Probing the specificity determinants of amino acid recognition by arginase

Structural and Biochemical Insights into Post-Translational Arginine-to-Ornithine Peptide Modifications by an Atypical Arginase

Landornamide A is a ribosomally synthesized and post-translationally modified peptide (RiPP) natural product with antiviral activity. Its biosynthetic gene cluster encodes─among other maturases─the peptide arginase OspR, which converts arginine to ornithine units in an unusual post-translational modification. Peptide arginases are a recently discovered RiPP maturase family with few characterized representatives. They show little sequence similarity to conventional arginases, a well-characterized enzyme family catalyzing the hydrolysis of free arginine to ornithine and urea. Peptide arginases are highly promiscuous and accept a variety of substrate sequences. The molecular basis for binding the large peptide substrate and for the high promiscuity of peptide arginases remains unclear. Here, we report the first crystal structure of a peptide arginase at a resolution of 2.6 Å. The three-dimensional structure reveals common features and differences between conventional arginases and the peptide arginase: the binuclear metal cluster and the active-site environment strongly resemble each other, while the quaternary structures diverge. Kinetic analyses of OspR with various substrates provide new insights into the order of biosynthetic reactions during the post-translational maturation of landornamide A. These results provide the basis for pathway engineering to generate derivatives of landornamide A and for the general application of peptide arginases as biosynthetic tools for peptide engineering.

Inhibiting Human and Leishmania Arginases Using Cannabis sativa as a Potential Therapy for Cutaneous Leishmaniasis: A Molecular Docking Study

Cutaneous leishmaniasis (CL), a vector-borne parasitic disease caused by the Leishmania protozoan, is a serious public health problem in Morocco. The treatment of this disease is still based on pentavalent antimonials as the primary therapy, but these have associated side effects. Thus, the development of effective, risk-free alternative therapeutics based on natural compounds against leishmaniasis is urgent. Arginase, the key enzyme in the polyamine biosynthetic pathway, plays a critical role in leishmaniasis outcome and has emerged as a potential therapeutic target. The objective of this study was to test Cannabis sativa's phytochemical components (cannabinoids and terpenoids) through molecular docking against Leishmania and human arginase enzymes. Our results showed that delta-9-tetrahydrocannabinol (THC) possessed the best binding energies of -6.02 and -6.35 kcal/mol with active sites of Leishmania and human arginases, respectively. Delta-9-THC interacted with Leishmania arginase through various amino acids including His139 and His 154 and linked to human arginase via His 126. In addition to delta-9-THC, caryophyllene oxide and cannabidiol (CBD) also showed a good inhibition of Leishmania and human arginases, respectively. Overall, the studied components were found to inhibit both arginases active sites via hydrogen bonds and hydrophobic interactions. These components may serve as therapeutic agents or in co-administrated therapy for leishmaniasis.

New Insights into the Determinants of Specificity in Human Type I Arginase: Generation of a Mutant That Is Only Active with Agmatine as Substrate

Arginase catalyzes the hydrolysis of L-arginine into L-ornithine and urea. This enzyme has several analogies with agmatinase, which catalyzes the hydrolysis of agmatine into putrescine and urea. However, this contrasts with the highlighted specificity that each one presents for their respective substrate. A comparison of available crystal structures for arginases reveals an important difference in the extension of two loops located in the entrance of the active site. The first, denominated loop A (I129-L140) contains the residues that interact with the alpha carboxyl group or arginine of arginase, and the loop B (D181-P184) contains the residues that interact with the alpha amino group of arginine. In this work, to determine the importance of these loops in the specificity of arginase, single, double, and triple arginase mutants in these loops were constructed, as well as chimeras between type I human arginase and E. coli agmatinase. In previous studies, the substitution of N130D in arginase (in loop A) generated a species capable of hydrolyzing arginine and agmatine. Now, the specificity of arginase is completely altered, generating a chimeric species that is only active with agmatine as a substrate, by substituting I129T, N130Y, and T131A together with the elimination of residues P132, L133, and T134. In addition, Quantum Mechanic/Molecular Mechanic (QM/MM) calculations were carried out to study the accommodation of the substrates in in the active site of this chimera. With these results it is concluded that this loop is decisive to discriminate the type of substrate susceptible to be hydrolyzed by arginase. Evidence was also obtained to define the loop B as a structural determinant for substrate affinity. Concretely, the double mutation D181T and V182E generate an enzyme with an essentially unaltered k_cat value, but with a significantly increased K_m value for arginine and a significant decrease in affinity for its product ornithine.

