Functional proteomic and structural insights into molecular recognition in the nitrilase family enzymes

A Proterozoic microbial origin of extant cyanide-hydrolyzing enzyme diversity

In addition to its role as a toxic environmental contaminant, cyanide has been hypothesized to play a key role in prebiotic chemistry and early biogeochemical evolution. While cyanide-hydrolyzing enzymes have been studied and engineered for bioremediation, the extant diversity of these enzymes remains underexplored. Additionally, the age and evolution of microbial cyanide metabolisms is poorly constrained. Here we provide comprehensive phylogenetic and molecular clock analyses of the distribution and evolution of the Class I nitrilases, thiocyanate hydrolases, and nitrile hydratases. Molecular clock analyses indicate that bacterial cyanide-reducing nitrilases were present by the Paleo- to Mesoproterozoic, and were subsequently horizontally transferred into eukaryotes. These results present a broad diversity of microbial enzymes that could be optimized for cyanide bioremediation.

The Rosetta Stone Hypothesis-Based Interaction of the Tumor Suppressor Proteins Nit1 and Fhit

In previous studies, we have identified the tumor suppressor proteins Fhit (fragile histidine triad) and Nit1 (Nitrilase1) as interaction partners of β-catenin both acting as repressors of the canonical Wnt pathway. Interestingly, in D. melanogaster and C. elegans these proteins are expressed as NitFhit fusion proteins. According to the Rosetta Stone hypothesis, if proteins are expressed as fusion proteins in one organism and as single proteins in others, the latter should interact physically and show common signaling function. Here, we tested this hypothesis and provide the first biochemical evidence for a direct association between Nit1 and Fhit. In addition, size exclusion chromatography of purified recombinant human Nit1 showed a tetrameric structure as also previously observed for the NitFhit Rosetta Stone fusion protein Nft-1 in C. elegans. Finally, in line with the Rosetta Stone hypothesis we identified Hsp60 and Ubc9 as other common interaction partners of Nit1 and Fhit. The interaction of Nit1 and Fhit may affect their enzymatic activities as well as interaction with other binding partners.

A novel efficient producer of human ω-amidase (Nit2) in Escherichia coli

Nit2/ω-amidase catalyzes the hydrolysis of α-ketoglutaramate (KGM, the α-keto acid analogue of glutamine) to α-ketoglutarate and ammonia. The enzyme also catalyzes the amide hydrolysis of monoamides of 4- and 5-C-dicarboxylates, including α-ketosuccinamate (KSM, the α-keto acid analogue of asparagine) and succinamate (SM). Here we describe an inexpensive procedure for high-yield expression of human Nit2 (hNit2) in Escherichia coli and purification of the expressed protein. This work includes: 1) the design of a genetic construct (pQE-Nit22) obtained from the previously described construct (pQE-Nit2) by replacing rare codons within an 81 bp-long DNA fragment "preferred" by E. coli near the translation initiation site; 2) methods for producing and maintaining the pQE-Nit22 construct; 3) purification of recombinant hNit2; and 4) activity measurements of the purified enzyme with KGM and SM. Important features of the hNit2 gene within the pQE-Nit22 construct are: 1) optimized codon composition, 2) the presence of an N-terminus His₆ tag immediately after the initiating codon ATG (Met) that permits efficient purification of the end-product on a Ni-NTA-agarose column. We anticipate that the availability of high yield hNit2/ω-amidase will be helpful in elucidating the normal and pathological roles of this enzyme and in the design of specific inhibitors.

Selectivity aspects of activity-based (chemical) probes

Selective chemical modulators are ideal tools to study the function of a protein. Yet, the poor ligandability of many proteins has hampered the development of specific chemical probes for numerous protein classes. Tools, such as covalent inhibitors and activity-based protein profiling, have enhanced our understanding of thus-far difficult-to-target proteins and have enabled correct assessment of the selectivity of small-molecule modulators. This also requires deeper knowledge of compound and target site reactivity, evaluation of binding to noncovalent targets and protein turnover. The availability of highly selective chemical probes, the evolution of activity-based probes, and the development of profiling methods will open a new era of drugging the undruggable proteome.

An Unsual Cys-Glu-Lys Catalytic Triad is Responsible for the Catalytic Mechanism of the Nitrilase Superfamily: A QM/MM Study on Nit2

Nitrilase 2 (Nit2) is a representative member of the nitrilase superfamily that catalyzes the hydrolysis of α-ketosuccinamate into oxaloacetate. It has been associated with the metabolism of rapidly dividing cells like cancer cells. The catalytic mechanism of Nit2 employs a catalytic triad formed by Cys191, Glu81 and Lys150. The Cys191 and Glu81 play an active role during the catalytic process while the Lys150 is shown to play only a secondary role. The results demonstrate that the catalytic mechanism of Nit2 involves four steps. The nucleophilic attack of Cys191 to the α-ketosuccinamate, the formation of two tetrahedral enzyme adducts and the hydrolysis of a thioacyl-enzyme intermediate, from which results the formation of oxaloacetate and enzymatic turnover. The rate limiting step of the catalytic process is the formation of the first tetrahedral intermediate with a calculated activation free energy of 18.4 kcal/mol, which agrees very well with the experimental k_cat (17.67 kcal/mol).

