Kinetics characterization of a low immunogenic recombinant l-asparaginase from Phaseolus vulgaris with cytotoxic activity against leukemia cells

l-asparaginases play a crucial role in the treatment of acute lymphoblastic leukemia (ALL), a type of cancer that mostly affects children and teenagers. However, it is common for these molecules to cause adverse reactions during treatment. These downsides ignite the search for novel asparaginases to mitigate these problems. Thus, this work aimed to produce and characterize a recombinant asparaginase from Phaseolus vulgaris (Asp-P). In this study, Asp-P was expressed in Escherichia coli with high yields and optimum activity at 40 °C, pH 9.0. The enzyme Km and Vmax values were 7.05 mM and 1027 U/mg, respectively. Asp-P is specific for l-asparagine, showing no activity against l-glutamine and other amino acids. The enzyme showed a higher cytotoxic effect against Raji than K562 cell lines, but only at high concentrations. In silico analysis indicated that Asp-P has lower immunogenicity than a commercial enzyme. Asp-P induced biofilm formation by Candida sp. due to sublethal dose, showing an underexplored potential of asparaginases. The absence of glutaminase activity, lower immunogenicity and optimal activity similar to physiological temperature conditions are characteristics that indicate Asp-P as a potential new commercial enzyme in the treatment of ALL and its underexplored application in the treatment of other diseases.

Towards a dependable data set of structures for L-asparaginase research

The Protein Data Bank (PDB) includes a carefully curated treasury of experimentally derived structural data on biological macromolecules and their various complexes. Such information is fundamental for a multitude of projects that involve large-scale data mining and/or detailed evaluation of individual structures of importance to chemistry, biology and, most of all, to medicine, where it provides the foundation for structure-based drug discovery. However, despite extensive validation mechanisms, it is almost inevitable that among the ∼215 000 entries there will occasionally be suboptimal or incorrect structure models. It is thus vital to apply careful verification procedures to those segments of the PDB that are of direct medicinal interest. Here, such an analysis was carried out for crystallographic models of L-asparaginases, enzymes that include approved drugs for the treatment of certain types of leukemia. The focus was on the adherence of the atomic coordinates to the rules of stereochemistry and their agreement with the experimental electron-density maps. Whereas the current clinical application of L-asparaginases is limited to two bacterial proteins and their chemical modifications, the field of investigations of such enzymes has expanded tremendously in recent years with the discovery of three entirely different structural classes and with numerous reports, not always quite reliable, of the anticancer properties of L-asparaginases of different origins.

Biochemical characterization of L-asparaginase isoforms from Rhizobium etli-the boosting effect of zinc

L-Asparaginases, divided into three structural Classes, catalyze the hydrolysis of L-asparagine to L-aspartic acid and ammonia. The members of Class 3, ReAIV and ReAV, encoded in the genome of the nitrogen fixing Rhizobium etli, have the same fold, active site, and quaternary structure, despite low sequence identity. In the present work we examined the biochemical consequences of this difference. ReAIV is almost twice as efficient as ReAV in asparagine hydrolysis at 37°C, with the kinetic KM, kcat parameters (measured in optimal buffering agent) of 1.5 mM, 770 s-1 and 2.1 mM, 603 s-1, respectively. The activity of ReAIV has a temperature optimum at 45°C-55°C, whereas the activity of ReAV, after reaching its optimum at 37°C, decreases dramatically at 45°C. The activity of both isoforms is boosted by 32 or 56%, by low and optimal concentration of zinc, which is bound three times more strongly by ReAIV then by ReAV, as reflected by the KD values of 1.2 and 3.3 μM, respectively. We also demonstrate that perturbation of zinc binding by Lys→Ala point mutagenesis drastically decreases the enzyme activity but also changes the mode of response to zinc. We also examined the impact of different divalent cations on the activity, kinetics, and stability of both isoforms. It appeared that Ni2+, Cu2+, Hg2+, and Cd2+ have the potential to inhibit both isoforms in the following order (from the strongest to weakest inhibitors) Hg2+ > Cu2+ > Cd2+ > Ni2+. ReAIV is more sensitive to Cu2+ and Cd2+, while ReAV is more sensitive to Hg2+ and Ni2+, as revealed by IC50 values, melting scans, and influence on substrate specificity. Low concentration of Cd2+ improves substrate specificity of both isoforms, suggesting its role in substrate recognition. The same observation was made for Hg2+ in the case of ReAIV. The activity of the ReAV isoform is less sensitive to Cl- anions, as reflected by the IC50 value for NaCl, which is eightfold higher for ReAV relative to ReAIV. The uncovered complementary properties of the two isoforms help us better understand the inducibility of the ReAV enzyme.

Probing the enzymatic activity and maturation process of the EcAIII Ntn-amidohydrolase using local random mutagenesis

This report describes a comprehensive approach to local random mutagenesis of the E. coli Ntn-amidohydrolase EcAIII, and supplements the results published earlier for the randomization series RDM1. Here, random mutagenesis was applied in the center of the EcAIII molecule, i.e., in the region important for substrate binding and its immediate neighborhood (series RDM2, RDM3, RDM7), in the vicinity of the catalytic threonine triplet (series RDM4, RDM5, RDM6), in the linker region (series RDM8), and in the sodium-binding (stabilization) loop (series RDM9). The results revealed that the majority of the new EcAIII variants have abolished or significantly reduced rate of autoprocessing, even if the mutation was not in a highly conserved sequence and structure regions. AlphaFold-predicted structures of the mutants suggest the role of selected residues in the positioning of the linker and stabilization of the scissile bond in precisely correct orientation, enabling the nucleophilic attack during the maturation process. The presented data highlight the details of EcAIII geometry that are important for the autoproteolytic maturation and for the catalytic mechanism in general, and can be treated as a guide for protein engineering experiments with other Ntn-hydrolases.