Targeting Metalloenzymes by Boron-Containing Metal-Binding Pharmacophores

Metalloenzymes have critical roles in a wide range of biological processes and are directly involved in many human diseases; hence, they are considered as important targets for therapeutic intervention. The specific characteristics of metal ion(s)-containing active sites make exploitation of metal-binding pharmacophores (MBPs) critical to inhibitor development targeting metalloenzymes. This Perspective focuses on boron-containing MBPs, which display unique binding modes with metalloenzyme active sites, particularly via mimicking native substrates or tetrahedral transition states. The design concepts regarding boron-containing MBPs are highlighted through the case analyses on five distinct classes of clinically relevant nucleophilic metalloenzymes from medicinal chemistry perspectives. The challenges (e.g., selectivity) faced by some boron-containing MBPs and possible strategies (e.g., bioisosteres) for metalloenzyme inhibitor transformation are also discussed.

Diversity, properties and functions of bacterial arginases

The metalloenzyme arginase hydrolyzes L-arginine to produce L-ornithine and urea. In bacteria, arginase has important functions in basic nitrogen metabolism and redistribution, production of the key metabolic precursor L-ornithine, stress resistance and pathogenesis. We describe the regulation and specific functions of the arginase pathway as well as summarize key characteristics of related arginine catabolic pathways. The use of arginase-derived ornithine as a precursor molecule is reviewed. We discuss the biochemical and transcriptional regulation of arginine metabolism, including arginase, with the latter topic focusing on the RocR and AhrC transcriptional regulators in the model organism Bacillus subtilis. Finally, we consider similarities and contrasts in the structure and catalytic mechanism of the arginases from Bacillus caldovelox and Helicobacter pylori. The overall aim of this review is to provide a panorama of the diversity of physiological functions, regulation, and biochemical features of arginases in a variety of bacterial species.

Butyrylcholinesterase-a potential plasma biomarker in manganese-induced neurobehavioral changes

Groundwater particularly drinking water contamination with metals has created an environmental disaster in Bangladesh. Manganese (Mn), an essential trace element, plays a key role in the development and function of the brain. Excess Mn exposure is reported to be associated with complex neurological disorders. Here, we have found a notably large extent of Mn above the permissive limit in the tube-well water of Rajshahi and Naogaon districts in Bangladesh. Higher levels of Mn in hair and nail samples, and a decreasing level of butyrylcholinesterase (BChE) activity were detected in plasma samples of the human subjects recruited from Naogaon district. Mn concentrations in water, hair, and nails were negatively correlated with the plasma BChE levels in Mn-exposed populations. To compare and validate these human studies, an animal model was used to determine the in vivo effects of Mn on neurobehavioral changes and blood BChE levels. In elevated plus maze, the time spent was significantly reduced in open arms and increased in closed arms of Mn-exposed mice compared to control group. The mean latency time to find the platform was declined significantly in control mice compared to Mn-treated group during 7 days in Morris water maze test, and Mn-exposed group also spent significantly less time in the desired quadrant as compared to the control group in probe trial. BChE activity was significantly reduced in Mn-exposed mice compared to control mice. Taken together, these results suggest that plasma BChE levels may serve as reliable biomarker of Mn-induced neurotoxicity related to behavioral changes.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

A manganese-salen complex as dipeptidyl peptidase IV inhibitor for the treatment of type 2 diabetes