High-throughput virtual screening of novel potent inhibitor(s) for Human Vanin-1 enzyme

Vanin-1 (VNN1) is a glycosylphosphatidylinositol (GPI)-anchored ectoenzyme which hydrolyzes pantetheine to pantothenic acid and cysteamine. It has emerged as a promising drug target for many human diseases associated with oxidative stress and inflammatory pathways. In the present study we used structure-based virtual screening approach for the identification of small molecule inhibitors of vanin-1. A chemical library consisting of natural compounds, synthetic compounds and RRV analogs were screened for drug-like molecules. The filtered molecules were subjected to molecular docking studies. Three potential hits-ZINC04073864 (Natural compound), CID227017 (synthetic compound) and CID129558381 (RRV analog)-were identified for the target enzyme. The molecules form good number of hydrogen bonds with the catalytic residues such as Glu79, Lys178 and Cys211. The apo-VNN1 and VNN1-ligand complexes were subjected to molecular dynamics (MD) simulation for 30 ns. The geometric properties such as root mean square deviation, radius of gyration, solvent accessible surface area, number of hydrogen bonds and the distance between the catalytic triad residues-Glu79, Lys178 and Cys211 were altered upon binding of the compounds. Essential dynamics and entropic studies further confirmed that the fluctuations in VNN1 decrease upon binding of the compounds. The lead molecules were stable throughout the simulation time period. Molecular Mechanics Poisson-Boltzmann Surface Area (MM/PBSA) studies showed that Van der Waals interaction energy contributes significantly to the total binding free energy. Thus, our study reveals three lead molecules-ZINC04073864, CID227017 and CID129558381 as potential inhibitors of Vanin-1 which can be validated through further studies. Communicated by Ramaswamy H. Sarma.

Activity-based Protein Profiling Approaches for Transplantation

Enzyme activity may be more pathophysiologically relevant than enzyme quantity and is regulated by changes in conformational status that are undetectable by traditional proteomic approaches. Further, enzyme activity may provide insights into rapid physiological responses to inflammation/injury that are not dependent on de novo protein transcription. Activity-based protein profiling (ABPP) is a chemical proteomic approach designed to characterize and identify active enzymes within complex biological samples. Activity probes have been developed to interrogate multiple enzyme families with broad applicability, including but not limited to serine hydrolases, cysteine proteases, matrix metalloproteases, nitrilases, caspases, and histone deacetylases. The goal of this overview is to describe the overall rationale, approach, methods, challenges, and potential applications of ABPP to transplantation research. To do so, we present a case example of urine serine hydrolase ABPP in kidney transplant rejection to illustrate the utility and workflow of this analytical approach. Ultimately, developing novel transplant therapeutics is critically dependent on understanding the pathophysiological processes that result in loss of transplant function. ABPP offers a new dimension for characterizing dynamic changes in clinical samples. The capacity to identify and measure relevant enzyme activities provides fresh opportunities for understanding these processes and may help identify markers of disease activity for the development of novel diagnostics and real-time monitoring of patients. Finally, these insights into enzyme activity may also help to identify new transplant therapeutics, such as enzyme-specific inhibitors.

Computational modeling for mutational analysis of nitrilase enzyme towards enhancement of binding empathy

Nitrilase enzyme (a green catalyst) is an industrially important enzyme which hydrolyses various nitrile compounds (containing -CN functional group) into amides and corresponding carboxylic acids. The current study explored the binding affinity and a method to enhance the catalysis activity of the enzyme using computational approaches. Four mutants were generated using sequential site-directed mutagenesis aiming that an increase in hydrogen bonds that will further increase binding efficiency towards the ligand. Molecular dynamics simulation was rigorously performed to check the stability of those mutants followed by docking to verify its interaction with the ligand. Various statistical dynamics analyses were performed to validate the structure. All the studies predict that built mutants are stable. Mutants 2 and 3 showed a better affinity towards acrylamide by forming the highest number of hydrogen bonds implying better catalysis. The binding affinity values of the Mutant 2 and Mutant 3 with acrylamide are -7.44 kcal/mol and -7.17 kcal/mol, respectively. This study may prove useful for the industry to develop efficient nitrilase enzymes with improved catalytic activity.Communicated by Ramaswamy H. Sarma.

Regulation of coenzyme A levels by degradation: the 'Ins and Outs'

Coenzyme A (CoA) is the predominant acyl carrier in mammalian cells and a cofactor that plays a key role in energy and lipid metabolism. CoA and its thioesters (acyl-CoAs) regulate a multitude of metabolic processes at different levels: as substrates, allosteric modulators, and via post-translational modification of histones and other non-histone proteins. Evidence is emerging that synthesis and degradation of CoA are regulated in a manner that enables metabolic flexibility in different subcellular compartments. Degradation of CoA occurs through distinct intra- and extracellular pathways that rely on the activity of specific hydrolases. The pantetheinase enzymes specifically hydrolyze pantetheine to cysteamine and pantothenate, the last step in the extracellular degradation pathway for CoA. This reaction releases pantothenate in the bloodstream, making this CoA precursor available for cellular uptake and de novo CoA synthesis. Intracellular degradation of CoA depends on specific mitochondrial and peroxisomal Nudix hydrolases. These enzymes are also active against a subset of acyl-CoAs and play a key role in the regulation of subcellular (acyl-)CoA pools and CoA-dependent metabolic reactions. The evidence currently available indicates that the extracellular and intracellular (acyl-)CoA degradation pathways are regulated in a coordinated and opposite manner by the nutritional state and maximize the changes in the total intracellular CoA levels that support the metabolic switch between fed and fasted states in organs like the liver. The objective of this review is to update the contribution of these pathways to the regulation of metabolism, physiology and pathology and to highlight the many questions that remain open.