Insights into the Distribution and Functional Properties of l-Asparaginase in the Archaeal Domain and Characterization of Picrophilus torridus Asparaginase Belonging to the Novel Family Asp2like1

l-Asparaginase catalyzes the hydrolysis of l-asparagine to aspartic acid and ammonia and is used in the medical and food industries. In this investigation, from the proteomes of 176 archaeal organisms (with completely sequenced genomes), 116 homologs of l-asparaginase were obtained from 86 archaeal organisms segregated into Asp1, Asp2, IaaA, Asp2like1, and Asp2like2 families based on the conserved domain. The similarities and differences in the structure of selected representatives from each family are discussed. From the two novel archaeal l-asparaginase families Asp2like1 and Asp2like2, a representative of Asp2like1 family Picrophilus torridus asparaginase (PtAsp2like1) was characterized in detail to find its suitability in therapeutics. PtAsp2like1 was a glutaminase-free asparaginase that showed the optimum activity at 80 °C and pH 10.0. The Km of PtAsp2like1 toward substrate l-asparagine was 11.69 mM. This study demonstrates the improved mapping of asparaginases in the archaeal domain, facilitating future focused research on archaeal asparaginases for therapeutic applications.

Massive annotation of bacterial L-asparaginases reveals their puzzling distribution and frequent gene transfer events

L-Asparaginases, which convert L-asparagine to L-aspartate and ammonia, come in five types, AI-AV. Some bacterial type AII enzymes are a key element in the treatment of acute lymphoblastic leukemia in children, but new L-asparaginases with better therapeutic properties are urgently needed. Here, we search publicly available bacterial genomes to annotate L-asparaginase proteins belonging to the five known types. We characterize taxonomic, phylogenetic, and genomic patterns of L-asparaginase occurrences pointing to frequent horizontal gene transfer (HGT) events, also occurring multiple times in the same recipient species. We show that the reference AV gene, encoding a protein originally found and structurally studied in Rhizobium etli, was acquired via HGT from Burkholderia. We also describe the sequence variability of the five L-asparaginase types and map the conservation levels on the experimental or predicted structures of the reference enzymes, finding the most conserved residues in the protein core near the active site, and the most variable ones on the protein surface. Additionally, we highlight the most common sequence features of bacterial AII proteins that may aid in selecting therapeutic L-asparaginases. Finally, we point to taxonomic units of bacteria that do not contain recognizable sequences of any of the known L-asparaginase types, implying that those microorganisms most likely contain new, as yet unknown types of L-asparaginases. Such novel enzymes, when properly identified and characterized, could hold promise as antileukemic drugs.

Structural and biophysical studies of new L-asparaginase variants: lessons from random mutagenesis of the prototypic Escherichia coli Ntn-amidohydrolase

This work reports the results of random mutagenesis of the Escherichia coli class 2 L-asparaginase EcAIII belonging to the Ntn-hydrolase family. New variants of EcAIII were studied using structural, biophysical and bioinformatic methods. Activity tests revealed that the L-asparaginase activity is abolished in all analyzed mutants with the absence of Arg207, but some of them retained the ability to undergo the autoproteolytic maturation process. The results of spectroscopic studies and the determined crystal structures showed that the EcAIII fold is flexible enough to accept different types of mutations; however, these mutations may have a diverse impact on the thermal stability of the protein. The conclusions from the experiments are grouped into six lessons focused on (i) the adaptation of the EcAIII fold to new substitutions, (ii) the role of Arg207 in EcAIII activity, (iii) a network of residues necessary for autoprocessing, (iv) the complexity of the autoprocessing reaction, (v) the conformational changes observed in enzymatically inactive variants and (vi) the cooperativity of the EcAIII dimer subunits. Additionally, the structural requirements (pre-maturation checkpoints) that are necessary for the initiation of the autocleavage of Ntn-hydrolases have been classified. The findings reported in this work provide useful hints that should be considered before planning enzyme-engineering experiments aimed at the design of proteins for therapeutic applications. This is especially important for L-asparaginases that can be utilized in leukemia therapy, as alternative therapeutics are urgently needed to circumvent the severe side effects associated with the currently used enzymes.

New Insight into Aspartate Metabolic Pathways in Populus: Linking the Root Responsive Isoenzymes with Amino Acid Biosynthesis during Incompatible Interactions of Fusarium solani

Integrating amino acid metabolic pathways into plant defense and immune systems provides the building block for stress acclimation and host-pathogen interactions. Recent progress in L-aspartate (Asp) and its deployed metabolic pathways highlighted profound roles in plant growth and defense modulation. Nevertheless, much remains unknown concerning the multiple isoenzyme families involved in Asp metabolic pathways in Populus trichocarpa, a model tree species. Here, we present comprehensive features of 11 critical isoenzyme families, representing biological significance in plant development and stress adaptation. The in silico prediction of the molecular and genetic patterns, including phylogenies, genomic structures, and chromosomal distribution, identify 44 putative isoenzymes in the Populus genome. Inspection of the tissue-specific expression demonstrated that approximately 26 isogenes were expressed, predominantly in roots. Based on the transcriptomic atlas in time-course experiments, the dynamic changes of the genes transcript were explored in Populus roots challenged with soil-borne pathogenic Fusarium solani (Fs). Quantitative expression evaluation prompted 12 isoenzyme genes (PtGS2/6, PtGOGAT2/3, PtAspAT2/5/10, PtAS2, PtAspg2, PtAlaAT1, PtAK1, and PtAlaAT4) to show significant induction responding to the Fs infection. Using high-performance liquid chromatography (HPLC) and non-target metabolomics assay, the concurrent perturbation on levels of Asp-related metabolites led to findings of free amino acids and derivatives (e.g., Glutamate, Asp, Asparagine, Alanine, Proline, and α-/γ-aminobutyric acid), showing marked differences. The multi-omics integration of the responsive isoenzymes and differential amino acids examined facilitates Asp as a cross-talk mediator involved in metabolite biosynthesis and defense regulation. Our research provides theoretical clues for the in-depth unveiling of the defense mechanisms underlying the synergistic effect of fine-tuned Asp pathway enzymes and the linked metabolite flux in Populus.