A manganese Schiff base complex with N,N'-1,2-phenylenediamine-bis(salicyladimine) was synthesized and characterized by X-ray crystallography. This complex was administered intragastrically to alloxan-diabetic mice 3 weeks. In vivo tests showed that the complex significantly lowered serum glucose levels in alloxan-diabetic mice at doses of 77 mg V kg^-1. Meanwhile, this complex was investigated as dipeptidyl peptidase IV (DPP-IV) inhibitor for the treatment of type 2 diabetes. The compound exhibit moderate inhibition against DPP-IV and possessed an IC₅₀ value of 30 μM. Lineweaver-Burk transformation of the inhibition kinetics data demonstrated that it was a noncompetitive inhibitor of DPP-IV and Ki value was 136.3 μM. Moreover, molecular modeling studies suggested that the complex could fit well within the active-site cleft of DPP-IV. An acute toxicity study showed that animals treated intragastically with complex 1 at a dose of 5.0 g/kg did not show any significantly abnormal signs. These preliminary results suggest that the manganese Schiff base complex can induce a hypoglycemic effect in alloxan-diabetic mice.

Evolution of biosynthetic diversity

Since the emergence of the last common ancestor from which all extant life evolved, the metabolite repertoire of cells has increased and diversified. Not only has the metabolite cosmos expanded, but the ways in which the same metabolites are made have diversified. Enzymes catalyzing the same reaction have evolved independently from different protein folds; the same protein fold can produce enzymes recognizing different substrates, and enzymes performing different chemistries. Genes encoding useful enzymes can be transferred between organisms and even between the major domains of life. Organisms that live in metabolite-rich environments sometimes lose the pathways that produce those same metabolites. Fusion of different protein domains results in enzymes with novel properties. This review will consider the major evolutionary mechanisms that generate biosynthetic diversity: gene duplication (and gene loss), horizontal and endosymbiotic gene transfer, and gene fusion. It will also discuss mechanisms that lead to convergence as well as divergence. To illustrate these mechanisms, one of the original metabolisms present in the last universal common ancestor will be employed: polyamine metabolism, which is essential for the growth and cell proliferation of archaea and eukaryotes, and many bacteria.

Manganese neurotoxicity: behavioral disorders associated with dysfunctions in the basal ganglia and neurochemical transmission

Manganese (Mn) is an essential element required for many physiological functions. While it is essential at physiological levels, excessive accumulation of Mn in the brain causes severe dysfunctions in the central nervous system known as manganism. Manganism is an extrapyramidal disorder characterized by motor disturbances associated with neuropsychiatric and cognitive disabilities similar to Parkinsonism. As the primary brain regions targeted by Mn are the basal ganglia, known to be involved in the pathophysiology of extrapyramidal disorders, this review will examine the impact of Mn exposure on the basal ganglia circuitry and neurotransmitters in relation to motor and non-motor disorders. The collected data from recent available studies in humans and experimental animal models provide new information about the mechanisms by which Mn affects behavior, neurotransmitters, and basal ganglia function observed in manganism. The effects of the alterations of metals on basal ganglia and neurochemical functioning are critical to develop effective modalities not only for the treatment of vulnerable populations (e.g., Mn-exposed workers) but also for understanding the etiology of neurodegenerative diseases where brain metal imbalances are involved, such as Parkinson's disease. We examine the impact of manganese (Mn) exposure on the basal ganglia circuitry and neurotransmitters in relation with motor and non-motor disorders. The collected data from available studies show that when accumulated in the globus pallidus, Mn influences the subthalamic (STN) and substantia nigra (SN) neurons, which are at the origin of changes in the thalamus and the cortex.

Enhancement of Inducible Nitric Oxide Synthase Activity by Low Molecular Weight Peptides Derived from Protamine: A Potential Therapy for Chronic Rhinosinusitis