NIT2 overexpression predicts poor prognosis in tongue squamous cell carcinoma patients

There is disputable on the role of nitrilase-like 2 (NIT2) in cancer. Its expression and its relationship with clinicopathological features in tongue squamous cell carcinoma (TSCC) are not yet clear. The purpose of this study is to investigate the expression of NIT2 in TSCC and its correlation with clinicopathological characteristics in TSCC patients. Through proteomic identification, we found that the protein NIT2 was related to the development of TSCC. q-PCR, western blot and immunohistochemistry techniques were applied to detect the expression of NIT2 in TSCC. The relationship between the expression of NIT2 and clinicopathological features was analyzed by Chi square tests. The results showed the expression of NIT2 in TSCC was significantly higher than that in normal tongue tissues (p < 0.05). Univariate and multivariate analysis showed that the positive expression of NIT2 and N classification were associated with decreased disease-free survival rate (DFS) and overall survival (OS) (p < 0.05). The results suggested that NIT2 is overexpressed in TSCC and NIT2 may be a potential therapeutic target for TSCC.

Plant nitrilase: a new job for an old enzyme

Nitrilases are versatile enzymes that hydrolyze nitriles to carboxylic acids and ammonia, but many members of this family lack defined biological functions. In plants, nitrilases have been associated with detoxification of cyanide-containing compounds and auxin biosynthesis; however, recent work suggests that the chemical versatility of these proteins contributes to metabolite repair. In this issue of the Biochemical Journal, Niehaus et al. demonstrate that the Nit1 nitrilase from Arabidopsis thaliana functions as a metabolite repair enzyme that removes deaminated glutathione from the cytoplasm and plastids.

A novel cysteine carbamoyl-switch is responsible for the inhibition of formamidase, a nitrilase superfamily member

Formamidases (EC 3.5.1.49) and amidases (EC 3.5.1.4) are paralogous cysteine-dependent enzymes which catalyze the conversion of amide substrates to ammonia and the corresponding carboxylic acid. Both enzymes have been suggested as an alternative pathway for ammonia production during urea shortage. Urea was proved key in the transcriptional regulation of formamidases/amidases, connecting urea level to amide metabolism. In addition, different amidases have also been shown to be inhibited by urea, pointing to urea-regulation at the enzymatic level. Although amidases have been widely studied due to its biotechnological application in the hydrolysis of aliphatic amides, up to date, only two formamidases have been extensively characterized, belonging to Helicobacter pylori (HpyAmiF) and Bacillus cereus (BceAmiF). In this work, we report the first structure of an acyl-intermediate of BceAmiF. We also report the inhibition of BceAmiF by urea, together with mass spectrometry studies confirming the S-carbamoylation of BceAmiF after urea treatment. X-ray studies of urea-soaked BceAmiF crystals showed short- and long-range rearrangements affecting oligomerization interfaces. Since cysteine-based switches are known to occur in the regulation of different metabolic and signaling pathways, our results suggest a novel S-carbamoylation-switch for the regulation of BceAmiF. This finding could relate to previous observations of unexplained modifications in the catalytic cysteine of different nitrilase superfamily members and therefore extending this regulation mechanism to the whole nitrilase superfamily.

Crystal structure and pH-dependent allosteric regulation of human β-ureidopropionase, an enzyme involved in anticancer drug metabolism

β-Ureidopropionase (βUP) catalyzes the third step of the reductive pyrimidine catabolic pathway responsible for breakdown of uracil-, thymine- and pyrimidine-based antimetabolites such as 5-fluorouracil. Nitrilase-like βUPs use a tetrad of conserved residues (Cys233, Lys196, Glu119 and Glu207) for catalysis and occur in a variety of oligomeric states. Positive co-operativity toward the substrate N-carbamoyl-β-alanine and an oligomerization-dependent mechanism of substrate activation and product inhibition have been reported for the enzymes from some species but not others. Here, the activity of recombinant human βUP is shown to be similarly regulated by substrate and product, but in a pH-dependent manner. Existing as a homodimer at pH 9, the enzyme increasingly associates to form octamers and larger oligomers with decreasing pH. Only at physiological pH is the enzyme responsive to effector binding, with N-carbamoyl-β-alanine causing association to more active higher molecular mass species, and β-alanine dissociation to inactive dimers. The parallel between the pH and ligand-induced effects suggests that protonation state changes play a crucial role in the allosteric regulation mechanism. Disruption of dimer-dimer interfaces by site-directed mutagenesis generated dimeric, inactive enzyme variants. The crystal structure of the T299C variant refined to 2.08 Å resolution revealed high structural conservation between human and fruit fly βUP, and supports the hypothesis that enzyme activation by oligomer assembly involves ordering of loop regions forming the entrance to the active site at the dimer-dimer interface, effectively positioning the catalytically important Glu207 in the active site.

