Nitrilases attract increasing attention due to their utility in the mild hydrolysis of nitriles. According to activity and gene screening, filamentous fungi are a rich source of nitrilases distinct in evolution from their widely examined bacterial counterparts. However, fungal nitrilases have been less explored than the bacterial ones. Nitrilases are typically heterogeneous in their quaternary structures, forming short spirals and extended filaments, these features making their structural studies difficult.
A nitrilase gene was amplified by PCR from the cDNA library of Aspergillus niger K10. The PCR product was ligated into expression vectors pET-30(+) and pRSET B to construct plasmids pOK101 and pOK102, respectively. The recombinant nitrilase (Nit-ANigRec) expressed in Escherichia coli BL21-Gold(DE3)(pOK101/pTf16) was purified with an about 2-fold increase in specific activity and 35% yield. The apparent subunit size was 42.7 kDa, which is approx. 4 kDa higher than that of the enzyme isolated from the native organism (Nit-ANigWT), indicating post-translational cleavage in the enzyme's native environment. Mass spectrometry analysis showed that a C-terminal peptide (Val327 - Asn356) was present in Nit-ANigRec but missing in Nit-ANigWT and Asp298-Val313 peptide was shortened to Asp298-Arg310 in Nit-ANigWT. The latter enzyme was thus truncated by 46 amino acids. Enzymes Nit-ANigRec and Nit-ANigWT differed in substrate specificity, acid/amide ratio, reaction optima and stability. Refolded recombinant enzyme stored for one month at 4°C was fractionated by gel filtration, and fractions were examined by electron microscopy. The late fractions were further analyzed by analytical centrifugation and dynamic light scattering, and shown to consist of a rather homogeneous protein species composed of 12-16 subunits. This hypothesis was consistent with electron microscopy and our modelling of the multimeric nitrilase, which supports an arrangement of dimers into helical segments as a plausible structural solution.
The nitrilase from Aspergillus niger K10 is highly homologous (≥86%) with proteins deduced from gene sequencing in Aspergillus and Penicillium genera. As the first of these proteins, it was shown to exhibit nitrilase activity towards organic nitriles. The comparison of the Nit-ANigRec and Nit-ANigWT suggested that the catalytic properties of nitrilases may be changed due to missing posttranslational cleavage of the former enzyme. Nit-ANigRec exhibits a lower tendency to form filaments and, moreover, the sample homogeneity can be further improved by in vitro protein refolding. The homogeneous protein species consisting of short spirals is expected to be more suitable for structural studies.
Nitrilase is an important member of the nitrilase superfamiliy. It has attracted substantial interest from academia and industry for its function of converting nitriles directly into the corresponding carboxylic acids in recent years. Thus nitrilase has played a crucial role in production of commercial carboxylic acids in chemical industry and detoxification of nitrile-contaminated wastes. However, conventional studies mainly focused on the bacterial nitrilase and the potential of fungal nitrilase has been far from being fully explored. Research on fungal nitrilase gene expression will advance our understanding for its biological function of fungal nitrilase in nitrile hydrolysis.
A fungal nitrilase gene from Gibberella intermedia was cloned through reverse transcription-PCR. The open reading frame consisted of 963 bp and potentially encoded a protein of 320 amino acid residues with a theoretical molecular mass of 35.94 kDa. Furthermore, the catalytic triad (Glu-45, Lys-127, and Cys-162) was proposed and confirmed by site-directed mutagenesis. The encoding gene was expressed in Escherichia coli Rosetta-gami (DE3) and the recombinant protein with His6-tag was purified to electrophoretic homogeneity. The purified enzyme exhibited optimal activity at 45°C and pH 7.8. This nitrilase was specific towards aliphatic and aromatic nitriles. The kinetic parameters Vmax and Km for 3-cyanopyridine were determined to be 0.81 µmol/min·mg and 12.11 mM through Hanes-Woolf plot, respectively. 3-Cyanopyridine (100 mM) could be thoroughly hydrolyzed into nicotinic acid within 10 min using the recombinant strain with the release of about 3% nicotinamide and no substrate was detected.
In the present study, a fungal nitrilase was cloned from the cDNA sequence of G. intermedia and successfully expressed in E. coli Rosetta-gami (DE3). The recombinant strain displayed good 3-cyanopyridine degradation efficiency and wide substrate spectrum. This fungal nitrilase might be a potential candidate for industrial applications in carboxylic acids production.