Temperature dependent autocleavage and applications of recombinant L-asparaginase from Thermococcus kodakarensis for acrylamide mitigation

This manuscript describes enhancement of soluble production, auto-cleavage analysis and assessment of acrylamide mitigation potential of Tk2246, a plant-type L-asparaginase from Thermococcus kodakarensis. The gene encoding Tk2246 was cloned and expressed in Escherichia coli. Recombinant Tk2246 was produced mainly in insoluble form. Various strategies were utilized to enhance the soluble production, which significantly increased the soluble yield. Interestingly, recombinant Tk2246 was produced even without addition of the inducer, though relatively in a lower amount. To our surprise, Tk2246 was produced in partially cleaved form when the inducer was not added in the culture. When applied for acrylamide mitigation, Tk2246 reduced the acrylamide formation more than 80% in French fries, chapati and yeast-leavened bread. In addition to acrylamide mitigation, Tk2246 exhibited antistaling activity without loss of sensory properties of the food. High activity, thermostability and efficient acrylamide reduction capability make Tk2246 a potential candidate for industrial applications.

Crystal structures of the elusive Rhizobium etli L-asparaginase reveal a peculiar active site

Rhizobium etli, a nitrogen-fixing bacterial symbiont of legume plants, encodes an essential L-asparaginase (ReAV) with no sequence homology to known enzymes with this activity. High-resolution crystal structures of ReAV show indeed a structurally distinct, dimeric enzyme, with some resemblance to glutaminases and β-lactamases. However, ReAV has no glutaminase or lactamase activity, and at pH 9 its allosteric asparaginase activity is relatively high, with Km for L-Asn at 4.2 mM and kcat of 438 s-1. The active site of ReAV, deduced from structural comparisons and confirmed by mutagenesis experiments, contains a highly specific Zn2+ binding site without a catalytic role. The extensive active site includes residues with unusual chemical properties. There are two Ser-Lys tandems, all connected through a network of H-bonds to the Zn center, and three tightly bound water molecules near Ser48, which clearly indicate the catalytic nucleophile.

Structural and biophysical aspects of l-asparaginases: a growing family with amazing diversity

l-Asparaginases have remained an intriguing research topic since their discovery ∼120 years ago, especially after their introduction in the 1960s as very efficient antileukemic drugs. In addition to bacterial asparaginases, which are still used to treat childhood leukemia, enzymes of plant and mammalian origin are now also known. They have all been structurally characterized by crystallography, in some cases at outstanding resolution. The structural data have also shed light on the mechanistic details of these deceptively simple enzymes. Yet, despite all this progress, no better therapeutic agents have been found to beat bacterial asparaginases. However, a new option might arise with the discovery of yet another type of asparaginase, those from symbiotic nitrogen-fixing Rhizobia, and with progress in the protein engineering of enzymes with desired properties. This review surveys the field of structural biology of l-asparaginases, focusing on the mechanistic aspects of the well established types and speculating about the potential of the new members of this amazingly diversified family.

Structural and functional diversity of asparaginases: Overview and recommendations for a revised nomenclature

Asparaginases (ASNases) are a large and structurally diverse group of enzymes ubiquitous amongst archaea, bacteria and eukaryotes, that catalyze hydrolysis of asparagine to aspartate and ammonia. Bacterial ASNases are important biopharmaceuticals for the treatment of acute lymphoblastic leukemia, although some patients experience adverse allergic side effects during treatment with these protein therapeutics. ASNases are currently divided into three families: plant-type ASNases, Rhizobium etli-type ASNases and bacterial-type ASNases. This system is outdated as both bacterial-type and plant-type families also include archaeal, bacterial and eukaryotic enzymes, each with their own distinct characteristics. Herein, phylogenetic studies allied to tertiary structural analyses are described with the aim of proposing a revised and more robust classification system that considers the biochemical diversity of ASNases. Accordingly, based on distinct peptide domains, phylogenetic data, structural analysis and functional characteristics, we recommend that ASNases now be divided into three new distinct classes containing subgroups according to structural and functional aspects. Using this new classification scheme, 25 ASNases were identified as candidates for future new lead discovery.

Heterologous gene expression and characterization of TK2246, a highly active and thermostable plant type l-asparaginase from Thermococcus kodakarensis

The genome sequence of the hyperthermophilic archaeon Thermococcus kodakarensis contains two putative genes, TK1656 and TK2246, annotated as l-asparaginases. TK1656 has been reported previously. The current report is focused on TK2246, a plant-type l-asparaginase, which consists of 918 nucleotides corresponding to a polypeptide of 306 amino acids. The gene was cloned, expressed in Escherichia coli and the purified gene product was used to determine the properties of the recombinant enzyme. TK2246 was optimally active at 85 °C and pH 7.0 with a specific activity of 767 μmol min^-1 mg^-1 towards l-asparagine. The enzyme exhibited a 10% activity towards d-asparagine as compared to 100% against l-asparagine. No detectable activity was observed towards l- or d-glutamine. Half-life of the enzyme was nearly 18 h at 85 °C. TK2246 exhibited apparent K_m and V_max values of 3.1 mM and 833 μmol min^-1 mg^-1, respectively. Activation energy of the reaction, determined from the Arrhenius plot, was 28.3 kJ mol^-1. To the best of our knowledge, this is the first characterization of a plant-type l-asparaginase from class Thermococci of phylum Euryarchaeota.

