the world's soils, plants have evolved with a complex network of genes and their regulatory mechanisms to cope with soil P deficiency. Among them, purple acid phosphatases (PAPs) are predominantly associated with P remobilization within the plant and acquisition from the soil by hydrolyzing organic P compounds. P in such compounds remains otherwise unavailable to plants for assimilation. PAPs are ubiquitous in plants, and similar enzymes exist in bacteria, fungi, mammals, and unicellular eukaryotes, but having some differences in their catalytic center. In the recent past, PAPs' roles have been extended to multiple plant processes like flowering, seed development, senescence, carbon metabolism, response to biotic and abiotic stresses, signaling, and root development. While new functions have been assigned to PAPs, the underlying mechanisms remained understood poorly. Here, we review the known functions of PAPs, the regulatory mechanisms, and their relevance in crop improvement for P-use-efficiency. We then discuss the mechanisms behind their functions and propose areas worthy of future research. Finally, we argue that PAPs could be a potential target for improving P utilization in crops. In turn, this is essential for sustainable agriculture.

Rational Design of Potent Inhibitors of a Metallohydrolase Using a Fragment-Based Approach

Metallohydrolases form a large group of enzymes that have fundamental importance in a broad range of biological functions. Among them, the purple acid phosphatases (PAPs) have gained attention due to their crucial role in the acquisition and use of phosphate by plants and also as a promising target for novel treatments of bone-related disorders and cancer. To date, no crystal structure of a mammalian PAP with drug-like molecules bound near the active site is available. Herein, we used a fragment-based design approach using structures of a mammalian PAP in complex with the Maybridge^TM fragment CC063346, the amino acid L-glutamine and the buffer molecule HEPES, as well as various solvent molecules to guide the design of highly potent and efficient mammalian PAP inhibitors. These inhibitors have improved aqueous solubility when compared to the clinically most promising PAP inhibitors available to date. Furthermore, drug-like fragments bound in newly discovered binding sites mapped out additional scaffolds for further inhibitor discovery, as well as scaffolds for the design of inhibitors with novel modes of action.

Insights into phosphatase-activated chemical defense in a marine sponge holobiont

Marine sponges often contain potent cytotoxic compounds, which in turn evokes the principle question of how marine sponges avoid self-toxicity. In a marine sponge Discodermia calyx, the highly toxic calyculin A is detoxified by the phosphorylation, which is catalyzed by the phosphotransferase CalQ of a producer symbiont, “Candidatus Entotheonella” sp. Here we show the activating mechanism to dephosphorylate the stored phosphocalyculin A protoxin. The phosphatase specific to phosphocalyculin A is CalL, which is also encoded in the calyculin biosynthetic gene cluster. CalL represents a new clade and unprecedently coordinates the heteronuclear metals Cu and Zn. CalL is localized in the periplasmic space of the sponge symbiont, where it is ready for the on-demand production of calyculin A in response to sponge tissue disruption.

The phosphatase that activates calyculin biogenesis in the sponge Discodermia calyx turned out to originate from the bacterial symbiont Entotheonella.

Identification, structure analysis, and transcript profiling of purple acid phosphatases under Pi deficiency in tomato (Solanum lycopersicum L.) and its wild relatives

Purple acid phosphatases (PAPs), a family of metallo-phosphoesterase enzymes, are involved in phosphorus nutrition in plants. In this study, we report that the tomato genome encodes 25 PAP members. Physio-biochemical analyses revealed relatively lower total root-associated acid phosphatase activity in the seedlings of Solanum pimpinellifolium than their cultivated tomato seedlings under Pi deficiency. Scrutiny of their transcript abundance shows that most of PAPs are activated, although to varying levels, under Pi deficiency in tomato. Further investigation demonstrates that the magnitude of induction of phosphate starvation inducible root-associated PAP homologs remains lower in the Pi-starved S. pimpinellifolium seedlings, hence, accounting for the lower acid phosphatase activity in this wild relative. Examination of their amino acid sequences revealed significant variation in their substrate-specificity defining residues. Among all members, only SlPAP15 possesses the critical lysine residue (R337) and atypical REKA motif in its C-terminal region. Homology modeling and docking studies revealed that ADP and ATP are preferred substrates of SlPAP15. We also identified other amino acid residues present in the vicinity of the active site, possibly facilitating such physical interactions. Altogether, the results presented here will help in the functional characterization of these genes in the tomato in the future.