Nitrilases are a large and diverse family of non-peptidic C-N hydrolases. The mammalian genome encodes eight nitrilase enzymes, several of which remain poorly characterized. Prominent among these are nitrilase-1 (Nit1) and nitrilase-2 (Nit2), which, despite having been shown to exert effects on cell growth and possibly serving as tumor suppressor genes, are without known substrates or selective inhibitors. In previous studies, we identified several nitrilases, including Nit1 and Nit2, as targets for dipeptide-chloroacetamide activity-based proteomics probes. Here, we have used these probes, in combination with high-resolution crystallography and molecular modeling, to systematically map the active site of Nit2 and identify residues involved in molecular recognition. We report the 1.4 Å crystal structure of mouse Nit2, and use this structure to identify residues that discriminate probe-labeling between the Nit1 and Nit2 enzymes. Interestingly, some of these residues are conserved across all vertebrate Nit2 enzymes and, conversely, not found in any vertebrate Nit1 enzymes, suggesting that they are key discriminators of molecular recognition between these otherwise highly homologous enzymes. Our findings thus point to a limited set of active site residues that establish distinct patterns of molecular recognition among nitrilases and provide chemical probes to selectively perturb the function of these enzymes in biological systems.
The nitrilase from Pseudomonas fluorescens EBC191 converted (R,S)-mandelonitrile with a low enantioselectivity to (R)-mandelic acid and (S)-mandeloamide in a ratio of about 4:1. In contrast, the same substrate was hydrolyzed by the homologous nitrilase from Alcaligenes faecalis ATCC 8750 almost exclusively to (R)-mandelic acid. A chimeric enzyme between both nitrilases was constructed, which represented in total 16 amino acid exchanges in the central part of the nitrilase from P. fluorescens EBC191. The chimeric enzyme clearly resembled the nitrilase from A. faecalis ATCC 8750 in its turnover characteristics for (R,S)-mandelonitrile and (R,S)-2-phenylpropionitrile (2-PPN) and demonstrated an even higher enantioselectivity for the formation of (R)-mandelic acid than the nitrilase from A. faecalis. An alanine residue (Ala165) in direct proximity to the catalytically active cysteine residue was replaced in the nitrilase from P. fluorescens by a tryptophan residue (as found in the nitrilase from A. faecalis ATCC 8750 and most other bacterial nitrilases) and several other amino acid residues. Those enzyme variants that possessed a larger substituent in position 165 (tryptophan, phenylalanine, tyrosine, or histidine) converted racemic mandelonitrile and 2-PPN to increased amounts of the R enantiomers of the corresponding acids. The enzyme variant Ala165His showed a significantly increased relative activity for mandelonitrile (compared to 2-PPN), and the opposite was found for the enzyme variants carrying aromatic residues in the relevant position. The mutant forms carrying an aromatic substituent in position 165 generally formed significantly reduced amounts of mandeloamide from mandelonitrile. The important effect of the corresponding amino acid residue on the reaction specificity and enantiospecificity of arylacetonitrilases was confirmed by the construction of a Trp164Ala variant of the nitrilase from A. faecalis ATCC 8750. This point mutation converted the highly R-specific nitrilase into an enzyme that converted (R,S)-mandelonitrile preferentially to (S)-mandeloamide.
The present report identifies the enzymatic substrates of a member of the mammalian nitrilase-like (Nit) family. Nit2, which is widely distributed in nature, has been suggested to be a tumor suppressor protein. The protein was assumed to be an amidase based on sequence homology to other amidases and on the presence of a putative amidase-like active site. This assumption was recently confirmed by the publication of the crystal structure of mouse Nit2. However, the in vivo substrates were not previously identified. Here we report that rat liver Nit2 is ω-amidodicarboxylate amidohydrolase (E.C. 126.96.36.199; abbreviated ω-amidase), a ubiquitously expressed enzyme that catalyzes a variety of amidase, transamidase, esterase and transesterification reactions. The in vivo amidase substrates are α-ketoglutaramate and α-ketosuccinamate, generated by transamination of glutamine and asparagine, respectively. Glutamine transaminases serve to salvage a number of α-keto acids generated through non-specific transamination reactions (particularly those of the essential amino acids). Asparagine transamination appears to be useful in mitochondrial metabolism and in photorespiration. Glutamine transaminases play a particularly important role in transaminating α-keto-γ-methiolbutyrate, a key component of the methionine salvage pathway. Some evidence suggests that excess α-ketoglutaramate may be neurotoxic. Moreover, α-ketosuccinamate is unstable and is readily converted to a number of hetero aromatic compounds that may be toxic. Thus, an important role of ω-amidase is to remove potentially toxic intermediates by converting α-ketoglutaramate and α-ketosuccinamate to biologically useful α-ketoglutarate and oxaloacetate, respectively. Despite its importance in nitrogen and sulfur metabolism, the biochemical significance of ω-amidase has been largely overlooked. Our report may provide clues regarding the nature of the biological amidase substrate(s) of Nit1 (another member of the Nit family), which is a well-established tumor suppressor protein), and emphasizes a) the crucial role of Nit2 in nitrogen and sulfur metabolism, and b) the possible link of Nit2 to cancer biology.