High-resolution structure of intramolecularly proteolyzed human mucin-1 SEA domain

SEA domains are ubiquitous in large proteins associated with highly glycosylated environments. Certain SEA domains undergo intramolecular proteolysis involving a nucleophilic attack of a serine hydroxyl group on the preceding glycine carbonyl. The mucin-1 (MUC1) SEA domain has been extensively investigated as a model of intramolecular proteolysis. Since neither a general base, a general acid, nor an oxyanion hole could be identified in MUC1 SEA, it has been suggested that proteolysis is accelerated by a non-planarity of the scissile peptide bond imposed by protein folding. A reactant distorted peptide bond has been also invoked to explain the autoproteolysis of several unrelated proteins. However, the only evidence of peptide distortion in MUC1 SEA stems from molecular dynamic simulations of the reactant modeled upon a single NMR structure of the cleaved product. We report the first high-resolution X-ray structure of cleaved MUC1 SEA. Structural comparison with uncleaved SEA domains suggests that the number of residues evolutionarily inserted in the cleaved loop of MUC1 SEA precludes the formation of a properly hydrogen-bonded beta turn. By sequence analysis, we show that this conformational frustration is shared by all known cleaved SEA domains. In addition, alternative conformations of the uncleaved precursor could be modeled in which the scissile peptide bond is planar. The implications of these structures for autoproteolysis are discussed in the light of the previous research on autoproteolysis.

Structural basis of the correct subunit assembly, aggregation, and intracellular degradation of nylon hydrolase

Nylon hydrolase (NylC) is initially expressed as an inactive precursor (36 kDa). The precursor is cleaved autocatalytically at Asn266/Thr267 to generate an active enzyme composed of an α subunit (27 kDa) and a β subunit (9 kDa). Four αβ heterodimers (molecules A-D) form a doughnut-shaped quaternary structure. In this study, the thermostability of the parental NylC was altered by amino acid substitutions located at the A/D interface (D122G/H130Y/D36A/L137A) or the A/B interface (E263Q) and spanned a range of 47 °C. Considering structural, biophysical, and biochemical analyses, we discuss the structural basis of the stability of nylon hydrolase. From the analytical centrifugation data obtained regarding the various mutant enzymes, we conclude that the assembly of the monomeric units is dynamically altered by the mutations. Finally, we propose a model that can predict whether the fate of the nascent polypeptide will be correct subunit assembly, inappropriate protein-protein interactions causing aggregation, or intracellular degradation of the polypeptide.

In silico characterization of a cyanobacterial plant-type isoaspartyl aminopeptidase/asparaginase

Asparaginases are found in a range of organisms, although those found in cyanobacteria have been little studied, in spite of their great potential for biotechnological application. This study therefore sought to characterize the molecular structure of an L-asparaginase from the cyanobacterium Limnothrix sp. CACIAM 69d, which was isolated from a freshwater Amazonian environment. After homology modeling, model validation was performed using a Ramachandran plot, VERIFY3D, and the RMSD. We also performed molecular docking and dynamics simulations based on binding free-energy analysis. Structural alignment revealed homology with the isoaspartyl peptidase/asparaginase (EcAIII) from Escherichia coli. When compared to the template, our model showed full conservation of the catalytic site. In silico simulations confirmed the interaction of cyanobacterial isoaspartyl peptidase/asparaginase with its substrate, β-Asp-Leu dipeptide. We also observed that the residues Thr154, Thr187, Gly207, Asp218, and Gly237 were fundamental to protein-ligand complexation. Overall, our results suggest that L-asparaginase from Limnothrix sp. CACIAM 669d has similar properties to E. coli EcAIII asparaginase. Our study opens up new perspectives for the biotechnological exploitation of cyanobacterial asparaginases.

Characterization of Three L-Asparaginases from Maritime Pine (Pinus pinaster Ait.)

Asparaginases (ASPG, EC 3.5.1.1) catalyze the hydrolysis of the amide group of L-asparagine producing L-aspartate and ammonium. Three ASPG, PpASPG1, PpASPG2, and PpASPG3, have been identified in the transcriptome of maritime pine (Pinus pinaster Ait.) that were transiently expressed in Nicotiana benthamiana by agroinfection. The three recombinant proteins were processed in planta to active enzymes and it was found that all mature forms exhibited double activity asparaginase/isoaspartyl dipeptidase but only PpASPG1 was able to catalyze efficiently L-asparagine hydrolysis. PpASPG1 contains a variable region of 77 amino acids that is critical for proteolytic processing of the precursor and is retained in the mature enzyme. Furthermore, the functional analysis of deletion mutants demonstrated that this protein fragment is required for specific recognition of the substrate and favors enzyme stability. Potassium has a limited effect on the activation of maritime pine ASPG what is consistent with the lack of a critical residue essential for interaction of cation. Taken together, the results presented here highlight the specific features of ASPG from conifers when compared to the enzymes from angiosperms.

Fluorescence-Activated Cell Sorting of Human l-asparaginase Mutant Libraries for Detecting Enzyme Variants with Enhanced Activity

Immunogenicity is one of the most common complications occurring during therapy making use of protein drugs of nonhuman origin. A notable example of such a case is bacterial l-asparaginases (L-ASNases) used for the treatment of acute lymphoblastic leukemia (ALL). The replacement of the bacterial enzymes by human ones is thought to set the basis for a major improvement of antileukemic therapy. Recently, we solved the crystal structure of a human enzyme possessing L-ASNase activity, designated hASNase-3. This enzyme is expressed as an inactive precursor protein and post-translationally undergoes intramolecular processing leading to the generation of two subunits which remain noncovalently, yet tightly associated and constitute the catalytically active form of the enzyme. We discovered that this intramolecular processing can be drastically and selectively accelerated by the free amino acid glycine. In the present study, we report on the molecular engineering of hASNase-3 aiming at the improvement of its catalytic properties. We created a fluorescence-activated cell sorting (FACS)-based high-throughput screening system for the characterization of rationally designed mutant libraries, capitalizing on the finding that free glycine promotes autoproteolytic cleavage, which activates the mutant proteins expressed in an E. coli strain devoid of aspartate biosynthesis. Successive screening rounds led to the isolation of catalytically improved variants showing up to 6-fold better catalytic efficiency as compared to the wild-type enzyme. Our work establishes a powerful strategy for further exploitation of the human asparaginase sequence space to facilitate the identification of in vitro-evolved enzyme species that will lay the basis for improved ALL therapy.