Structural elements that modulate the substrate specificity of plant purple acid phosphatases: Avenues for improved phosphorus acquisition in crops

Phosphate acquisition by plants is an essential process that is directly implicated in the optimization of crop yields. Purple acid phosphatases (PAPs) are ubiquitous metalloenzymes, which catalyze the hydrolysis of a wide range of phosphate esters and anhydrides. While some plant PAPs display a preference for ATP as the substrate, others are efficient in hydrolyzing phytate or 2-phosphoenolpyruvate (PEP). PAP from red kidney bean (rkbPAP) is an efficient ATP- and ADPase, but has no activity towards phytate. Crystal structures of this enzyme in complex with ATP analogues (to 2.20 and 2.60 Å resolution, respectively) complement the recent structure of rkbPAP with a bound ADP analogue (ChemBioChem 20 (2019) 1536). Together these complexes provide the first structural insight of a PAP in complex with molecules that mimic biologically relevant substrates. Homology modeling was used to generate three-dimensional structures for the active sites of PAPs from tobacco (NtPAP) and thale cress (AtPAP26) that are efficient in hydrolyzing phytate and PEP as preferred substrates, respectively. The combining of crystallographic data, substrate docking simulations and a phylogenetic analysis of 49 plant PAP sequences (including the first PAP sequences reported from Eucalyptus) resulted in the identification of several active site residues that are important in defining the substrate specificities of plant PAPs; of particular relevance is the identification of a motif ("REKA") that is characteristic for plant PAPs that possess phytase activity. These results may inform bioengineering studies aimed at identifying and incorporating suitable plant PAP genes into crops to improve phosphorus acquisition and use efficiency. Organic phosphorus sources increasingly supplement or replace inorganic fertilizer, and efficient phosphorus use of crops will lower the environmental footprint of agriculture while enhancing food production.

Synthesis and biological activities of drugs for the treatment of osteoporosis

Osteoporosis is an asymptomatic progressive disease. With the improvement of people's living standard and the aging of population, osteoporosis and its fracture have become one of the main diseases threatening the aging society. The serious medical and social burden caused by this has aroused wide public concern. Osteoporosis is listed as one of the three major diseases of the elderly. At present, the drugs for osteoporosis include bone resorption inhibitors and bone formation promoters. The purpose of these anti-osteoporosis drugs is to balance osteoblast bone formation and osteoclast bone resorption. With the development of anti-osteoporosis drugs, new anti osteoporosis drugs have been designed and synthesized. There are many kinds of new compounds with anti osteoporosis activity, but most of them are concentrated on the original drugs with anti osteoporosis activity, or the natural products with anti-osteoporosis activity are extracted from the natural products for structural modification to obtain the corresponding derivatives or analogues. These target compounds showed good ALP activity in vitro and in vivo, promoted osteoblast differentiation and mineralization, or had anti TRAP activity, inhibited osteoclast absorption. This work attempts to systematically review the studies on the synthesis and bioactivity of anti-osteoporosis drugs in the past 10 years. The structure-activity relationship was discussed, which provided a reasonable idea for the design and development of new anti-osteoporosis drugs.