Nitric oxide (NO) is a key immune defense agent that is produced from l-arginine in the airways by leukocytes and airway epithelial cells, primarily via inducible nitric oxide synthase (iNOS). Deficiencies in nasal NO levels have been associated with diseases such as primary ciliary dyskinesia and chronic rhinosinusitis. Herein, we demonstrate a proof-of-concept regarding a potential new therapeutic approach for such disorders. We show that arginine-rich low molecular weight peptides (LMWPs) derived from the FDA-approved protamine (obtained from salmon sperm) are effective at significantly raising NO production in both RAW 264.7 mouse macrophage and LA4 mouse epithelial cell lines. LMWP is produced using a stable, easily produced immobilized thermolysin gel column followed by size-exclusion purification. Monomeric l-arginine induces concentration-dependent increases in NO production in stimulated RAW 264.7 and LA4 cells, as measured by stable nitrite in the cell media. In stimulated RAW 264.7 cells, LMWP significantly increases iNOS expression and total NO production 12-24 h post-treatment compared to cells given equivalent levels of monomeric l-arginine. For stimulated LA4 cells, LMWPs are effective in significantly increasing NO production compared to equivalent l-arginine monomer concentrations over 24 h but do not substantially enhance iNOS expression. The use of the arginase inhibitor S-boronoethyl-l-cysteine in combination with LMWPs results in even higher NO production by stimulated RAW 264.7 cells and LA4 cells. Increases in NO due to LMWPs, compared to l-arginine, occur only after 4 h, which may be due to iNOS elevation rather than increased substrate availability.

Crystal structure of an arginase-like protein from Trypanosoma brucei that evolved without a binuclear manganese cluster

The X-ray crystal structure of an arginase-like protein from the parasitic protozoan Trypanosoma brucei, designated TbARG, is reported at 1.80 and 2.38 Å resolution in its reduced and oxidized forms, respectively. The oxidized form of TbARG is a disulfide-linked hexamer that retains the overall architecture of a dimer of trimers in the reduced form. Intriguingly, TbARG does not contain metal ions in its putative active site, and amino acid sequence comparisons indicate that all but one of the residues required for coordination to the catalytically obligatory binuclear manganese cluster in other arginases are substituted here with residues incapable of metal ion coordination. Therefore, the structure of TbARG is the first of a member of the arginase/deacetylase superfamily that is not a metalloprotein. Although we show that metal binding activity is easily reconstituted in TbARG by site-directed mutagenesis and confirmed in X-ray crystal structures, it is curious that this protein and its parasitic orthologues evolved away from metal binding function. Knockout of the TbARG gene from the genome demonstrated that its function is not essential to cultured bloodstream-form T. brucei, and metabolomics analysis confirmed that the enzyme has no role in the conversion of l-arginine to l-ornithine in these cells. While the molecular function of TbARG remains enigmatic, the fact that the T. brucei genome encodes only this protein and not a functional arginase indicates that the parasite must import l-ornithine from its host to provide a source of substrate for ornithine decarboxylase in the polyamine biosynthetic pathway, an active target for the development of antiparasitic drugs.

Crystal structure of Schistosoma mansoni arginase, a potential drug target for the treatment of schistosomiasis

The X-ray crystal structure of arginase from Schistosoma mansoni (SmARG) and the structures of its complexes with several amino acid inhibitors have been determined at atomic resolution. SmARG is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea, and this enzyme is upregulated in all forms of the parasite that interact with the human host. Current hypotheses suggest that parasitic arginases could play a role in host immune evasion by depleting pools of substrate l-arginine that would otherwise be utilized for NO biosynthesis and NO-dependent processes in the immune response. Although the amino acid sequence of SmARG is only 42% identical with that of human arginase I, residues important for substrate binding and catalysis are strictly conserved. In general, classical amino acid inhibitors such as 2(S)-amino-6-boronohexanoic acid (ABH) tend to bind more weakly to SmARG than to human arginase I despite identical inhibitor binding modes in each enzyme active site. The identification of a patch on the enzyme surface capable of accommodating the additional Cα substitutent of an α,α-disubstituted amino acid inhibitor suggests that such inhibitors could exhibit higher affinity and biological activity. The structures of SmARG complexed with two different α,α-disubstituted derivatives of ABH are presented and provide a proof of concept for this approach in the enhancement of enzyme-inhibitor affinity.