Nit1 is a metabolite repair enzyme that hydrolyzes deaminated glutathione

The mammalian gene Nit1 (nitrilase-like protein 1) encodes a protein that is highly conserved in eukaryotes and is thought to act as a tumor suppressor. Despite being ∼35% sequence identical to ω-amidase (Nit2), the Nit1 protein does not hydrolyze efficiently α-ketoglutaramate (a known physiological substrate of Nit2), and its actual enzymatic function has so far remained a puzzle. In the present study, we demonstrate that both the mammalian Nit1 and its yeast ortholog are amidases highly active toward deaminated glutathione (dGSH; i.e., a form of glutathione in which the free amino group has been replaced by a carbonyl group). We further show that Nit1-KO mutants of both human and yeast cells accumulate dGSH and the same compound is excreted in large amounts in the urine of Nit1-KO mice. Finally, we show that several mammalian aminotransferases (transaminases), both cytosolic and mitochondrial, can form dGSH via a common (if slow) side-reaction and provide indirect evidence that transaminases are mainly responsible for dGSH formation in cultured mammalian cells. Altogether, these findings delineate a typical instance of metabolite repair, whereby the promiscuous activity of some abundant enzymes of primary metabolism leads to the formation of a useless and potentially harmful compound, which needs a suitable "repair enzyme" to be destroyed or reconverted into a useful metabolite. The need for a dGSH repair reaction does not appear to be limited to eukaryotes: We demonstrate that Nit1 homologs acting as excellent dGSH amidases also occur in Escherichia coli and other glutathione-producing bacteria.

Structural Investigations of N-carbamoylputrescine Amidohydrolase from Medicago truncatula: Insights into the Ultimate Step of Putrescine Biosynthesis in Plants

Putrescine, 1,4-diaminobutane, is an intermediate in the biosynthesis of more complexed polyamines, spermidine and spermine. Unlike other eukaryotes, plants have evolved a multistep pathway for putrescine biosynthesis that utilizes arginine. In the final reaction, N-carbamoylputrescine is hydrolyzed to putrescine by N-carbamoylputrescine amidohydrolase (CPA, EC 3.5.1.53). During the hydrolysis, consecutive nucleophilic attacks on the substrate by Cys158 and water lead to formation of putrescine and two by-products, ammonia and carbon dioxide. CPA from the model legume plant, Medicago truncatula (MtCPA), was investigated in this work. Four crystal structures were determined: the wild-type MtCPA in complex with the reaction intermediate, N-(dihydroxymethyl)putrescine as well as with cadaverine, which is a longer analog of putrescine; and also structures of MtCPA-C158S mutant unliganded and with putrescine. MtCPA assembles into octamers, which resemble an incomplete left-handed helical twist. The active site of MtCPA is funnel-like shaped, and its entrance is walled with a contribution of the neighboring protein subunits. Deep inside the catalytic cavity, Glu48, Lys121, and Cys158 form the catalytic triad. In this studies, we have highlighted the key residues, highly conserved among the plant kingdom, responsible for the activity and selectivity of MtCPA toward N-carbamoylputrescine. Moreover, since, according to previous reports, a close MtCPA relative from Arabidopsis thaliana, along with several other nitrilase-like proteins, are subjected to allosteric regulation by substrates, we have used the structural information to indicate a putative secondary binding site. Based on the docking experiment, we postulate that this site is adjacent to the entrance to the catalytic pocket.

Protein engineering of a nitrilase from Burkholderia cenocepacia J2315 for efficient and enantioselective production of (R)-o-chloromandelic acid

The nitrilase-mediated pathway has significant advantages in the production of optically pure aromatic α-hydroxy carboxylic acids. However, low enantioselectivity and activity are observed on hydrolyzing o-chloromandelonitrile to produce optically pure (R)-o-chloromandelic acid. In the present study, a protein engineering approach was successfully used to enhance the performance of nitrilase obtained from Burkholderia cenocepacia strain J2315 (BCJ2315) in hydrolyzing o-chloromandelonitrile. Four hot spots (T49, I113, Y199, and T310) responsible for the enantioselectivity and activity of BCJ2315 were identified by random mutagenesis. An effective double mutant (I113M/Y199G [encoding the replacement of I with M at position 113 and Y with G at position 199]), which demonstrated remarkably enhanced enantioselectivity (99.1% enantiomeric excess [ee] compared to 89.2% ee for the wild type) and relative activity (360% of the wild type), was created by two rounds of site saturation mutagenesis, first at each of the four hot spots and subsequently at position 199 for combination with the selected beneficial mutation I113M. Notably, this mutant also demonstrated dramatically enhanced enantioselectivity and activity toward other mandelonitrile derivatives and, thus, broadened the substrate scope of this nitrilase. Using an ethyl acetate-water (1:9) biphasic system, o-chloromandelonitrile (500 mM) was completely hydrolyzed in 3 h by this mutant with a small amount of biocatalyst (10 g/liter wet cells), resulting in a high concentration of (R)-o-chloromandelic acid with 98.7% ee, to our knowledge the highest ever reported. This result highlights a promising method for industrial production of optically pure (R)-o-chloromandelic acid. Insight into the source of enantioselectivity and activity was gained by homology modeling and molecular docking experiments.

Role of the Vnn1 pantetheinase in tissue tolerance to stress

Pantetheinase is an ubiquitous enzyme which hydrolyses D-pantetheine into cysteamine and pantothenate (vitamin B5) on the dissimilative pathway of CoA. Pantetheinase isoforms are encoded by the Vnn (vanin) genes and Vnn1 is the predominant tissue isoform in mice and humans. In the present article, we review the results showing the regulation of Vnn1 expression during developmental, repair and inflammatory situations and the impact of a Vnn1 deficiency in mouse models of pathologies. We document the involvement of the Vnn1 pantetheinase in situations of increased tissue needs and propose that Vnn1 through recycling of pantothenate and release of cysteamine in tissues participates in the adaptive response of the tissue to stress.