Key Structural Motifs Balance Metal Binding and Oxidative Reactivity in a Heterobimetallic Mn/Fe Protein

Heterobimetallic Mn/Fe proteins represent a new cofactor paradigm in bioinorganic chemistry and pose countless outstanding questions. The assembly of the active site defies common chemical convention by contradicting the Irving-Williams series, while the scope of reactivity remains unexplored. In this work, the assembly and C-H bond activation process in the Mn/Fe R2-like ligand-binding oxidase (R2lox) protein is investigated using a suite of biophysical techniques, including time-resolved optical spectroscopy, global kinetic modeling, X-ray crystallography, electron paramagnetic resonance spectroscopy, protein electrochemistry, and mass spectrometry. Selective metal binding is found to be under thermodynamic control, with the binding sites within the apo-protein exhibiting greater Mn^II affinity than Fe^II affinity. The comprehensive analysis of structure and reactivity of wild-type R2lox and targeted primary and secondary sphere mutants indicate that the efficiency of C-H bond activation directly correlates with the Mn/Fe cofactor reduction potentials and is inversely related to divalent metal binding affinity. These findings suggest the R2lox active site is precisely tuned for achieving both selective heterobimetallic binding and high levels of reactivity and offer a mechanism to examine the means by which proteins achieve appropriate metal incorporation.

A glycoform of the secreted purple acid phosphatase AtPAP26 co-purifies with a mannose-binding lectin (AtGAL1) upregulated by phosphate-starved Arabidopsis

The purple acid phosphatase AtPAP26 plays a central role in Pi-scavenging by Pi-starved (-Pi) Arabidopsis. Mass spectrometry (MS) of AtPAP26-S1 and AtPAP26-S2 glycoforms secreted by -Pi suspension cells demonstrated that N-glycans at Asn³⁶⁵ and Asn⁴²² were modified in AtPAP26-S2 to form high-mannose glycans. A 55-kDa protein that co-purified with AtPAP26-S2 was identified as a Galanthus nivalis agglutinin-related and apple domain lectin-1 (AtGAL1; At1g78850). MS revealed that AtGAL1 was bisphosphorylated at Tyr³⁸ and Thr³⁹ and glycosylated at four conserved Asn residues. When AtGAL was incubated in the presence of a thiol-reducing reagent prior to immunoblotting, its cross-reactivity with anti-AtGAL1-IgG was markedly attenuated (consistent with three predicted disulfide bonds in AtGAL1's apple domain). Secreted AtGAL1 polypeptides were upregulated to a far greater extent than AtGAL1 transcripts during Pi deprivation, indicating posttranscriptional control of AtGAL1 expression. Growth of a -Pi atgal1 mutant was unaffected, possibly due to compensation by AtGAL1's closest paralog, AtGAL2 (At1g78860). Nevertheless, AtGAL1's induction by numerous stresses combined with the broad distribution of AtGAL1-like lectins in diverse species implies an important function for AtGAL1 orthologs within the plant kingdom. We hypothesize that binding of AtPAP26-S2's high-mannose glycans by AtGAL1 enhances AtPAP26 function to facilitate Pi-scavenging by -Pi Arabidopsis.

Synthesis and evaluation of novel purple acid phosphatase inhibitors

Transgenic studies in animals have demonstrated a direct association between the level of expression of purple acid phosphatase (PAP; also known as tartrate-resistant acid phosphatase) and the progression of osteoporosis. Consequently, PAP has emerged as a promising target for the development of novel therapeutic agents to treat this debilitating disorder. PAPs are binuclear hydrolases that catalyse the hydrolysis of phosphorylated substrates under acidic to neutral conditions. A series of phenyltriazole carboxylic acids, prepared by the reactions of azide derivatives with propiolic acid through copper(i)-catalysed azide-alkyne cycloaddition click reactions, has been assessed for their inhibitory effect on the catalytic activity of pig and red kidney bean PAPs. The binding mode of most of these compounds is purely uncompetitive with K _iuc values as low as ∼23 μM for the mammalian enzyme. Molecular modelling has been used to examine the binding modes of these triazole compounds in the presence of a substrate in the active site of the enzyme in order to rationalise their activities and to design more potent and specific derivatives.

