Different ways to transport ammonia in human and Mycobacterium tuberculosis NAD+ synthetases.

NAD+ synthetase is an essential enzyme of de novo and recycling pathways of NAD+ biosynthesis in Mycobacterium tuberculosis but not in humans. This bifunctional enzyme couples the NAD+ synthetase and glutaminase activities through an ammonia tunnel but free ammonia is also a substrate. Here we show that the Homo sapiens NAD+ synthetase (hsNadE) lacks substrate specificity for glutamine over ammonia and displays a modest activation of the glutaminase domain compared to tbNadE. We report the crystal structures of hsNadE and NAD+ synthetase from M. tuberculosis (tbNadE) with synthetase intermediate analogues. Based on the observed exclusive arrangements of the domains and of the intra- or inter-subunit tunnels we propose a model for the inter-domain communication mechanism for the regulation of glutamine-dependent activity and NH3 transport. The structural and mechanistic comparison herein reported between hsNadE and tbNadE provides also a starting point for future efforts in the development of anti-TB drugs.

Design, synthesis, and evaluation of substituted nicotinamide adenine dinucleotide (NAD+) synthetase inhibitors as potential antitubercular agents.

Nicotinamide adenine dinucleotide (NAD+) synthetase catalyzes the last step in NAD+ biosynthesis. Depletion of NAD+ is bactericidal for both active and dormant Mycobacterium tuberculosis (Mtb). By inhibiting NAD+ synthetase (NadE) from Mtb, we expect to eliminate NAD+ production which will result in cell death in both growing and nonreplicating Mtb. NadE inhibitors have been investigated against various pathogens, but few have been tested against Mtb. Here, we report on the expansion of a series of urea-sulfonamides, previously reported by Brouillette et al. Guided by docking studies, substituents on a terminal phenyl ring were varied to understand the structure-activity-relationships of substituents on this position. Compounds were tested as inhibitors of both recombinant Mtb NadE and Mtb whole cells. While the parent compound displayed very weak inhibition against Mtb NadE (IC50=1000µM), we observed up to a 10-fold enhancement in potency after optimization. Replacement of the 3,4-dichloro group on the phenyl ring of the parent compound with 4-nitro yielded 4f, the most potent compound of the series with an IC50 value of 90µM against Mtb NadE. Our modeling results show that these urea-sulfonamides potentially bind to the intramolecular ammonia tunnel, which transports ammonia from the glutaminase domain to the active site of the enzyme. This hypothesis is supported by data showing that, even when treated with potent inhibitors, NadE catalysis is restored when treated with exogenous ammonia. Most of these compounds also inhibited Mtb cell growth with MIC values of 19-100µg/mL. These results improve our understanding of the SAR of the urea-sulfonamides, their mechanism of binding to the enzyme, and of Mtb NadE as a potential antitubercular drug target.

Identification of the dioxygenase-generated intermediate formed during biosynthesis of the dihydropyrrole moiety common to anthramycin and sibiromycin.

A description of pyrrolo[1,4]benzodiazepine (PBD) biosynthesis is a prerequisite for engineering production of analogs with enhanced antitumor activity. Predicted dioxygenases Orf12 and SibV associated with dihydropyrrole biosynthesis in PBDs anthramycin and sibiromycin, respectively, were expressed and purified for activity studies. UV-visible spectroscopy revealed that these enzymes catalyze the regiospecific 2,3-extradiol dioxygenation of l-3,4-dihydroxyphenylalanine (l-DOPA) to form l-2,3-secodopa (λmax=368 nm). (1)H NMR spectroscopy indicates that l-2,3-secodopa cyclizes into the α-keto acid tautomer of l-4-(2-oxo-3-butenoic-acid)-4,5-dihydropyrrole-2-carboxylic acid (λmax=414 nm). Thus, the dioxygenases are key for establishing the scaffold of the dihydropyrrole moiety. Kinetic studies suggest the dioxygenase product is relatively labile and is likely consumed rapidly by subsequent biosynthetic steps. The enzymatic product and dimeric state of these dioxygenases are conserved in dioxygenases involved in dihydropyrrole and pyrrolidine biosynthesis within both PBD and non-PBD pathways.