Downregulation of NIT2 inhibits colon cancer cell proliferation and induces cell cycle arrest through the caspase-3 and PARP pathways

Colorectal cancer, also known as colon cancer is the most devastating malignancy worldwide. Previous studies have reported that Nit2, a member of the nitrilase superfamily, is a potential tumor suppressor, although its function remains elusive in colon cancer. In the present study, we employed an RNA interference lentivirus system to silence endogenous NIT2 expression in the colon cancer cell line, HCT116. The knockdown efficiency was determined by RT-qPCR and western blot analysis. The depletion of NIT2 markedly inhibited colon cancer cell proliferation and colony formation and induced cell cycle arrest in the G0/G1 phase, as shown by MTT assay, colony formation assay and flow cytometric analysis, respectively. Further investigation with an intracellular signaling array demonstrated that the depletion of NIT2 triggered the apoptosis of colon cancer cells through the caspase-3 and poly(ADP-ribose) polymerase (PARP) pathways. Our findings suggest that NIT2 may be an oncogene in human colon cancer and may thus serve as a promising therapeutic target for the treatment of colon cancer.

α-Ketoglutaramate: an overlooked metabolite of glutamine and a biomarker for hepatic encephalopathy and inborn errors of the urea cycle

Glutamine metabolism is generally regarded as proceeding via glutaminase-catalyzed hydrolysis to glutamate and ammonia, followed by conversion of glutamate to α-ketoglutarate catalyzed by glutamate dehydrogenase or by a glutamate-linked aminotransferase (transaminase). However, another pathway exists for the conversion of glutamine to α-ketoglutarate that is often overlooked, but is widely distributed in nature. This pathway, referred to as the glutaminase II pathway, consists of a glutamine transaminase coupled to ω-amidase. Transamination of glutamine results in formation of the corresponding α-keto acid, namely, α-ketoglutaramate (KGM). KGM is hydrolyzed by ω-amidase to α-ketoglutarate and ammonia. The net glutaminase II reaction is: L - Glutamine + α - keto acid + H2O → α - ketoglutarate + L - amino acid + ammonia. In this mini-review the biochemical importance of the glutaminase II pathway is summarized, with emphasis on the key component KGM. Forty years ago it was noted that the concentration of KGM is increased in the cerebrospinal fluid (CSF) of patients with hepatic encephalopathy (HE) and that the level of KGM in the CSF correlates well with the degree of encephalopathy. In more recent work, we have shown that KGM is markedly elevated in the urine of patients with inborn errors of the urea cycle. It is suggested that KGM may be a useful biomarker for many hyperammonemic diseases including hepatic encephalopathy, inborn errors of the urea cycle, citrin deficiency and lysinuric protein intolerance.

The structure of vanin 1: a key enzyme linking metabolic disease and inflammation

Although part of the coenzyme A pathway, vanin 1 (also known as pantetheinase) sits on the cell surface of many cell types as an ectoenzyme, catalyzing the breakdown of pantetheine to pantothenic acid (vitamin B5) and cysteamine, a strong reducing agent. Vanin 1 was initially discovered as a protein involved in the homing of leukocytes to the thymus. Numerous studies have shown that vanin 1 is involved in inflammation, and more recent studies have shown a key role in metabolic disease. Here, the X-ray crystal structure of human vanin 1 at 2.25 Å resolution is presented, which is the first reported structure from the vanin family, as well as a crystal structure of vanin 1 bound to a specific inhibitor. These structures illuminate how vanin 1 can mediate its biological roles by way of both enzymatic activity and protein-protein interactions. Furthermore, it sheds light on how the enzymatic activity is regulated by a novel allosteric mechanism at a domain interface.

Structural insights into enzymatic activity and substrate specificity determination by a single amino acid in nitrilase from Syechocystis sp. PCC6803

Nitrilases are enzymes widely expressed in prokaryotes and eukaryotes that utilize a Cys–Glu–Lys catalytic triad to hydrolyze non-peptide carbon–nitrogen bonds. Nitrilase from Syechocystis sp. Strain PCC6803 (Nit6803) shows hydrolysis activity towards a broad substrate spectrum, ranging from mononitriles to dinitriles and from aromatic nitriles to aliphatic nitriles. Yet, the structural principle of the substrate specificity of this nitrilase is still unknown. We report the crystal structure of Nit6803 at 3.1 Å resolution and propose a structural mechanism of substrate selection. Our mutagenesis data exhibited that the aromaticity of the amino acid at position 146 of Nit6803 is absolutely required for its nitrilase activity towards any substrates tested. Moreover, molecular docking and dynamic simulation analysis indicated that the distance between the sulfhydryl group of the catalytic cysteine residue and the cyano carbon of the substrate plays a crucial role in determining the nitrilase catalytic activity of Nit6803 and its mutants towards different nitrile substrates.