A New Mixed-Valence Mn(II)Mn(III) Compound With Catalase and Superoxide Dismutase Activities

The synthesis, X-ray molecular structure, physico-chemical characterization and dual antioxidant activity (catalase and superoxide dismutase) of a new polymeric mixed valence Mn(III)Mn(II) complex, containing the ligand H₂BPClNOL (N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)[(3-chloro)(2-hydroxy)] propylamine) is described. The monomeric unit is composed of a dinuclear Mn(II)Mn(III) moiety, [Mn(III)(μ-HBPClNOL)(μ-BPClNOL)Mn(II)(Cl)](ClO₄)·2H₂O, 1, in which the Mn ions are connected by two different bridging groups provided by two molecules of the ligand H₂BPClNOL, a phenoxide and an alkoxide group. In the solid state, this mixed valence dinuclear unit is connected to its neighbors through chloro bridges. Magnetic measurements indicated the presence of ferromagnetic [J = +0.076(13) cm⁻¹] and antiferromagnetic [J = −5.224(13) cm⁻¹] interactions. The compound promotes O2•- dismutation in aqueous solution (IC₅₀ = 0.370 μmol dm⁻³, k_cat = 3.6x10⁶ M⁻¹ s⁻¹). EPR studies revealed that a high-valent Mn(III)-O-Mn(IV) species is involved in the superoxide dismutation catalytic cycle. Complex 1 shows catalase activity only in the presence of a base, e.g., piperazine or triethylamine. Kinetic studies were carried out in the presence of piperazine and employing two different methods, resulting in k_cat values of 0.58 ± 0.03 s⁻¹ (detection of O₂ production employing a Clark electrode) and 2.59 ± 0.12 s⁻¹ (H₂O₂ consuption recorded via UV-Vis). EPR and ESI-(+)-MS studies indicate that piperazine induces the oxidation of 1, resulting in the formation of the catalytically active Mn(III)-O-Mn(IV) species.

Synthesis, Magnetic Properties, and Catalytic Properties of a Nickel(II)-Dependent Biomimetic of Metallohydrolases

A dinickel(II) complex of the ligand 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol (HL1) has been prepared and characterized to generate a functional model for nickel(II) phosphoesterase enzymes. The complex, [Ni2(L1)(μ-OAc)(H2O)2](ClO4)2·H2O, was characterized by microanalysis, X-ray crystallography, UV-visible, and IR absorption spectroscopy and solid state magnetic susceptibility measurements. Susceptibility studies show that the complex is antiferromagnetically coupled with the best fit parameters J = -27.4 cm-1, g = 2.29, D = 28.4 cm-1, comparable to corresponding values measured for the analogous dicobalt(II) complex [Co2(L1)(μ-OAc)](ClO4)2·0.5 H2O (J = -14.9 cm-1 and g = 2.16). Catalytic measurements with the diNi(II) complex using the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) demonstrated activity toward hydrolysis of the phosphoester substrate with K m ~10 mM, and k cat ~0.025 s-1. The combination of structural and catalytic studies suggests that the likely mechanism involves a nucleophilic attack on the substrate by a terminal nucleophilic hydroxido moiety.

Identification of Purple Acid Phosphatases in Chickpea and Potential Roles of CaPAP7 in Seed Phytate Accumulation

Purple acid phosphatases (PAPs) play important roles in phosphate (Pi) acquisition and utilization. These PAPs hydrolyze organic Phosphorus (P) containing compounds in rhizosphere as well as inside the plant cell. However, roles of PAPs in one of the most widely cultivated legumes, chickpea (Cicer arietnum L.), have not been unraveled so far. In the present study, we identified 25 putative PAPs in chickpea (CaPAPs) which possess functional PAP motifs and domains. Differential regulation of CaPAPs under different nutrient deficiencies revealed their roles under multiple nutrient stresses including Pi deficiency. Interestingly, most of the CaPAPs were prominently expressed in flowers and young pods indicating their roles in flower and seed development. Association mapping of SNPs underlying CaPAPs with seed traits revealed significant association of low Pi inducible CaPAP7 with seed weight and phytate content. Biochemical characterization of recombinant CaPAP7 established it to be a functional acid phosphatase with highest activity on most abundant organic-P substrate, phytate. Exogenous application of recombinant CaPAP7 enhanced biomass and Pi content of Arabidopsis seedlings supplemented with phytate as sole P source. Taken together, our results uncover the PAPs in chickpea and potential roles of CaPAP7 in seed phytate accumulation.