Asparagine Metabolic Pathways in Arabidopsis

Inorganic nitrogen in the form of ammonium is assimilated into asparagine via multiple steps involving glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AspAT) and asparagine synthetase (AS) in Arabidopsis. The asparagine amide group is liberated by the reaction catalyzed by asparaginase (ASPG) and also the amino group of asparagine is released by asparagine aminotransferase (AsnAT) for use in the biosynthesis of amino acids. Asparagine plays a primary role in nitrogen recycling, storage and transport in developing and germinating seeds, as well as in vegetative and senescence organs. A small multigene family encodes isoenzymes of each step of asparagine metabolism in Arabidopsis, except for asparagine aminotransferase encoded by a single gene. The aim of this study is to highlight the structure of the genes and encoded enzyme proteins involved in asparagine metabolic pathways; the regulation and role of different isogenes; and kinetic and physiological properties of encoded enzymes in different tissues and developmental stages.

Structural Characterization of the Loop at the Alpha-Subunit C-Terminus of the Mixed Lineage Leukemia Protein Activating Protease Taspase1

Type 2 asparaginases, a subfamily of N-terminal nucleophile (Ntn) hydrolases, are activated by limited proteolysis. This activation yields a heterodimer and a loop region at the C-terminus of the α-subunit is released. Since this region is unresolved in all type 2 asparaginase crystal structures but is close to the active site residues, we explored this loop region in six members of the type 2 asparaginase family using homology modeling. As the loop model for the childhood cancer-relevant protease Taspase1 differed from the other members, Taspase1 activation as well as the conformation and dynamics of the 56 amino acids loop were investigated by CD and NMR spectroscopy. We propose a helix-turn-helix motif, which can be exploited as novel anticancer target to inhibit Taspase1 proteolytic activity.

Reassimilation of ammonium in Lotus japonicus

This review summarizes the most recent results obtained in the analysis of two important metabolic pathways involved in the release of internal sources of ammonium in the model legume Lotus japonicus: photorespiratory metabolism and asparagine breakdown mediated by aparaginase (NSE). The use of photorespiratory mutants deficient in plastidic glutamine synthetase (GS2) enabled us to investigate the transcriptomics and metabolomic changes associated with photorespiratory ammonium accumulation in this plant. The results obtained indicate the existence of a coordinate regulation of genes involved in photorespiratory metabolism. Other types of evidence illustrate the multiple interconnections existing among the photorespiratory pathway and other processes such as intermediate metabolism, nodule function, and secondary metabolism in this plant, all of which are substantially affected in GS2-deficient mutants because of the impairment of the photorespiratory cycle. Finally, the importance of asparagine metabolism in L. japonicus is highlighted because of the fact that asparagine constitutes the vast majority of the reduced nitrogen translocated between different organs of this plant. The different types of NSE enzymes and genes which are present in L. japonicus are described. There is a particular focus on the most abundant K(+)-dependent LjNSE1 isoform and how TILLING mutants were used to demonstrate by reverse genetics the importance of this particular isoform in plant growth and seed production.

Exploring the role of conformational heterogeneity in cis-autoproteolytic activation of ThnT

In the past decade, there have been major achievements in understanding the relationship between enzyme catalysis and protein structural plasticity. In autoprocessing systems, however, there is a sparsity of direct evidence of the role of conformational dynamics, which are complicated by their intrinsic chemical reactivity. ThnT is an autoproteolytically activated enzyme involved in the biosynthesis of the β-lactam antibiotic thienamycin. Conservative mutation of ThnT results in multiple conformational states that can be observed via X-ray crystallography, establishing ThnT as a representative and revealing system for studing how conformational dynamics control autoactivation at a molecular level. Removal of the nucleophile by mutation to Ala disrupts the population of a reactive state and causes widespread structural changes from a conformation that promotes autoproteolysis to one associated with substrate catalysis. Finer probing of the active site polysterism was achieved by EtHg derivatization of the nucleophile, which indicates the active site and a neighboring loop have coupled dynamics. Disruption of these interactions by mutagenesis precludes the ability to observe a reactive state through X-ray crystallography, and application of this insight to other autoproteolytically activated enzymes offers an explanation for the widespread crystallization of inactive states. We suggest that the N → O(S) acyl shift in cis-autoproteolysis might occur through a si-face attack, thereby unifying the fundamental chemistry of these enzymes through a common mechanism.