Structures of enzyme-intermediate complexes of yeast Nit2: insights into its catalytic mechanism and different substrate specificity compared with mammalian Nit2

The Nit (nitrilase-like) protein subfamily constitutes branch 10 of the nitrilase superfamily. Nit proteins are widely distributed in nature. Mammals possess two members of the Nit subfamily, namely Nit1 and Nit2. Based on sequence similarity, yeast Nit2 (yNit2) is a homologue of mouse Nit1, a tumour-suppressor protein whose substrate specificity is not yet known. Previous studies have shown that mammalian Nit2 (also a putative tumour suppressor) is identical to ω-amidase, an enzyme that catalyzes the hydrolysis of α-ketoglutaramate (α-KGM) and α-ketosuccinamate (α-KSM) to α-ketoglutarate (α-KG) and oxaloacetate (OA), respectively. In the present study, crystal structures of wild-type (WT) yNit2 and of WT yNit2 in complex with α-KG and with OA were determined. In addition, the crystal structure of the C169S mutant of yNit2 (yNit2-C169S) in complex with an endogenous molecule of unknown structure was also solved. Analysis of the structures revealed that α-KG and OA are covalently bound to Cys169 by the formation of a thioester bond between the sulfhydryl group of the cysteine residue and the γ-carboxyl group of α-KG or the β-carboxyl group of OA, reflecting the presumed reaction intermediates. However, an enzymatic assay suggests that α-KGM is a relatively poor substrate of yNit2. Finally, a ligand was found in the active site of yNit2-C169S that may be a natural substrate of yNit2 or an endogenous regulator of enzyme activity. These crystallographic analyses provide information on the mode of substrate/ligand binding at the active site of yNit2 and insights into the catalytic mechanism. These findings suggest that yNit2 may have broad biological roles in yeast, especially in regard to nitrogen homeostasis, and provide a framework for the elucidation of the substrate specificity and biological role of mammalian Nit1.

Screening and Improving the Recombinant Nitrilases and Application in Biotransformation of Iminodiacetonitrile to Iminodiacetic Acid

In this study, several nitrilase genes from phylogenetically distinct organisms were expressed and purified in E. coli in order to study their ability to mediate the biotransformation of nitriles. We identified three nitrilases: Acidovorax facilis nitrilase (AcN); Alcaligenes fecalis nitrilase (AkN); and Rhodococcus rhodochrous nitrilase (RkN), which catalyzed iminodiacetonitrile (IDAN) to iminodiacetic acid (IDA). AcN demonstrated 8.8-fold higher activity for IDAN degradation as compared to AkN and RkN. Based on homology modeling and previously described ‘hot spot’ mutations, several AcN mutants were screened for improved activity. One mutant M3 (F168V/L201N/S192F) was identified, which demonstrates a 41% enhancement in the conversion as well as a 2.4-fold higher catalytic efficiency towards IDAN as compared to wild-type AcN.

Target deconvolution techniques in modern phenotypic profiling

The past decade has seen rapid growth in the use of diverse compound libraries in classical phenotypic screens to identify modulators of a given process. The subsequent process of identifying the molecular targets of active hits, also called 'target deconvolution', is an essential step for understanding compound mechanism of action and for using the identified hits as tools for further dissection of a given biological process. Recent advances in 'omics' technologies, coupled with in silico approaches and the reduced cost of whole genome sequencing, have greatly improved the workflow of target deconvolution and have contributed to a renaissance of 'modern' phenotypic profiling. In this review, we will outline how both new and old techniques are being used in the difficult process of target identification and validation as well as discuss some of the ongoing challenges remaining for phenotypic screening.

Structural insights into the catalytic active site and activity of human Nit2/ω-amidase: kinetic assay and molecular dynamics simulation

Human nitrilase-like protein 2 (hNit2) is a putative tumor suppressor, recently identified as ω-amidase. hNit2/ω-amidase plays a crucial metabolic role by catalyzing the hydrolysis of α-ketoglutaramate (the α-keto analog of glutamine) and α-ketosuccinamate (the α-keto analog of asparagine), yielding α-ketoglutarate and oxaloacetate, respectively. Transamination between glutamine and α-keto-γ-methiolbutyrate closes the methionine salvage pathway. Thus, hNit2/ω-amidase links sulfur metabolism to the tricarboxylic acid cycle. To elucidate the catalytic specificity of hNit2/ω-amidase, we performed molecular dynamics simulations on the wild type enzyme and its mutants to investigate enzyme-substrate interactions. Binding free energies were computed to characterize factors contributing to the substrate specificity. The predictions resulting from these computations were verified by kinetic analyses and mutational studies. The activity of hNit2/ω-amidase was determined with α-ketoglutaramate and succinamate as substrates. We constructed three catalytic triad mutants (E43A, K112A, and C153A) and a mutant with a loop 116-128 deletion to validate the role of key residues and the 116-128 loop region in substrate binding and turnover. The molecular dynamics simulations successfully verified the experimental trends in the binding specificity of hNit2/ω-amidase toward various substrates. Our findings have revealed novel structural insights into the binding of substrates to hNit2/ω-amidase. A catalytic triad and the loop residues 116-128 of hNit2 play an essential role in supporting the stability of the enzyme-substrate complex, resulting in the generation of the catalytic products. These observations are predicted to be of benefit in the design of new inhibitors or activators for research involving cancer and hyperammonemic diseases.

Drug target deconvolution by chemical proteomics

Drug target deconvolution is a process where the action of a drug, a small molecule, is characterised by identifying the proteins binding the drug and initiating the biological effect. The biological relevant target has to be extracted, or deconvoluted, from a list of proteins identified in such an approach. Beside the medically desired action of the drug, the identification of other proteins binding the drug can help to identify side effects and toxicity at a very early stage of drug development. The current approach to identify the proteins binding to the drug is an affinity-enrichment based approach, where the drug molecule is immobilised to a matrix through a linker and the proteins binding to the drug are identified by proteomics.