Analysis of novel ARG1 mutations causing hyperargininemia and correlation with arginase I activity in erythrocytes

Hyperargininemia (HA) is an autosomal recessive disease that typically has a clinical presentation that is distinct from other urea cycle disorders. It is caused by the deficient activity of the enzyme arginase I, encoded by the gene ARG1. We screened for ARG1 mutations and measured erythrocyte enzyme activity in a series of 16 Brazilian HA patients. Novel mutations, in addition to previously described missense mutations, were analysed for their effect on the structure, stability and/or function of arginase I (ARG1) using bioinformatics tools. Three previously reported mutations were found (p.R21X; p.I11T and p.W122X), and five novel mutations were identified (p.G27D; p.G74V; p.T134I; p.R308Q; p.I174fs179). The p.T134I mutation was the most frequent in the Brazilian population. Patients carrying the p.R308Q mutation had higher residual ARG1 decreased activity, but presented no distinguishable phenotype compared to the other patients. Bioinformatics analyses revealed that missense mutations (1) affect the ARG1 active site, (2) interfere with the stability of the ARG1 folded conformation or (3) alter the quaternary structure of the ARG1. Our study reinforced the role of Arg308 residue for assembly of the ARG1 homotrimer. The panel of heterogeneous ARG1 mutations that cause HA was expanded, nevertheless a clear genotype-phenotype correlation was not observed in our series.

Binding of the unreactive substrate analog L-2-amino-3-guanidinopropionic acid (dinor-L-arginine) to human arginase I

Human arginase I (HAI) is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to form L-ornithine and urea through a metal-activated hydroxide mechanism. Since HAI regulates L-Arg bioavailability for NO biosynthesis, it is a potential drug target for the treatment of cardiovascular diseases such as atherosclerosis. X-ray crystal structures are now reported of the complexes of Mn(2)(2+)-HAI and Co(2)(2+)-HAI with L-2-amino-3-guanidinopropionic acid (AGPA; also known as dinor-L-arginine), an amino acid bearing a guanidinium side chain two methylene groups shorter than that of L-arginine. Hydrogen bonds to the α-carboxylate and α-amino groups of AGPA dominate enzyme-inhibitor recognition; the guanidinium group does not interact directly with the metal ions.

SCHEMA-designed variants of human Arginase I and II reveal sequence elements important to stability and catalysis

Arginases catalyze the divalent cation-dependent hydrolysis of L-arginine to urea and L-ornithine. There is significant interest in using arginase as a therapeutic antineogenic agent against L-arginine auxotrophic tumors and in enzyme replacement therapy for treating hyperargininemia. Both therapeutic applications require enzymes with sufficient stability under physiological conditions. To explore sequence elements that contribute to arginase stability we used SCHEMA-guided recombination to design a library of chimeric enzymes composed of sequence fragments from the two human isozymes Arginase I and II. We then developed a novel active learning algorithm that selects sequences from this library that are both highly informative and functional. Using high-throughput gene synthesis and our two-step active learning algorithm, we were able to rapidly create a small but highly informative set of seven enzymatically active chimeras that had an average variant distance of 40 mutations from the closest parent arginase. Within this set of sequences, linear regression was used to identify the sequence elements that contribute to the long-term stability of human arginase under physiological conditions. This approach revealed a striking correlation between the isoelectric point and the long-term stability of the enzyme to deactivation under physiological conditions.

Expression, purification and characterization of arginase from Helicobacter pylori in its apo form

Arginase, a manganese-dependent enzyme that widely distributed in almost all creatures, is a urea cycle enzyme that catalyzes the hydrolysis of L-arginine to generate L-ornithine and urea. Compared with the well-studied arginases from animals and yeast, only a few eubacterial arginases have been characterized, such as those from H. pylori and B. anthracis. However, these enzymes used for arginase activity assay were all expressed with LB medium, as low concentration of Mn²⁺ was detectable in the medium, protein obtained were partially Mn²⁺ bonded, which may affect the results of arginase activity assay. In the present study, H. pylori arginase (RocF) was expressed in a Mn²⁺ and Co²⁺ free minimal medium, the resulting protein was purified through affinity and gel filtration chromatography and the apo-form of RocF was confirmed by flame photometry analysis. Gel filtration indicates that the enzyme exists as monomer in solution, which was unique as compared with homologous enzymes. Arginase activity assay revealed that apo-RocF had an acidic pH optimum of 6.4 and exhibited metal preference of Co²⁺>Ni²⁺>Mn²⁺. We also confirmed that heat-activation and reducing regents have significant impact on arginase activity of RocF, and inhibits S-(2-boronoethyl)-L-Cysteine (BEC) and Nω-hydroxy-nor-Arginine (nor-NOHA) inhibit the activity of RocF in a dose-dependent manner.