Na⁺/K⁺ exchange switches the catalytic apparatus of potassium-dependent plant L-asparaginase

Plant-type L-asparaginases, which are a subclass of the Ntn-hydrolase family, are divided into potassium-dependent and potassium-independent enzymes with different substrate preferences. While the potassium-independent enzymes have already been well characterized, there are no structural data for any of the members of the potassium-dependent group to illuminate the intriguing dependence of their catalytic mechanism on alkali-metal cations. Here, three crystal structures of a potassium-dependent plant-type L-asparaginase from Phaseolus vulgaris (PvAspG1) differing in the type of associated alkali metal ions (K(+), Na(+) or both) are presented and the structural consequences of the different ions are correlated with the enzyme activity. As in all plant-type L-asparaginases, immature PvAspG1 is a homodimer of two protein chains, which both undergo autocatalytic cleavage to α and β subunits, thus creating the mature heterotetramer or dimer of heterodimers (αβ)2. The αβ subunits of PvAspG1 are folded similarly to the potassium-independent enzymes, with a sandwich of two β-sheets flanked on each side by a layer of helices. In addition to the `sodium loop' (here referred to as the `stabilization loop') known from potassium-independent plant-type asparaginases, the potassium-dependent PvAspG1 enzyme contains another alkali metal-binding loop (the `activation loop') in subunit α (residues Val111-Ser118). The active site of PvAspG1 is located between these two metal-binding loops and in the immediate neighbourhood of three residues, His117, Arg224 and Glu250, acting as a catalytic switch, which is a novel feature that is identified in plant-type L-asparaginases for the first time. A comparison of the three PvAspG1 structures demonstrates how the metal ion bound in the activation loop influences its conformation, setting the catalytic switch to ON (when K(+) is coordinated) or OFF (when Na(+) is coordinated) to respectively allow or prevent anchoring of the reaction substrate/product in the active site. Moreover, it is proposed that Ser118, the last residue of the activation loop, is involved in the potassium-dependence mechanism. The PvAspG1 structures are discussed in comparison with those of potassium-independent L-asparaginases (LlA, EcAIII and hASNase3) and those of other Ntn-hydrolases (AGA and Tas1), as well as in the light of noncrystallographic studies.

Elucidation of the specific function of the conserved threonine triad responsible for human L-asparaginase autocleavage and substrate hydrolysis

Our long-term goal is the design of a human l-asparaginase (hASNase3) variant, suitable for use in cancer therapy without the immunogenicity problems associated with the currently used bacterial enzymes. Asparaginases catalyze the hydrolysis of the amino acid asparagine to aspartate and ammonia. The key property allowing for the depletion of blood asparagine by bacterial asparaginases is their low micromolar KM value. In contrast, human enzymes have a millimolar KM for asparagine. Toward the goal of engineering an hASNase3 variant with micromolar KM, we conducted a structure/function analysis of the conserved catalytic threonine triad of this human enzyme. As a member of the N-terminal nucleophile family, to become enzymatically active, hASNase3 must undergo autocleavage between residues Gly167 and Thr168. To determine the individual contribution of each of the three conserved active-site threonines (threonine triad Thr168, Thr186, Thr219) for the enzyme-activating autocleavage and asparaginase reactions, we prepared the T168S, T186V and T219A/V mutants. These mutants were tested for their ability to cleave and to catalyze asparagine hydrolysis, in addition to being examined structurally. We also elucidated the first N-terminal nucleophile plant-type asparaginase structure in the covalent intermediate state. Our studies indicate that, while not all triad threonines are required for the cleavage reaction, all are essential for the asparaginase activity. The increased understanding of hASNase3 function resulting from these studies reveals the key regions that govern cleavage and the asparaginase reaction, which may inform the design of variants that attain a low KM for asparagine.

Structural and kinetic characterization of guinea pig L-asparaginase type III

We investigated whether an uncharacterized protein from guinea pig could be the enzyme behind Kidd's serendipitous discovery, made over 60 years ago, that guinea pig serum has cell killing ability. It has been long known that an enzyme with l-asparaginase activity is responsible for cell killing, although astonishingly, its identity remains unclear. Bacterial asparaginases with similar cell killing properties have since become a mainstay therapy of certain cancers such as acute lymphoblastic leukemia. By hydrolyzing asparagine to aspartate and ammonia, these drugs deplete the asparagine present in the blood, killing cancer cells that rely on extracellular asparagine uptake for survival. However, bacterial asparaginases can elicit an adverse immune response. We propose that replacement of bacterial enzymes with the guinea pig asparaginase responsible for serum activity, by its virtue of being more closely related to human enzymes, will be less immunogenic. To this goal, we investigated whether an uncharacterized protein from guinea pig with putative asparaginase activity, which we call gpASNase3, could be that enzyme. We examined its self-activation process (gpASNase3 requires autocleavage to become active), kinetically characterized it for asparaginase and β-aspartyl dipeptidase activity, and elucidated its crystal structure in both the uncleaved and cleaved states. This work reveals that gpASNase3 is not the enzyme responsible for the antitumor effects of guinea pig serum. It exhibits a low affinity for asparagine as measured by a high Michaelis constant, KM, in the millimolar range, in contrast to the low KM (micromolar range) required for asparaginase to be effective as an anticancer agent.

A computational study of the glycylserine hydrolysis at physiological pH: a zwitterionic versus anionic mechanism

The hydrolysis of GlySer at physiological pH was investigated by modeling the most feasible reaction mechanisms in aqueous phase at the MP2/6-311+(2df,2p)//SMD-M06/6-311+(2df,2p) level of the theory. To refine the energies of the most relevant transition states along the reaction paths the cluster-continuum concept was adopted. The hydrolytic process could proceed through two competitive mechanisms involving either the zwitterionic or the anionic form of GlySer. The calculations suggest that at physiological pH the actual mechanism is most probably mixed, anionic-zwitterionic. In this reaction scheme the first stage of N→O acyl transfer involves the anionic form whereas the second stage, during which the resultant ester is hydrolyzed, most likely involves the zwitterionic ester form of GlySer. The energy requirement for the first reaction stage is estimated to be slightly lower than for the second one. The calculated activation parameters (e.g. ΔG(#) = 27.8 kcal mol(-1)) for the nucleophilic addition of a water molecule to the ester carbonyl group of the zwitterionic ester are in good agreement with the experimentally determined values at pD 7.4 (ΔG(#) = 28.7 kcal mol(-1)).