ω-Amidase; Nit2; α-ketoglutaramate; glutamine transaminases; methionine salvage pathway
Nitrilases are important in the biosphere as participants in synthesis and degradation pathways for naturally occurring, as well as xenobiotically derived, nitriles. Because of their inherent enantioselectivity, nitrilases are also attractive as mild, selective catalysts for setting chiral centers in fine chemical synthesis. Unfortunately, <20 nitrilases have been reported in the scientific and patent literature, and because of stability or specificity shortcomings, their utility has been largely unrealized. In this study, 137 unique nitrilases, discovered from screening of >600 biotope-specific environmental DNA (eDNA) libraries, were characterized. Using culture-independent means, phylogenetically diverse genomes were captured from entire biotopes, and their genes were expressed heterologously in a common cloning host. Nitrilase genes were targeted in a selection-based expression assay of clonal populations numbering 106 to 1010 members per eDNA library. A phylogenetic analysis of the novel sequences discovered revealed the presence of at least five major sequence clades within the nitrilase subfamily. Using three nitrile substrates targeted for their potential in chiral pharmaceutical synthesis, the enzymes were characterized for substrate specificity and stereospecificity. A number of important correlations were found between sequence clades and the selective properties of these nitrilases. These enzymes, discovered using a high-throughput, culture-independent method, provide a catalytic toolbox for enantiospecific synthesis of a variety of carboxylic acid derivatives, as well as an intriguing library for evolutionary and structural analyses.
Nitrilase from Rhodococcus rhodochrous ATCC 33278 hydrolyses both aliphatic and aromatic nitriles. Replacing Tyr-142 in the wild-type enzyme with the aromatic amino acid phenylalanine did not alter specificity for either substrate. However, the mutants containing non-polar aliphatic amino acids (alanine, valine and leucine) at position 142 were specific only for aromatic substrates such as benzonitrile, m-tolunitrile and 2-cyanopyridine, and not for aliphatic substrates. These results suggest that the hydrolysis of substrates probably involves the conjugated π-electron system of the aromatic ring of substrate or Tyr-142 as an electron acceptor. Moreover, the mutants containing charged amino acids such as aspartate, glutamate, arginine and asparagine at position 142 displayed no activity towards any nitrile, possibly owing to the disruption of hydrophobic interactions with substrates. Thus aromaticity of substrate or amino acid at position 142 in R. rhodochrous nitrilase is required for enzyme activity.
aliphatic nitrile; aromatic nitrile; nitrilase; Rhodococcus rhodochrous; substrate specificity; LB, Luria–Bertani
The `black yeast' Exophiala oligosperma R1 can utilise various
organic nitriles under acidic conditions as nitrogen sources. The induction of
a phenylacetonitrile converting activity was optimised by growing the strain
in the presence of different nitriles and /or complex or inorganic nitrogen
sources. The highest nitrile hydrolysing activity was observed with cells
grown with 2-cyanopyridine and NaNO3. The cells metabolised the
inducer and grew with 2-cyanopyridine as sole source of nitrogen. Cell
extracts converted various (substituted) benzonitriles and
phenylacetonitriles. They usually converted the isomers carrying a substituent
in the meta-position with higher relative activities than the
corresponding para- or ortho-substituted isomers. Aliphatic
substrates such as acrylonitrile and 2-hydroxy-3-butenenitrile were also
hydrolysed. The highest specific activity was detected with 4-cyanopyridine.
Most nitriles were almost exclusively converted to the corresponding acids and
no or only low amounts of the corresponding amides were formed. The cells
hydrolysed amides only with extremely low activities. It was therefore
concluded that the cells harboured a nitrilase activity. The specific
activities of whole cells and cell extracts were compared for different
nitriles and evidence obtained for limitation in the substrate-uptake by whole
cells. The conversion of 2-hydroxy-3-butenenitrile to 2-hydroxy-3-butenoic
acid at pH 4 demonstrated the unique ability of cells of E.
oligosperma R1 to hydrolyse aliphatic α-hydroxynitriles under
acidic conditions. The organism could grow with phenylacetonitrile as sole
source of carbon, energy and nitrogen. The degradation of phenylacetonitrile
presumably proceeds via phenylacetic acid, 2-hydroxyphenylacetic acid,
2,5-dihydroxyphenylacetic acid (homogentisate), maleylacetoacetate and
Acidotolerance; biotransformation; black yeasts; Exophiala; homogentisate pathway; induction; nitrilase
The weak-acid-inducible locus inaA in Escherichia coli was mapped to 48.6 min by P1 cotransduction of inaA Mud lac fusions and linked Tn10 insertions. The inaA1::lac fusion tested negative for phenotypes characteristic of mutations in the nearby locus ubiG. Sequence analysis of a fragment amplified by polymerase chain reaction located the inaA1::lac fusion joint within an open reading frame 311 nucleotides downstream of nrdB, transcribed in the opposite direction, encoding a 168-amino-acid polypeptide. Constitutive mutant strains identified on lactose MacConkey revealed a novel regulatory locus unlinked to inaA, which mapped at 34 min (designated inaR). Expression of inaA1::lac increased slightly with external acidification; the presence of benzoate, a membrane-permeant weak acid, greatly increased the acid effect. The expression at various combinations of benzoate and external pH correlated with the decrease in intracellular pH. The uncouplers salicylate and dinitrophenol also caused acid-dependent induction of inaA, but substantial induction was seen at external pH values higher than the internal pH; this effect cannot be caused by internal acidification. Nondissociating analogs of benzoate and salicylate, benzyl alcohol and salicyl alcohol, did not induce inaA. Expression of inaA was inversely related to growth temperature over the range of 30 to 45 degrees C. The inaA1::lac fusion was transferred to a strain defective for K+ uptake (kdpABC trkA trkD) in which pH homeostasis was shown to depend on the external K+ concentration. In this construct, inaA1::lac retained pH-dependent induction by benzoate but was not induced at low K+ concentrations. Induction of inaA appears to involve several factors in addition to internal pH. inaR may be related to the nearby locus marA/soxQ, which is inducible by acidic benzyl derivatives.