Crystal structures of complexes with cobalt-reconstituted human arginase I

The binuclear manganese metalloenzyme human arginase I (HAI) is a potential protein drug for cancer chemotherapy, in that it is capable of depleting extracellular l-Arg levels in the microenvironment of tumor cells that require this nutrient to thrive. Substitution of the native Mn(2+)(2) cluster with a Co(2+)(2) cluster in the active site yields an enzyme with enhanced catalytic activity at physiological pH (∼7.4) that could serve as an improved protein drug for L-Arg depletion therapy [Stone, E. M., Glazer, E. S., Chantranupong, L., Cherukuri, P., Breece, R. M., Tierney, D. L., Curley, S. A., Iverson, B. L., and Georgiou, G. (2010) ACS Chem. Biol. 5, 333-342]. A different catalytic mechanism is proposed for Co(2+)(2)-HAI compared with that of Mn(2+)(2)-HAI, including an unusual Nε-Co(2+) coordination mode, to rationalize the lower K(M) value of L-Arg and the lower K(i) value of L-Orn. However, we now report that no unusual metal coordination modes are observed in the cobalt-reconstituted enzyme. The X-ray crystal structures of unliganded Co(2+)(2)-HAI determined at 2.10 Å resolution (pH 7.0) and 1.97 Å resolution (pH 8.5), as well as the structures of Co(2+)(2)-HAI complexed with the reactive substrate analogue 2(S)-amino-6-boronohexanoic acid (ABH, pH 7.0) and the catalytic product L-Orn (pH 7.0) determined at 1.85 and 1.50 Å resolution, respectively, are essentially identical to the corresponding structures of Mn(2+)(2)-HAI. Therefore, in the absence of significant structural differences between Co(2+)(2)-HAI and Mn(2+)(2)-HAI, we suggest that a higher concentration of metal-bridging hydroxide ion at physiological pH for Co(2+)(2)-HAI, a consequence of the lower pK(a) of a Co(2+)-bound water molecule compared with a Mn(2+)-bound water molecule, strengthens electrostatic interactions with cationic amino acids and accounts for enhanced affinity as reflected in the lower K(M) value of L-Arg and the lower K(i) value of L-Orn.

Binding of α,α-disubstituted amino acids to arginase suggests new avenues for inhibitor design

Arginase is a binuclear manganese metalloenzyme that hydrolyzes L-arginine to form L-ornithine and urea, and aberrant arginase activity is implicated in various diseases such as erectile dysfunction, asthma, atherosclerosis, and cerebral malaria. Accordingly, arginase inhibitors may be therapeutically useful. Continuing our efforts to expand the chemical space of arginase inhibitor design and inspired by the binding of 2-(difluoromethyl)-L-ornithine to human arginase I, we now report the first study of the binding of α,α-disubstituted amino acids to arginase. Specifically, we report the design, synthesis, and assay of racemic 2-amino-6-borono-2-methylhexanoic acid and racemic 2-amino-6-borono-2-(difluoromethyl)hexanoic acid. X-ray crystal structures of human arginase I and Plasmodium falciparum arginase complexed with these inhibitors reveal the exclusive binding of the L-stereoisomer; the additional α-substituent of each inhibitor is readily accommodated and makes new intermolecular interactions in the outer active site of each enzyme. Therefore, this work highlights a new region of the protein surface that can be targeted for additional affinity interactions, as well as the first comparative structural insights on inhibitor discrimination between a human and a parasitic arginase.