Free glycine accelerates the autoproteolytic activation of human asparaginase

Human asparaginase 3 (hASNase3), which belongs to the N-terminal nucleophile hydrolase superfamily, is synthesized as a single polypeptide that is devoid of asparaginase activity. Intramolecular autoproteolytic processing releases the amino group of Thr168, a moiety required for catalyzing asparagine hydrolysis. Recombinant hASNase3 purifies as the uncleaved, asparaginase-inactive form and undergoes self-cleavage to the active form at a very slow rate. Here, we show that the free amino acid glycine selectively acts to accelerate hASNase3 cleavage both in vitro and in human cells. Other small amino acids such as alanine, serine, or the substrate asparagine are not capable of promoting autoproteolysis. Crystal structures of hASNase3 in complex with glycine in the uncleaved and cleaved enzyme states reveal the mechanism of glycine-accelerated posttranslational processing and explain why no other amino acid can substitute for glycine.

Uncoupling intramolecular processing and substrate hydrolysis in the N-terminal nucleophile hydrolase hASRGL1 by circular permutation

The human asparaginase-like protein 1 (hASRGL1) catalyzes the hydrolysis of l-asparagine and isoaspartyl-dipeptides. As an N-terminal nucleophile (Ntn) hydrolase superfamily member, the active form of hASRGL1 is generated by an intramolecular cleavage step with Thr168 as the catalytic residue. However, in vitro, autoprocessing is incomplete (~50%), fettering the biophysical characterization of hASRGL1. We circumvented this obstacle by constructing a circularly permuted hASRGL1 that uncoupled the autoprocessing reaction, allowing us to kinetically and structurally characterize this enzyme and the precursor-like hASRGL1-Thr168Ala variant. Crystallographic and biochemical evidence suggest an activation mechanism where a torsional restraint on the Thr168 side chain helps drive the intramolecular processing reaction. Cleavage and formation of the active site releases the torsional restriction on Thr168, which is facilitated by a small conserved Gly-rich loop near the active site that allows the conformational changes necessary for activation.

Structures of apo and product-bound human L-asparaginase: insights into the mechanism of autoproteolysis and substrate hydrolysis

Asparaginases catalyze the hydrolysis of the amino acid asparagine to aspartate and ammonia. Bacterial asparaginases are used in cancer chemotherapy to deplete asparagine from the blood, because several hematological malignancies depend on extracellular asparagine for growth. To avoid the immune response against the bacterial enzymes, it would be beneficial to replace them with human asparaginases. However, unlike the bacterial asparaginases, the human enzymes have a millimolar K(m) value for asparagine, making them inefficient in depleting the amino acid from blood. To facilitate the development of human variants suitable for therapeutic use, we determined the structure of human l-asparaginase (hASNase3). This asparaginase is an N-terminal nucleophile (Ntn) family member that requires autocleavage between Gly167 and Thr168 to become catalytically competent. For most Ntn hydrolases, this autoproteolytic activation occurs efficiently. In contrast, hASNas3 is relatively stable in its uncleaved state, and this allowed us to observe the structure of the enzyme prior to cleavage. To determine the structure of the cleaved state, we exploited our discovery that the free amino acid glycine promotes complete cleavage of hASNase3. Both enzyme states were elucidated in the absence and presence of the product aspartate. Together, these structures provide insight into the conformational changes required for cleavage and the precise enzyme-substrate interactions. The new understanding of hASNase3 will serve to guide the design of variants that possess a decreased K(m) value for asparagine, making the human enzyme a suitable replacement for the bacterial asparaginases in cancer therapy.

Role of asparaginase variable loop at the carboxyl terminal of the alpha subunit in the determination of substrate preference in plants

Structural determinants responsible for the substrate preference of the potassium-independent (ASPGA1) and -dependent (ASPGB1) asparaginases from Arabidopsis thaliana have been investigated. Like ASPGA1, ASPGB1 was found to be catalytically active with both L: -Asn and β-Asp-His as substrates, contrary to a previous report. However, ASPGB1 had a 45-fold higher specific activity with Asn as substrate than ASPGA1. A divergent sequence between the two enzymes forms a variable loop at the C-terminal of the alpha subunit. The results of dynamic simulations have previously implicated a movement of the C-terminus in the allosteric transduction of K(+)-binding at the surface of LjNSE1 asparaginase. In the crystal structure of Lupinus luteus asparaginase, most residues in this segment cannot be visualized due to a weak electron density. Exchanging the variable loop in ASPGA1 with that from ASPGB1 increased the affinity for Asn, with a 320-fold reduction in K (m) value. Homology modeling identified a residue specific to ASPGB1, Phe(162), preceding the variable loop, whose side chain is located in proximity to the beta-carboxylate group of the product aspartate, and to Gly(246), a residue participating in an oxyanion hole which stabilizes a negative charge forming on the side chain oxygen of asparagine during catalysis. Replacement with the corresponding leucine from ASPGA1 specifically lowered the V (max) value with Asn as substrate by 8.4-fold.

Insights into cis-autoproteolysis reveal a reactive state formed through conformational rearrangement

ThnT is a pantetheine hydrolase from the DmpA/OAT superfamily involved in the biosynthesis of the β-lactam antibiotic thienamycin. We performed a structural and mechanistic investigation into the cis-autoproteolytic activation of ThnT, a process that has not previously been subject to analysis within this superfamily of enzymes. Removal of the γ-methyl of the threonine nucleophile resulted in a rate deceleration that we attribute to a reduction in the population of the reactive rotamer. This phenomenon is broadly applicable and constitutes a rationale for the evolutionary selection of threonine nucleophiles in autoproteolytic systems. Conservative substitution of the nucleophile (T282C) allowed determination of a 1.6-Å proenzyme ThnT crystal structure, which revealed a level of structural flexibility not previously observed within an autoprocessing active site. We assigned the major conformer as a nonreactive state that is unable to populate a reactive rotamer. Our analysis shows the system is activated by a structural rearrangement that places the scissile amide into an oxyanion hole and forces the nucleophilic residue into a forbidden region of Ramachandran space. We propose that conformational strain may drive autoprocessing through the destabilization of nonproductive states. Comparison of our data with previous reports uncovered evidence that many inactivated structures display nonreactive conformations. For penicillin and cephalosporin acylases, this discrepancy between structure and function may be resolved by invoking the presence of a hidden conformational state, similar to that reported here for ThnT.