Photo-activation of the hydrophobic membrane probe 1, 5 iodonaphthylazide (INA) by irradiation with UV light (310–380 nm) results in the covalent modification of transmembrane anchors of membrane proteins. This unique selectivity of INA towards the transmembrane anchor has been exploited to specifically label proteins inserted in membranes. Previously, we have demonstrated that photo-activation of INA in enveloped viruses resulted in the inhibition of viral membrane protein-induced membrane fusion and viral entry into cells. In this study we show that photo-activation of INA in various cell lines, including those over-expressing the multi-drug resistance transporters MRP1 or Pgp, leads to cell death. We analyzed mechanisms of cell killing by INA-UV treatment. The effects of INA-UV treatment on signaling via various cell surface receptors, on the activity of the multi-drug resistance transporter MRP1 and on membrane protein lateral mobility were also investigated.
INA treatment of various cell lines followed by irradiation with UV light (310–380 nm) resulted in loss of cell viability in a dose dependent manner. The mechanism of cell death appeared to be apoptosis as indicated by phosphatidylserine exposure, mitochondrial depolarization and DNA fragmentation. Inhibition by pan-caspase inhibitors and cleavage of caspase specific substrates indicated that at low concentrations of INA apoptosis was caspase dependent. The INA-UV treatment showed similar cell killing efficacy in cells over-expressing MRP1 function as control cells. Efflux of an MRP1 substrate was blocked by INA-UV treatment of the MRP1-overexpressing cells. Although INA-UV treatment resulted in inhibition of calcium mobilization triggered by chemokine receptor signaling, Akt phosphorylation triggered by IGF1 receptor signaling was enhanced. Furthermore, fluorescence recovery after photobleaching experiments indicated that INA-UV treatment resulted in reduced lateral mobility of a seven transmembrane G protein-coupled receptor.
INA is a photo-activable agent that induces apoptosis in various cancer cell lines. It reacts with membrane proteins to alter the normal physiological function resulting in apoptosis. This activity of INA maybe exploited for use as an anti-cancer agent.
A nitrilase-mediated pathway has significant advantages in the production of optically pure (R)-(−)-mandelic acid. However, unwanted byproduct, low enantioselectivity, and specific activity reduce its value in practical applications. An ideal nitrilase that can efficiently hydrolyze mandelonitrile to optically pure (R)-(−)-mandelic acid without the unwanted byproduct is needed.
A novel nitrilase (BCJ2315) was discovered from Burkholderia cenocepacia J2315 through phylogeny-based enzymatic substrate specificity prediction (PESSP). This nitrilase is a mandelonitrile hydrolase that could efficiently hydrolyze mandelonitrile to (R)-(−)-mandelic acid, with a high enantiomeric excess of 98.4%. No byproduct was observed in this hydrolysis process. BCJ2315 showed the highest identity of 71% compared with other nitrilases in the amino acid sequence. BCJ2315 possessed the highest activity toward mandelonitrile and took mandelonitrile as the optimal substrate based on the analysis of substrate specificity. The kinetic parameters Vmax, Km, Kcat, and Kcat/Km toward mandelonitrile were 45.4 μmol/min/mg, 0.14 mM, 15.4 s-1, and 1.1×105 M-1s-1, respectively. The recombinant Escherichia coli M15/BCJ2315 had a strong substrate tolerance and could completely hydrolyze mandelonitrile (100 mM) with fewer amounts of wet cells (10 mg/ml) within 1 h.
PESSP is an efficient method for discovering an ideal mandelonitrile hydrolase. BCJ2315 has high affinity and catalytic efficiency toward mandelonitrile. This nitrilase has great advantages in the production of optically pure (R)-(−)-mandelic acid because of its high activity and enantioselectivity, strong substrate tolerance, and having no unwanted byproduct. Thus, BCJ2315 has great potential in the practical production of optically pure (R)-(−)-mandelic acid in the industry.