Catalytic scaffolds for phosphoryl group transfer

A single genome encodes a large number of phosphoryl hydrolases for the purposes of phosphate recycling, primary and secondary metabolism, signal transduction and regulation, and protection from xenobiotics. Phosphate monoester hydrolysis faces a high kinetic barrier, yet there are multiple solutions to the problem both in terms of catalytic mechanisms and three-dimensional structure of the hydrolases. Recent structural and mechanistic findings highlight the trigonal-bipyramidal nature of the transition state for enzyme promoted phosphate monoester hydrolysis and the evolution and role of inserted loops/domains in governing substrate specificity and promiscuity. Important questions remain as to how electrostatics modulate water networks and critical proton-transfer events. How substrate targeting and catalysis is achieved by the independently evolved catalytic platforms is compared and contrasted in this article.

Metal Ions Play an Essential Catalytic Role in the Mechanism of Ketol-Acid Reductoisomerase

Ketol-acid reductoisomerase (KARI) is a Mg(2+) -dependent enzyme in the branched-chain amino acid biosynthesis pathway. It catalyses a complex two-part reaction: an alkyl migration followed by a NADPH-dependent reduction. Both reactions occur within the one active site, but in particular, the mechanism of the isomerisation step is poorly understood. Here, using a combination of kinetic, thermodynamic and spectroscopic techniques, the reaction mechanisms of both Escherichia coli and rice KARI have been investigated. We propose a conserved mechanism of catalysis, whereby a hydroxide, bridging the two Mg(2+) ions in the active site, initiates the reaction by abstracting a proton from the C2 alcohol group of the substrate. While the μ-hydroxide-bridged dimetallic centre is pre-assembled in the bacterial enzyme, in plant KARI substrate binding leads to a reduction of the metal-metal distance with the concomitant formation of a hydroxide bridge. Only Mg(2+) is capable of promoting the isomerisation reaction, likely to be due to non-competent substrate binding in the presence of other metal ions.

Divergent assembly mechanisms of the manganese/iron cofactors in R2lox and R2c proteins

A manganese/iron cofactor which performs multi-electron oxidative chemistry is found in two classes of ferritin-like proteins, the small subunit (R2) of class Ic ribonucleotide reductase (R2c) and the R2-like ligand-binding oxidase (R2lox). It is unclear how a heterodimeric Mn/Fe metallocofactor is assembled in these two related proteins as opposed to a homodimeric Fe/Fe cofactor, especially considering the structural similarity and proximity of the two metal-binding sites in both protein scaffolds and the similar first coordination sphere ligand preferences of Mn^II and Fe^II. Using EPR and Mössbauer spectroscopies as well as X-ray anomalous dispersion, we examined metal loading and cofactor activation of both proteins in vitro (in solution). We find divergent cofactor assembly mechanisms for the two systems. In both cases, excess Mn^II promotes heterobimetallic cofactor assembly. In the absence of Fe^II, R2c cooperatively binds Mn^II at both metal sites, whereas R2lox does not readily bind Mn^II at either site. Heterometallic cofactor assembly is favored at substoichiometric Fe^II concentrations in R2lox. Fe^II and Mn^II likely bind to the protein in a stepwise fashion, with Fe^II binding to site 2 initiating cofactor assembly. In R2c, however, heterometallic assembly is presumably achieved by the displacement of Mn^II by Fe^II at site 2. The divergent metal loading mechanisms are correlated with the putative in vivo functions of R2c and R2lox, and most likely with the intracellular Mn^II/Fe^II concentrations in the host organisms from which they were isolated.