Biochemical and mutational studies of the Bacillus cereus CECT 5050T formamidase support the existence of a C-E-E-K tetrad in several members of the nitrilase superfamily

Formamidases (EC 3.5.1.49) are poorly characterized proteins. In spite of this scarce knowledge, ammonia has been described as playing a central role in the pathogenesis of human pathogens such as Helicobacter pylori, for which formamidase has been shown to participate in the nitrogen metabolic pathway. Sequence analysis has revealed that at least two different groups of formamidases are classified as EC 3.5.1.49: on the one hand, the derivatives of the FmdA-AmdA superfamily, which are the best studied to date, and on the other hand, the derivatives of Helicobacter pylori AmiF. Here we present the cloning, purification, and characterization of a recombinant formamidase from Bacillus cereus CECT 5050T (BceAmiF), the second member of the AmiF subfamily to be characterized, showing new features of the enzyme further supporting its relationship with aliphatic amidases. We also present homology modeling-based mutational studies confirming the importance of the Glu140 and Tyr191 residues in the enzymatic activities of the AmiF family. Moreover, we can conclude that a second glutamate residue is critical in several members of the nitrilase superfamily, meaning that what has consistently been identified as a C-E-K triad is in fact a C-E-E-K tetrad.

Heterologous expression, purification and characterization of nitrilase from Aspergillus niger K10

BACKGROUND

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.

RESULTS

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.

CONCLUSIONS

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.

Hits, Fhits and Nits: beyond enzymatic function

We have briefly summarized what is known about these proteins, but in closing wish to feature the outstanding questions. Hint1 was discovered mistakenly as an inhibitor of Protein Kinase C and designated Pkci, a designation that still confuses the literature. The other Hint family members were discovered by homology to Hint1. Aprataxin was discovered as a result of the hunt for a gene responsible for AOA1. Fhit was discovered through cloning of a familial chromosome translocation breakpoint on chromosome 3 that interrupts the large FHIT gene within an intron, in the FRA3B chromosome region (Ohta et al., 1996), now known to be the region of the human genome most susceptible to DNA damage due to replication stress (Durkin et al., 2008). The NitFhit fusion genewas discovered during searches for Fhit homologs in flies and worms because the fly/worm Nit polypeptide is fused to the 5'-end of the Fhit gene; the mammalian Nit gene family was discovered because of the NitFhit fusion gene, in searches for homologs to the Nit polypeptide of the NitFhit gene. Each of the Hit family member proteins is reported to have enzymatic activities toward putative substrates involving nucleosides or dinucleosides. Most surprisingly, each of the Hit family proteins discussed has been implicated in important DNA damage response pathways and/or tumor suppression pathways. And for each of them it has been difficult to assign definite substrates, to know if the substrates and catalytic products have biological functions, to know if that function is related to the DNA damage response and suppressor functions, and to precisely define the pathways through which tumor suppression occurs. When the fly Nit sequence was found at the 5'-end of the fly Fhit gene, this gene was hailed as a Rosetta stone gene/protein that would help in discovery of the function of Fhit, because the Nit protein should be in the same signal pathway (Pace et al., 2000). However, the mammalian Nit family proteins have turned out to be at least as mysterious as the Fhit proteins, with the Nit1 substrate still unknown and the surprising finding that Nit proteins also appear to behave as tumor suppressor proteins. Whether the predicted enzymatic functions of these proteins are relevant to the observed biological functions, remain among the outstanding unanswered puzzles and raise the question: have these mammalian proteins evolved beyond the putative original enzymatic purpose, such that the catalytic function is now vestigial and subservient to signal pathways that use the protein-substrate complexes in pathways that signal apoptosis or DNA damage response? Or can these proteins be fulfilling catalytic functions independently but in parallel with signal pathway functions, as perhaps observed for Aprataxin? Or is the catalytic function indeed part of the observed biological functions, such as apoptosis and tumor suppression? Perhaps the recent, post-genomic focus on metabolomics and genome-wide investigations of signal pathway networks will lead to answers to some of these outstanding questions.

Activity-based protein profiling identifies a host enzyme, carboxylesterase 1, which is differentially active during hepatitis C virus replication

Hepatitis C virus (HCV) relies on many interactions with host cell proteins for propagation. Successful HCV infection also requires enzymatic activity of host cell enzymes for key post-translational modifications. To identify such enzymes, we have applied activity-based protein profiling to examine the activity of serine hydrolases during HCV replication. Profiling of hydrolases in Huh7 cells replicating HCV identified CES1 (carboxylesterase 1) as a differentially active enzyme. CES1 is an endogenous liver protein involved in processing of triglycerides and cholesterol. We observe that CES1 expression and activity were altered in the presence of HCV. The knockdown of CES1 with siRNA resulted in lower levels of HCV replication, and up-regulation of CES1 was observed to favor HCV propagation, implying an important role for this host cell protein. Experiments in HCV JFH1-infected cells suggest that CES1 facilitates HCV release because less intracellular HCV core protein was observed, whereas HCV titers remained high. CES1 activity was observed to increase the size and density of lipid droplets, which are necessary for the maturation of very low density lipoproteins, one of the likely vehicles for HCV release. In transgenic mice containing human-mouse chimeric livers, HCV infection also correlates with higher levels of endogenous CES1, providing further evidence that CES1 has an important role in HCV propagation.