(R)-(−)-mandelic acid; Nitrilase; Burkholderia cenocepacia J2315; Substrate specificity prediction; Enantioselective hydrolysis
cAMP-activated Na+ current (INa,cAMP) was studied in voltage-clamped neurons of the seaslug Pleurobranchaea californica. The current response to injected cAMP varied in both time course and amplitude as the tip of an intracellular injection electrode was moved from the periphery to the center of the neuron soma. The latency from injection to peak response was dependent on the amount of cAMP injected unless the electrode was centered within the cell. Decay of the INa,cAMP response was slowed by phosphodiesterase inhibition. These observations suggest that the kinetics of the INa,cAMP response are governed by cAMP diffusion and degradation. Phosphodiesterase inhibition induced a persistent inward current. At lower concentrations of inhibitor, INa,cAMP response amplitude increased as expected for decreased hydrolysis rate of injected cAMP. Higher inhibitor concentrations decreased INa,cAMP response amplitude, suggesting that inhibitor- induced increase in native cAMP increased basal INa,cAMP and thus caused partial saturation of the current. The Hill coefficient estimated from the plot of injected cAMP to INa,cAMP response amplitude was close to 1.0. An equation modeling INa,cAMP incorporated terms for diffusion and degradation. In it, the first-order rate constant of phosphodiesterase activity was taken as the rate constant of the exponential decay of the INa,cAMP response. The stoichiometry of INa,cAMP activation was inferred from the Hill coefficient as 1 cAMP/channel. The equation closely fitted the INa,cAMP response and simulated changes in the waveform of the response induced by phosphodiesterase inhibition. With modifications to accommodate asymmetric INa,cAMP activation, the equation also simulated effects of eccentric electrode position. The simple reaction-diffusion model of the kinetics of INa,cAMP may provide a useful conceptual framework within which to investigate the modulation of INa,cAMP by neuromodulators, intracellular regulatory factors, and pharmacological agents.
Chemical and biological properties of the ice nucleating sites of Pseudomonas syringae, strain C-9, and Erwinia herbicola have been characterized. The ice nucleating activity (INA) for both bacteria was unchanged in buffers ranging from pH 5.0 to 9.2, suggesting that there were no essential groups for which a change in charge in this range was critical. The INA of both bacteria was also unaffected by the addition of metal chelating compounds. Borate compounds and certain lectins markedly inhibited the INA of both types of bacterial cells. Butyl borate was not an inhibitor, but borate, phenyl borate, and m-nitrophenyl borate were, in order, increasingly potent inhibitors. These compounds have a similar order of affinity for cis hydroxyls, particularly for those found on sugars. Lentil lectin and fava bean lectin, which have binding sites for mannose or glucose, inhibited the INA of both bacteria. All other lectins examined had no effect. The inhibition of INA by these two types of reagents indicate that sugar-like groups are at or near the ice nucleating site. Sulfhydryl reagents were potent inhibitors of the INA of both bacteria. When treated with N-ethylmaleimide, p-hydroxymercuribenzoate, or iodoacetamide, the INA was irreversibly inhibited by 99%. The kinetics of inactivation with N-ethylmaleimide suggested that E. herbicola cells have at least two separate ice nucleating sites, whereas P. syringae cells have possibly four or more separate sites. The effect of infection with a virulent phage (Erh 1) on the INA of E. herbicola was examined. After multiple infection of a bacterial culture the INA was unchanged until 40 to 45 min, which was midway through the 95-min latent period. At that time, the INA activity began falling and 99% of the INA was lost by 55 min after infection, well before any cells had lysed. This decrease in INA before lysis is attributed to phage-induced changes in the cell wall.
The nitrile hydratase (NHase, EC 188.8.131.52) activity of Rhodococcus rhodochrous PA-34 was explored for the conversion of 3-cyanopyridine to nicotinamide. The NHase activity (∼18 U/mg dry cell weight, dcw) was observed in 0.1 M phosphate buffer, pH 8.0 containing 1M 3-cyanopyridine as substrate, and 0.75 mg of resting cells (dry cell weight) per ml reaction mixture at 40°C. However, 25°C was more suitable for prolonged batch reaction at high substrate (3-cyanopyridine) concentration. In a batch reaction (1 liter), 7M 3-cyanopyridine (729 g) was completely converted to nicotinamide (855 g) in 12h at 25°C using 9.0 g resting cells (dry cell weight) of R. rhodochrous PA-34.