Metallohydrolase biomimetics with catalytic and structural flexibility

The phosphatase activity of zinc complexes containing six- and seven-dentate ligands was evaluated through kinetic and³¹P NMR studies.

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.

A hexanuclear manganese(ii) complex: synthesis, characterization and catalytic activity toward organic sulfide oxidation

A hexanuclear [Mn^II₆] wheel-like assembly with naphthalene-1,8-dicarboxylate and 1,10-phenanthroline was reported. The compound acts as a catalyst toward organic sulfide oxidation.

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

An enhanced understanding of the metal ion binding and active site structural features of phosphoesterases such as the glycerophosphodiesterase from Enterobacter aerogenes (GpdQ), and the organophosphate degrading agent from Agrobacterium radiobacter (OpdA) have important consequences for potential applications. Coupled with investigations of the metalloenzymes, programs of study to synthesise and characterise model complexes based on these metalloenzymes can add to our understanding of structure and function of the enzymes themselves. This review summarises some of our work and illustrates the significance and contributions of model studies to knowledge in the area.

Purple Acid Phosphatase5 is required for maintaining basal resistance against Pseudomonas syringae in Arabidopsis

BACKGROUND

Plants have evolved an array of constitutive and inducible defense strategies to restrict pathogen ingress. However, some pathogens still manage to invade plants and impair growth and productivity. Previous studies have revealed several key regulators of defense responses, and efforts have been made to use this information to develop disease resistant crop plants. These efforts are often hampered by the complexity of defense signaling pathways. To further elucidate the complexity of defense responses, we screened a population of T-DNA mutants in Colombia-0 background that displayed altered defense responses to virulent Pseudomonas syringae pv. tomato DC3000 (Pst DC3000).

RESULTS

In this study, we demonstrated that the Arabidopsis Purple Acid Phosphatse5 (PAP5) gene, induced under prolonged phosphate (Pi) starvation, is required for maintaining basal resistance to certain pathogens. The expression of PAP5 was distinctly induced only under prolonged Pi starvation and during the early stage of Pst DC3000 infection (6 h.p.i). T-DNA tagged mutant pap5 displayed enhanced susceptibility to the virulent bacterial pathogen Pst DC3000. The pap5 mutation greatly reduced the expression of pathogen inducible gene PR1 compared to wild-type plants. Similarly, other defense related genes including ICS1 and PDF1.2 were impaired in pap5 plants. Moreover, application of BTH (an analog of SA) restored PR1 expression in pap5 plants.

CONCLUSION

Taken together, our results demonstrate the requirement of PAP5 for maintaining basal resistance against Pst DC3000. Furthermore, our results provide evidence that PAP5 acts upstream of SA accumulation to regulate the expression of other defense responsive genes. We also provide the first experimental evidence indicating the role PAP5 in plant defense responses.

Binuclear metallohydrolases: complex mechanistic strategies for a simple chemical reaction