Crystal structures of Pseudomonas aeruginosa guanidinobutyrase and guanidinopropionase, members of the ureohydrolase superfamily

Pseudomonas aeruginosa guanidinobutyrase (GbuA) and guanidinopropionase (GpuA) catalyze the hydrolysis of 4-guanidinobutyrate and 3-guanidinopropionate, respectively. They belong to the ureohydrolase superfamily, which includes arginase, agmatinase, proclavaminate amidinohydrolase, and formiminoglutamase. In this study, we have determined the crystal structures of GbuA and GpuA from P. aeruginosa to provide a structural insight into their substrate specificity. Although GbuA and GpuA share a common structural fold of the typical ureohydrolase superfamily, they exhibit significant variations in two active site loops. Mutagenesis of Met161 of GbuA and Tyr157 of GpuA, both of which are located in the active site loop 1 and predicted to be involved in substrate recognition, significantly affected their enzymatic properties, implying their important roles in catalysis.

Crystallization of an apo form of human arginase: using all the tools in the toolbox simultaneously

Arginase (EC 3.5.3.1) is an aminohydrolase that acts on L-arginine to produce urea and ornithine. Two isotypes of the enzyme are found in humans. Type I is predominantly produced in the liver and is a homotrimer of 35 kDa subunits. Human arginase (hArginase) I is seen to be up-regulated in many diseases and is a potential therapeutic target for many diverse indications. Previous reports of crystallization and structure determination of hArginase have always included inhibitors of the enzyme: here, the first case of a true apo crystal form of the enzyme which is suitable for small-molecule soaking is reported. The crystals belonged to space group P2(1)2(1)2(1) and have approximate unit-cell parameters a=53, b=67.5, c=250 Å. The crystals showed slightly anisotropic diffraction to beyond 2.0 Å resolution.

Inhibition of human arginase I by substrate and product analogues

Human arginase I is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to generate L-ornithine and urea. We demonstrate that N-hydroxy-L-arginine (NOHA) binds to this enzyme with K(d)=3.6 microM, and nor-N-hydroxy-L-arginine (nor-NOHA) binds with K(d)=517 nM (surface plasmon resonance) or K(d) approximately 50 nM (isothermal titration calorimetry). Crystals of human arginase I complexed with NOHA and nor-NOHA afford 2.04 and 1.55 A resolution structures, respectively, which are significantly improved in comparison with previously-determined structures of the corresponding complexes with rat arginase I. Higher resolution structures clarify the binding interactions of the inhibitors. Finally, the crystal structure of the complex with L-lysine (K(d)=13 microM) is reported at 1.90 A resolution. This structure confirms the importance of hydrogen bond interactions with inhibitor alpha-carboxylate and alpha-amino groups as key specificity determinants of amino acid recognition in the arginase active site.

Manganese and its role in Parkinson's disease: from transport to neuropathology

The purpose of this review is to highlight recent advances in the neuropathology associated with Mn exposures. We commence with a discussion on occupational manganism and clinical aspects of the disorder. This is followed by novel considerations on Mn transport (see also chapter by Yokel, this volume), advancing new hypotheses on the involvement of several transporters in Mn entry into the brain. This is followed by a brief description of the effects of Mn on neurotransmitter systems that are putative modulators of dopamine (DA) biology (the primary target of Mn neurotoxicity), as well as its effects on mitochondrial dysfunction and disruption of cellular energy metabolism. Next, we discuss inflammatory activation of glia in neuronal injury and how disruption of synaptic transmission and glial-neuronal communication may serve as underlying mechanisms of Mn-induced neurodegeneration commensurate with the cross-talk between glia and neurons. We conclude with a discussion on therapeutic aspects of Mn exposure. Emphasis is directed at treatment modalities and the utility of chelators in attenuating the neurodegenerative sequelae of exposure to Mn. For additional reading on several topics inherent to this review as well as others, the reader may wish to consult Aschner and Dorman (Toxicological Review 25:147-154, 2007) and Bowman et al. (Metals and neurodegeneration, 2009).