Rhodococcus rhodochrous PA-34; bioconversion; nitrile hydratase (NHase); 3-cyanopyridine; nicotinamide
Indole-3-acetic acid (IAA) is a fundamental phytohormone with the ability to control many aspects of plant growth and development. Pseudomonas sp. strain UW4 is a rhizospheric plant growth-promoting bacterium that produces and secretes IAA. While several putative IAA biosynthetic genes have been reported in this bacterium, the pathways leading to the production of IAA in strain UW4 are unclear. Here, the presence of the indole-3-acetamide (IAM) and indole-3-acetaldoxime/indole-3-acetonitrile (IAOx/IAN) pathways of IAA biosynthesis is described, and the specific role of two of the enzymes (nitrilase and nitrile hydratase) that mediate these pathways is assessed. The genes encoding these two enzymes were expressed in Escherichia coli, and the enzymes were isolated and characterized. Substrate-feeding assays indicate that the nitrilase produces both IAM and IAA from the IAN substrate, while the nitrile hydratase only produces IAM. The two nitrile-hydrolyzing enzymes have very different temperature and pH optimums. Nitrilase prefers a temperature of 50°C and a pH of 6, while nitrile hydratase prefers 4°C and a pH of 7.5. Based on multiple sequence alignments and motif analyses, physicochemical properties and enzyme assays, it is concluded that the UW4 nitrilase has an aromatic substrate specificity. The nitrile hydratase is identified as an iron-type metalloenzyme that does not require the help of a P47K activator protein to be active. These data are interpreted in terms of a preliminary model for the biosynthesis of IAA in this bacterium.
Immobilized cells of a recombinant Escherichia coli expressing nitrilase from Pseudomonas putida were used to catalyze the hydrolysis of mandelonitrile (2-hydroxy-2-phenylacetonitrile) to (R)-(−)-mandelic acid. The cells had been immobilized by entrapment in an alginate matrix. Conditions for the hydrolysis reaction were optimized in shake flasks and in a packed bed reactor. In shake flasks the best conditions for the reaction were a temperature of 40 °C, pH 8, biocatalyst bead diameter of 4.3 mm, sodium alginate concentration in the gel matrix of 2 % (w/v, g/100 mL), a cell dry mass concentration in the bead matrix of 20 mg/mL, an initial substrate concentration of 50 mM and a reaction time of 60 min. Under these conditions, the conversion of mandelonitrile was nearly 95 %. In the packed bed reactor, a feed flow rate of 20 mL/h at a substrate concentration of 200 mM proved to be the best at 40 °C, pH 8, using 4.3 mm beads (2 % w/v sodium alginate in the gel matrix, 20 mg dry cell concentration per mL of gel matrix). This feed flow rate corresponded to a residence time of 0.975 h in the packed bed.
Nitrilase; Mandelic acid; Mandelonitrile; Packed bed reactor; Immobilized cells
The arylacetonitrilase from Pseudomonas fluorescens EBC191 differs from previously studied arylacetonitrilases by its low enantiospecificity during the turnover of mandelonitrile and by the large amounts of amides that are formed in the course of this reaction. In the sequence of the nitrilase from P. fluorescens, a cysteine residue (Cys163) is present in direct neighborhood (toward the amino terminus) to the catalytic active cysteine residue, which is rather unique among bacterial nitrilases. Therefore, this cysteine residue was exchanged in the nitrilase from P. fluorescens EBC191 for various amino acid residues which are present in other nitrilases at the homologous position. The influence of these mutations on the reaction specificity and enantiospecificity was analyzed with (R,S)-mandelonitrile and (R,S)-2-phenylpropionitrile as substrates. The mutants obtained demonstrated significant differences in their amide-forming capacities. The exchange of Cys163 for asparagine or glutamine residues resulted in significantly increased amounts of amides formed. In contrast, a substitution for alanine or serine residues decreased the amounts of amides formed. The newly discovered mutation was combined with previously identified mutations which also resulted in increased amide formation. Thus, variants which possessed in addition to the mutation Cys163Asn also a deletion at the C terminus of the enzyme and/or the modification Ala165Arg were constructed. These constructs demonstrated increased amide formation capacity in comparison to the mutants carrying only single mutations. The recombinant plasmids that encoded enzyme variants which formed large amounts of mandeloamide or that formed almost stoichiometric amounts of mandelic acid from mandelonitrile were used to transform Escherichia coli strains that expressed a plant-derived (S)-hydroxynitrile lyase. The whole-cell biocatalysts obtained in this way converted benzaldehyde plus cyanide either to (S)-mandeloamide or (S)-mandelic acid with high yields and enantiopurities.
Nesterenkonia strain AN1 was isolated from a screening program for nitrile- and amide-hydrolyzing microorganisms in Antarctic desert soil samples. Strain AN1 showed significant 16S rRNA sequence identity to known members of the genus. Like known Nesterenkonia species, strain AN1 was obligately alkaliphilic (optimum environmental pH, 9 to 10) and halotolerant (optimum environmental Na+ content, 0 to 15% [wt/vol]) but was also shown to be an obligate psychrophile with optimum growth at approximately 21°C. The partially sequenced genome of AN1 revealed an open reading frame (ORF) encoding a putative protein member of the nitrilase superfamily, referred to as NitN (264 amino acids). The protein crystallized readily as a dimer and the atomic structure of all but 10 amino acids of the protein was determined, confirming that the enzyme had an active site and a fold characteristic of the nitrilase superfamily. The protein was screened for activity against a variety of nitrile, carbamoyl, and amide substrates and was found to have only amidase activity. It had highest affinity for propionamide but demonstrated a low catalytic rate. NitN had maximal activity at 30°C and between pH 6.5 and 7.5, conditions which are outside the optimum growth range for the organism.