Binuclear metallohydrolases are a large family of enzymes that require two closely spaced transition metal ions to carry out a plethora of hydrolytic reactions. Representatives include purple acid phosphatases (PAPs), enzymes that play a role in bone metabolism and are the only member of this family with a heterovalent binuclear center in the active form (Fe(3+)-M(2+), M = Fe, Zn, Mn). Other members of this family are urease, which contains a di-Ni(2+) center and catalyzes the breakdown of urea, arginase, which contains a di-Mn(2+) center and catalyzes the final step in the urea cycle, and the metallo-β-lactamases, which contain a di-Zn(2+) center and are virulence factors contributing to the spread of antibiotic-resistant pathogens. Binuclear metallohydrolases catalyze numerous vital reactions and are potential targets of drugs against a wide variety of human disorders including osteoporosis, various cancers, antibiotic resistance, and erectile dysfunctions. These enzymes also tend to catalyze more than one reaction. An example is an organophosphate (OP)-degrading enzyme from Enterobacter aerogenes (GpdQ). Although GpdQ is part of a pathway that is used by bacteria to degrade glycerolphosphoesters, it hydrolyzes a variety of other phosphodiesters and displays low levels of activity against phosphomono- and triesters. Such a promiscuous nature may have assisted the apparent recent evolution of some binuclear metallohydrolases to deal with situations created by human intervention such as OP pesticides in the environment. OP pesticides were first used approximately 70 years ago, and therefore the enzymes that bacteria use to degrade them must have evolved very quickly on the evolutionary time scale. The promiscuous nature of enzymes such as GpdQ makes them ideal candidates for the application of directed evolution to produce new enzymes that can be used in bioremediation and against chemical warfare. In this Account, we review the mechanisms employed by binuclear metallohydrolases and use PAP, the OP-degrading enzyme from Agrobacterium radiobacter (OPDA), and GpdQ as representative systems because they illustrate both the diversity and similarity of the reactions catalyzed by this family of enzymes. The majority of binuclear metallohydrolases utilize metal ion-activated water molecules as nucleophiles to initiate hydrolysis, while some, such as alkaline phosphatase, employ an intrinsic polar amino acid. Here we only focus on catalytic strategies applied by the former group.

A molecular description of acid phosphatase

Acid phosphatase is ubiquitous in distribution in various organisms. Although it catalyzes simple hydrolytic reactions, it is considered as an interesting enzyme in biological systems due to its involvement in different physiological activities. However, earlier reviews on acid phosphatase reveal some fragmentary information and do not give a holistic view on this enzyme. So, the present review summarizes studies on biochemical properties, structure, catalytic mechanism, and applications of acid phosphatase. Recent advancement of acid phosphatase in agricultural and clinical fields is emphasized where it is presented as potent agent for sustainable agricultural practices and diagnostic marker in bone metabolic disorders. Also, its significance in prostate cancer therapies as a therapeutic target has been discussed. At the end, current studies and prospects of immobilized acid phosphatase are included.

Preparation, structure, and properties of tetranuclear vanadium(III) and (IV) complexes bridged by diphenyl phosphate or phosphate

Three novel tetranuclear vanadium(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V(III)(2)(μ-hpnbpda)}(2){μ-(C(6)H(5)O)(2)PO(2)}(2)(μ-O)(2)]·6CH(3)OH (1), [{V(III)(2)(μ-tphpn)(μ-η(3)-HPO(4))}(2)(μ-η(4)-PO(4))](ClO(4))(3)·4.5H(2)O (2), and [{(V(IV)O)(2)(μ-tphpn)}(2)(μ-η(4)-PO(4))](ClO(4))(3)·H(2)O (3), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H(3)hpnbpda represents 2-hydroxypropane-1,3-diamino-N,N'-bis(2-pyridylmethyl)-N,N'-diacetic acid, and Htphpn represents N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium(IV) complex without a phosphate bridge, [(VO)(2)(μ-tphpn)(H(2)O)(2)](ClO(4))(3)·2H(2)O (4), was also prepared and structurally characterized for comparison. The vanadium(III) center in 1 adopts a hexacoordinate structure while that in 2 adopts a heptacoordinate structure. In 1, the two dinuclear vanadium(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In 2, the two vanadium(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex 2 is oxidized in aerobic solution to yield complex 3, in which two of the three phosphate groups in 2 are substituted by oxo groups.