Three-dimensional structure of nylon hydrolase and mechanism of nylon-6 hydrolysis

We performed x-ray crystallographic analyses of the 6-aminohexanoate oligomer hydrolase (NylC) from Agromyces sp. at 2.0 Å-resolution. This enzyme is a member of the N-terminal nucleophile hydrolase superfamily that is responsible for the degradation of the nylon-6 industry byproduct. We observed four identical heterodimers (27 kDa + 9 kDa), which resulted from the autoprocessing of the precursor protein (36 kDa) and which constitute the doughnut-shaped quaternary structure. The catalytic residue of NylC was identified as the N-terminal Thr-267 of the 9-kDa subunit. Furthermore, each heterodimer is folded into a single domain, generating a stacked αββα core structure. Amino acid mutations at subunit interfaces of the tetramer were observed to drastically alter the thermostability of the protein. In particular, four mutations (D122G/H130Y/D36A/E263Q) of wild-type NylC from Arthrobacter sp. (plasmid pOAD2-encoding enzyme), with a heat denaturation temperature of T(m) = 52 °C, enhanced the protein thermostability by 36 °C (T(m) = 88 °C), whereas a single mutation (G111S or L137A) decreased the stability by ∼10 °C. We examined the enzymatic hydrolysis of nylon-6 by the thermostable NylC mutant. Argon cluster secondary ion mass spectrometry analyses of the reaction products revealed that the major peak of nylon-6 (m/z 10,000-25,000) shifted to a smaller range, producing a new peak corresponding to m/z 1500-3000 after the enzyme treatment at 60 °C. In addition, smaller fragments in the soluble fraction were successively hydrolyzed to dimers and monomers. Based on these data, we propose that NylC should be designated as nylon hydrolase (or nylonase). Three potential uses of NylC for industrial and environmental applications are also discussed.

Structural analysis of K+ dependence in L-asparaginases from Lotus japonicus

The molecular features responsible for the existence in plants of K+-dependent asparaginases have been investigated. For this purpose, two different cDNAs were isolated in Lotus japonicus, encoding for K+-dependent (LjNSE1) or K+-independent (LjNSE2) asparaginases. Recombinant proteins encoded by these cDNAs have been purified and characterized. Both types of asparaginases are composed by two different subunits, α (20 kDa) and β (17 kDa), disposed as (αβ)₂ quaternary structure. Major differences were found in the catalytic efficiency of both enzymes, due to the fact that K+ is able to increase by tenfold the enzyme activity and lowers the K(m) for asparagine specifically in LjNSE1 but not in LjNSE2 isoform. Optimum LjNSE1 activity was found at 5-50 mM K+, with a K(m) for K+ of 0.25 mM. Na+ and Rb+ can, to some extent, substitute for K+ on the activating effect of LjNSE1 more efficiently than Cs+ and Li+ does. In addition, K+ is able to stabilize LjNSE1 against thermal inactivation. Protein homology modelling and molecular dynamics studies, complemented with site-directed mutagenesis, revealed the key importance of E248, D285 and E286 residues for the catalytic activity and K+ dependence of LjNSE1, as well as the crucial relevance of K+ for the proper orientation of asparagine substrate within the enzyme molecule. On the other hand, LjNSE2 but not LjNSE1 showed β-aspartyl-hydrolase activity (K(m) = 0.54 mM for β-Asp-His). These results are discussed in terms of the different physiological significance of these isoenzymes in plants.

Crystallographic snapshot of glycosylasparaginase precursor poised for autoprocessing

Glycosylasparaginase belongs to a family of N-terminal nucleophile hydrolases that autoproteolytically generate their mature enzymes from single-chain protein precursors. Previously, based on a precursor structure paused at pre-autoproteolysis stage by a reversible inhibitor (glycine), we proposed a mechanism of intramolecular autoproteolysis. A key structural feature, a highly strained conformation at the scissile peptide bond, had been identified and was hypothesized to be critical for driving autoproteolysis through an N-O acyl shift. To examine this "twist-and-break" hypothesis, we report here a 1. 9-Å-resolution structure of an autoproteolysis-active precursor (a T152C mutant) that is free of inhibitor or ligand and is poised to undergo autoproteolysis. The current crystallographic study has provided direct evidence for the natural conformation of the glycosylasparaginase autocatalytic site without influence from any inhibitor or ligand. This finding has confirmed our previous proposal that conformational strain is an intrinsic feature of an active precursor.

The human asparaginase-like protein 1 hASRGL1 is an Ntn hydrolase with beta-aspartyl peptidase activity

Herein we report the bacterial expression, purification, and enzymatic characterization of the human asparaginase-like protein 1 (hASRGL1). We present evidence that hASRGL1 exhibits beta-aspartyl peptidase activity consistent with enzymes designated as plant-type asparaginases, which had thus far been found in only plants and bacteria. Similar to nonmammalian plant-type asparaginases, hASRGL1 is shown to be an Ntn hydrolase for which Thr168 serves as the essential N-terminal nucleophile for intramolecular processing and catalysis, corroborated in part by abolishment of both activities through the Thr168Ala point mutation. In light of the activity profile reported here, ASRGL1s may act synergistically with protein l-isoaspartyl methyl transferase to relieve accumulation of potentially toxic isoaspartyl peptides in mammalian brain and other tissues.