Nit1 and Fhit tumor suppressor activities are additive

The fragile histidine triad gene (human FHIT, mouse Fhit) has been shown to act as a tumor suppressor gene. Nit1 and Fhit form a fusion protein, encoded by the NitFhit gene in flies and worms, suggesting that mammalian Nit1 and Fhit proteins, which are encoded by genes on different chromosomes in mammals, may function in the same signal pathway(s). A previous study showed that Nit1 deficiency in knockout mice confers a cancer prone phenotype, as does Fhit deficiency. We have now assessed the tumor susceptibility of Fhit(-/-)Nit1(-/-) mice and observed that double knockout mice develop more spontaneous and carcinogen-induced tumors than Fhit(-/-) mice, suggesting that the extent of tumor susceptibility due to Nit1 and Fhit deficiency is additive, and that Nit1 and Fhit affect distinct signal pathways in mammals. Nit1, like Fhit, is present in cytoplasm and mitochondria but not nuclei. Because Fhit deficiency affects responses to replicative and oxidative stress, we sought evidence for Nit1 function in response to such stresses in tissues and cultured cells: when treated with hydroxyurea, the normal kidney-derived double-deficient cells appear not to activate the pChk2 pathway and when treated with H(2)O(2), show little evidence of DNA damage, compared with wild type and Fhit(-/-) cells. The relevance of Nit1 deficiency to human cancers was examined in human esophageal cancer tissues, and loss of Nit1 expression was observed in 48% of esophageal adenocarcinomas.

Molecular identification of omega-amidase, the enzyme that is functionally coupled with glutamine transaminases, as the putative tumor suppressor Nit2

Our purpose was to identify the sequence of omega-amidase, which hydrolyses the amide group of alpha-ketoglutaramate, a product formed by glutamine transaminases. In the Bacillus subtilis genome, the gene encoding a glutamine transaminase (mtnV) is flanked by a gene encoding a putative 'carbon-nitrogen hydrolase'. The closest mammalian homolog of this putative bacterial omega-amidase is 'nitrilase 2', whose size and amino acid composition were in good agreement with those reported for purified rat liver omega-amidase. Mouse nitrilase 2 was expressed in Escherichia coli, purified and shown to catalyse the hydrolysis of alpha-ketoglutaramate and other known substrates of omega-amidase. No such activity was observed with mouse nitrilase 1. We conclude that mammalian nitrilase 2 is omega-amidase.

Identification of the putative tumor suppressor Nit2 as omega-amidase, an enzyme metabolically linked to glutamine and asparagine transamination

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 omega-amidodicarboxylate amidohydrolase (E.C. 3.5.1.3; abbreviated omega-amidase), a ubiquitously expressed enzyme that catalyzes a variety of amidase, transamidase, esterase and transesterification reactions. The in vivo amidase substrates are alpha-ketoglutaramate and alpha-ketosuccinamate, generated by transamination of glutamine and asparagine, respectively. Glutamine transaminases serve to salvage a number of alpha-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 alpha-keto-gamma-methiolbutyrate, a key component of the methionine salvage pathway. Some evidence suggests that excess alpha-ketoglutaramate may be neurotoxic. Moreover, alpha-ketosuccinamate is unstable and is readily converted to a number of hetero-aromatic compounds that may be toxic. Thus, an important role of omega-amidase is to remove potentially toxic intermediates by converting alpha-ketoglutaramate and alpha-ketosuccinamate to biologically useful alpha-ketoglutarate and oxaloacetate, respectively. Despite its importance in nitrogen and sulfur metabolism, the biochemical significance of omega-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.

Assay and purification of omega-amidase/Nit2, a ubiquitously expressed putative tumor suppressor, that catalyzes the deamidation of the alpha-keto acid analogues of glutamine and asparagine

omega-Amidase (omega-amidodicarboxylate amidohydrolase, EC 3.5.1.3) isolated from rat liver cytosol is a versatile enzyme that catalyzes a large number of amidase, transamidase, and ester hydrolysis reactions. omega-Amidase activity toward alpha-ketoglutaramate and alpha-ketosuccinamate (the alpha-keto acid analogues of glutamine and asparagine, respectively) is present in mammalian tissues, tumors, plants, bacteria, and fungi. Despite its versatility, widespread occurrence, and high specific activity, the enzyme has been little studied, possibly because the assay procedure previously required a substrate (alpha-ketoglutaramate) that is not commercially available. Here we report a simplified method for preparing alpha-ketoglutaramate and an assay procedure that measures alpha-ketoglutarate formation from alpha-ketoglutaramate in a 96-well plate format. We also describe a 96-well plate assay procedure that measures omega-amidase-catalyzed hydroxaminolysis of commercially available succinamic acid. The product, succinyl hydroxamate, yields a stable brown color in the presence of acidic ferric chloride that can be quantitated spectrophotometrically with negligible background interference. The two assay procedures (hydrolysis of alpha-ketoglutaramate and hydroxaminolysis of succinamate) were employed in purifying omega-amidase approximately 3600-fold from rat liver cytosol. The ratio of alpha-ketoglutaramate hydrolysis to succinamate hydroxaminolysis remained constant during the purification. omega-Amidase has recently been shown to be identical to Nit2, a putative tumor suppressor protein. It is anticipated that these new assay procedures will help to characterize the function of omega-amidase/Nit2 in tumor suppression, will provide the basis of high-throughput procedures to search for potent inhibitors and enhancers of omega-amidase, and will assist in identifying biological interactions between nitrogen metabolism and tumor biology.