Promiscuity comes at a price: catalytic versatility vs efficiency in different metal ion derivatives of the potential bioremediator GpdQ

The glycerophosphodiesterase from Enterobacter aerogenes (GpdQ) is a highly promiscuous dinuclear metallohydrolase with respect to both substrate specificity and metal ion composition. While this promiscuity may adversely affect the enzyme's catalytic efficiency its ability to hydrolyse some organophosphates (OPs) and by-products of OP degradation have turned GpdQ into a promising candidate for bioremedial applications. Here, we investigated both metal ion binding and the effect of the metal ion composition on catalysis. The prevalent in vivo metal ion composition for GpdQ is proposed to be of the type Fe(II)Zn(II), a reflection of natural abundance rather than catalytic optimisation. The Fe(II) appears to have lower binding affinity than other divalent metal ions, and the catalytic efficiency of this mixed metal center is considerably smaller than that of Mn(II), Co(II) or Cd(II)-containing derivatives of GpdQ. Interestingly, metal ion replacements do not only affect catalytic efficiency but also the optimal pH range for the reaction, suggesting that different metal ion combinations may employ different mechanistic strategies. These metal ion-triggered modulations are likely to be mediated via an extensive hydrogen bond network that links the two metal ion binding sites via residues in the substrate binding pocket. The observed functional diversity may be the cause for the modest catalytic efficiency of wild-type GpdQ but may also be essential to enable the enzyme to evolve rapidly to alter substrate specificity and enhance k(cat) values, as has recently been demonstrated in a directed evolution experiment. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.

Monoesterase activity of a purple acid phosphatase mimic with a cyclam platform

The synthesis and characterization of a novel dinucleating ligand L (L=4,11-dimethyl-1,8-bis{2-[N-(di-2-pyridylmethyl)amino]ethyl}cyclam) and its μ-oxo-bridged diferric complex [(H(2)L){Fe(III)(2)(O)}(Cl)(4)](2+) are reported. This diiron(III) complex is the first example of a truly functional purple acid phosphatase (PAP) mimic as it accelerates the hydrolysis of the activated phosphomonoester 2,4-dinitrophenyl phosphate (DNPP). The spectroscopic and kinetic data indicate that only substrates that are monodentately bound to one of the two ferric ions can be attacked by a suitable nucleophile. This is, most probably, a terminal iron(III)-bound hydroxide. DFT calculations support this assumption and also highlight the importance of secondary interactions, exerted by the protonated cyclam platform, for the positioning and activation of the iron(III)-bound substrate. Similar effects are postulated in the native enzyme but addressed in PAP mimics for the first time.

A Potentially Polymerizable Heterodinuclear FeIIIZnII Purple Acid Phosphatase Mimic. Synthesis, Characterization, and Phosphate Ester Hydrolysis Studies

An analogue of the purple acid phosphatase biomimetic 2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-(((2-hydroxybenzyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol has been synthesized. The analogue, 2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-(((2-hydroxy-4-(4-vinylbenzyloxy)benzyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (H2BPBPMPV) possesses a pendant olefin suitable for copolymerization. Complexation with FeIII/ZnII resulted in the complex [FeIIIZnII(BPBPMPV)(CH3COO)2](ClO4), characterized with mass spectrometry, microanalysis, UV/vis, and IR spectrometry. The catalytic activity of the complex toward bis-(2,4-dinitrophenyl) phosphate was determined, resulting in Km of 4.1 ± 0.6 mM, with kcat 3.8 ± 0.2 × 10–3 s–1 and a bell-shaped pH–rate profile with pKa values of 4.31, 5.66, 8.96, the profile exhibiting residual activity above pH 9.5.

The divalent metal ion in the active site of uteroferrin modulates substrate binding and catalysis

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 Fe(3+)-Fe(2+) pig PAP (uteroferrin; Uf) and its Fe(3+)-Mn(2+) derivative to investigate the effect of metal ion substitution on the mechanism of catalysis. Replacement of the Fe(2+) by Mn(2+) 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 Fe(3+) site. These data also indicate that in both metal forms of Uf, phenyl phosphate hydrolysis occurs faster than formation of a mu-1,3 phosphate complex. The slower rate of interaction between substrate and the Fe(3+) 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 Fe(3+). The faster rates of interaction with the Fe(3+) site in the Fe(3+)-Mn(2+) derivative than the native Fe(3+)-Fe(2+) 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.