The nitrilase from Pseudomonas fluorescens EBC191 converted 2-methyl-2-phenylpropionitrile, which contains a quaternary carbon atom in the α-position toward the nitrile group, and also similar sterically demanding substrates, such as 2-hydroxy-2-phenylpropionitrile (acetophenone cyanohydrin) or 2-acetyloxy-2-methylphenylacetonitrile. 2-Methyl-2-phenylpropionitrile was hydrolyzed to almost stoichiometric amounts of the corresponding acid. Acetophenone cyanohydrin was transformed to the corresponding acid (atrolactate) and amide (atrolactamide) at a ratio of about 3.4:1. The (R)-acid and the (S)-amide were formed preferentially from acetophenone cyanohydrin. A homology model of the nitrilase suggested that steric hindrance with amino acid residue Tyr54 could impair the binding or conversion of sterically demanding substrates. Therefore, several enzyme variants that carried mutations in the respective residues were generated and subsequently analyzed for the substrate specificity and enantioselectivity of the reactions. Enzyme variants that demonstrated increased relative activities for the conversion of acetophenone cyanohydrin were identified. The chiral analysis of these reactions demonstrated peculiar reaction kinetics, which suggested that the enzyme variants converted the nonpreferred (S)-enantiomer of acetophenone cyanohydrin with a higher reaction rate than that of the (preferred) (R)-enantiomer. Recombinant whole-cell catalysts that simultaneously produced the nitrilase from P. fluorescens EBC191 and a plant-derived (S)-oxynitrilase from cassava (Manihot esculenta) converted acetophenone plus cyanide at pH 4.5 to (S)-atrolactate and (S)-atrolactamide. These recombinant cells are promising catalysts for the synthesis of stable chiral quaternary carbon centers from ketones.
Endogenous serine proteases have been reported to control the reabsorption of Na+ by kidney- and lung-derived epithelial cells via stimulation of electrogenic Na+ transport mediated by the epithelial Na+ channel (ENaC). In this study we investigated the effects of aprotinin on ENaC single channel properties using transepithelial fluctuation analysis in the amphibian kidney epithelium, A6. Aprotinin caused a time- and concentration-dependent inhibition (84 ± 10.5%) in the amiloride-sensitive sodium transport (INa) with a time constant of 18 min and half maximal inhibition constant of 1 μM. Analysis of amiloride analogue blocker–induced fluctuations in INa showed linear rate–concentration plots with identical blocker on and off rates in control and aprotinin-inhibited conditions. Verification of open-block kinetics allowed for the use of a pulse protocol method (Helman, S.I., X. Liu, K. Baldwin, B.L. Blazer-Yost, and W.J. Els. 1998. Am. J. Physiol. 274:C947–C957) to study the same cells under different conditions as well as the reversibility of the aprotinin effect on single channel properties. Aprotinin caused reversible changes in all three single channel properties but only the change in the number of open channels was consistent with the inhibition of INa. A 50% decrease in INa was accompanied by 50% increases in the single channel current and open probability but an 80% decrease in the number of open channels. Washout of aprotinin led to a time-dependent restoration of INa as well as the single channel properties to the control, pre-aprotinin, values. We conclude that protease regulation of INa is mediated by changes in the number of open channels in the apical membrane. The increase in the single channel current caused by protease inhibition can be explained by a hyperpolarization of the apical membrane potential as active Na+ channels are retrieved. The paradoxical increase in channel open probability caused by protease inhibition will require further investigation but does suggest a potential compensatory regulatory mechanism to maintain INa at some minimal threshold value.
Nitrilase enzymes (nitrilases) catalyse the hydrolysis of nitrile compounds to the corresponding carboxylic acid and ammonia, and have a wide range of industrial and biotechnological applications, including the synthesis of industrially important carboxylic acids and bioremediation of cyanide and toxic nitriles. Nitrilases are produced by a wide range of organisms, including plants, bacteria and fungi, but despite their biotechnological importance, the role of these enzymes in living organisms is relatively underexplored. Current research suggests that nitrilases play important roles in a range of biological processes. In the context of plant–microbe interactions they may have roles in hormone synthesis, nutrient assimilation and detoxification of exogenous and endogenous nitriles. Nitrilases are produced by both plant pathogenic and plant growth‐promoting microorganisms, and their activities may have a significant impact on the outcome of plant–microbe interactions. In this paper we review current knowledge of the role of nitriles and nitrilases in plants and plant‐associated microorganisms, and discuss how greater understanding of the natural functions of nitrilases could be applied to benefit both industry and agriculture.
The persistent Na+ current (INaP) is important for neuronal functions and can play a role in several pathologies, although it is small compared to the transient Na+ current (INaT). Notably, INaP is not a real persistent current because it undergoes inactivation with kinetics in the order of tens of seconds, but this property has often been overlooked. Na+ channel blockers, drugs used for treating epilepsy and other diseases, can inhibit INaP, but the mechanism of this action and the conditions in which INaP can be actually inhibited have not been completely clarified yet. We evaluated the action of phenytoin (PHT), a prototype anti-epileptic Na+ channel blocker, on INaP inactivation in pyramidal neurons of rat sensorimotor cortical slices at different concentrations, from 5 to 100 µM. PHT did not modify INaP evoked with depolarizing voltage ramps of 50 or 100 mVs−1, but decreased INaP evoked by slower voltage ramps (10 mVs−1). However, at all of the tested concentrations, PHT decreased INaP evoked by faster ramps when they were preceded by inactivating pre-pulses. Moreover, PHT shifted towards negative potentials the voltage-dependence of INaP inactivation and accelerated its kinetics of development also at depolarized potentials (+40 mV), not consistently with a simple inactivated state stabilizer. Therefore, our study shows a prominent PHT effect on INaP inactivation rather than an open channel block, which is instead often implied. INaP is inhibited by PHT only in conditions that induce major INaP inactivation. These results highlight the importance of INaP inactivation not only for physiological functions but also as drug target, which could be shared by other therapeutic drugs. Through this action PHT can reduce INaP-induced long-lasting pathological depolarisations and intracellular sodium overload, whereas shorter INaP actions should not be modified. These properties set the conditions of efficacy and the limits of PHT as INaP inhibitor.
Nitrilases represent a very important class of enzymes having an array of applications. In the present scenario, where the indepth information about nitrilases is limited, the present work is an attempt to shed light on the residues crucial for the nitrilase activity. The nitrilase sequences demonstrating varying degree of identity with P. putida nitrilase were explored. A stretch of residues, fairly conserved throughout the range of higher (96%) to lower (27%) sequence identity among different nitrilases was selected and investigated for the possible functional role in nitrilase enzyme system. Subsequently, the alanine substitution mutants (T48A, W49A, L50A, P51A, G52A, Y53A and P54A) were generated. Substitution of the rationally selected conserved residues altered the substrate recognition ability, catalysis and affected the substrate specificity but had very little impact on enantioselectivity and pattern of nitrile hydrolysis.
Nitrilase; catalysis; sequence identity; site directed mutagenesis; mutants; conserved residues
Nitrilases have found wide use in the pharmaceutical industry for the production of fine chemicals, and it is important to have a method by which to screen libraries of isolated or engineered nitrilase variants (including bacteria and fungi). The conventional methods, such as high-performance liquid chromatography, liquid chromatography-mass spectrometry, capillary electrophoresis, or gas chromatography, are tedious and time-consuming. Therefore, a direct and sensitive readout of the nitrilase's activity has to be considered. In this paper, we report a novel time-resolved luminescent probe: o-hydroxybenzonitrile derivatives could be applied to detect the activity of the nitrilases. By the action of nitrilases, o-hydroxybenzonitrile derivatives can be transformed to the corresponding salicylic acid derivatives, which, upon binding Tb3+, serve as a photon antenna and sensitize Tb3+ luminescence. Because of the time-resolved property of the luminescence, the background from the other proteins (especially in the fermentation system) in the assay could be reduced and, therefore, the sensitivity was increased. Moreover, because the detection was performed on a 96- or 384-well plate, the activity of the nitrilases from microorganisms could be determined quickly. Based on this strategy, the best fermentation conditions for nitrilase-producing strains were obtained.
Ranolazine is an antianginal drug that inhibits the cardiac late Na+ current (INa). The selectivity of ranolazine to block late INa relative to peak INa at rapid heart rates has not been determined, but is potentially important to drug efficacy and safety.
To quantify use-dependent block (UDB) of cardiac peak and late INa by ranolazine.
Wild-type (WT) and LQT3 mutation R1623Q channels were expressed in HEK293 cells and studied using whole-cell patch-clamp technique. Ranolazine (1–300 μM) caused tonic (0.1 Hz) and UDB (1, 2 and 5 Hz) of WT and R1623Q peak INa. The IC50 values for block WT and R1623Q peak INa at 0.1, 1, 2 and 5 Hz were 430, 260, 160 and 150μM, and 95, 78, 37 and 25μM, respectively. The IC50 values for block of R1623Q late INa at 0.1, 1, 2 and 5 Hz were 7.5, 7.3, 2.2 and 1.9 μM, respectively. Ranolazine (10 μM) caused a hyperpolarizing shift of WT and R1623Q peak INa steady-state inactivation without affecting steady-state activation, suggesting that ranolazine interacts with inactivated states of the channels. Ranolazine (30 μM) significantly slowed the recovery from inactivation of peak INa of both WT and R1623Q and late INa of R1623Q.
Ranolazine slowed recovery of late INa from inactivation and thus caused UDB of late INa. These data suggest that the effect of ranolazine to block late INa may be increased, and the selectivity to block late INa relative to peak INa may be retained, during tachycardia.
Angina; sodium channel; late sodium; ranolazine; tachycardia