Mutasynthesis of a potent anticancer sibiromycin analogue.

Pursuit of the actinomycete pyrrolobenzodiazepine natural product sibiromycin as a chemotherapeutic agent has been limited by its cardiotoxicity. Among pyrrolobenzodiazepines, cardiotoxicity is associated with hydroxylation at position 9. Deletion of the methyltransferase gene sibL abolishes the production of sibiromycin. Supplementation of growth media with 4-methylanthranilic acid can substitute for its native 3-hydroxy congener. Cultures grown in this fashion yielded 9-deoxysibiromycin. In this study, we characterize the structure and biological activity of sibiromycin and 9-deoxysibiromycin methyl carbinolamines. Preliminary in vitro evidence suggests that 9-deoxysibiromycin exhibits reduced cardiotoxicity while gaining antitumor activity. These results strongly support further exploration of the production and evaluation of monomeric and dimeric glycosylated pyrrolobenzodiazepine analogues of sibiromycin.

Regulation of the intersubunit ammonia tunnel in Mycobacterium tuberculosis glutamine-dependent NAD+ synthetase.

Glutamine-dependent NAD+ synthetase is an essential enzyme and a validated drug target in Mycobacterium tuberculosis (mtuNadE). It catalyses the ATP-dependent formation of NAD+ from NaAD+ (nicotinic acid-adenine dinucleotide) at the synthetase active site and glutamine hydrolysis at the glutaminase active site. An ammonia tunnel 40 Å (1 Å=0.1 nm) long allows transfer of ammonia from one active site to the other. The enzyme displays stringent kinetic synergism; however, its regulatory mechanism is unclear. In the present paper, we report the structures of the inactive glutaminase C176A variant in an apo form and in three synthetase-ligand complexes with substrates (NaAD+/ATP), substrate analogue {NaAD+/AMP-CPP (adenosine 5'-[α,ß-methylene]triphosphate)} and intermediate analogues (NaAD+/AMP/PPi), as well as the structure of wild-type mtuNadE in a product complex (NAD+/AMP/PPi/glutamate). This series of structures provides snapshots of the ammonia tunnel during the catalytic cycle supported also by kinetics and mutagenesis studies. Three major constriction sites are observed in the tunnel: (i) at the entrance near the glutaminase active site; (ii) in the middle of the tunnel; and (iii) at the end near the synthetase active site. Variation in the number and radius of the tunnel constrictions is apparent in the crystal structures and is related to ligand binding at the synthetase domain. These results provide new insight into the regulation of ammonia transport in the intermolecular tunnel of mtuNadE.

Biosynthesis, synthesis, and biological activities of pyrrolobenzodiazepines.

Pyrrolobenzodiazepines (PBDs) are sequence selective DNA alkylating agents with remarkable antineoplastic activity. They are either naturally produced by actinomycetes or synthetically produced. The remarkable broad spectrum of activities of the naturally produced PBDs encouraged the synthesis of several PBDs, including dimeric and hybrid PBDs yielding to an improvement in the DNA-binding sequence specificity and in the potency of this class of compounds. However, limitation in the chemical synthesis prevented the testing of one of the most potent PBDs, sibiromycin, a naturally produced glycosylated PBDs. Only recently, the biosynthetic gene clusters for PBDs have been identified opening the doors to the production of glycosylated PBDs by mutasynthesis and biosynthetic engineering. This review describes the recent studies on the biosynthesis of naturally produced pyrrolobenzodiazepines. In addition, it provides an overview on the isolation and characterization of naturally produced PBDs, chemical synthesis of PBDs, mechanism of DNA alkylation, and DNA-binding affinity and cytotoxic properties of both naturally produced and synthetic pyrrolobenzodiazepines.

A heme peroxidase with a functional role as an L-tyrosine hydroxylase in the biosynthesis of anthramycin.

We report the first characterization and classification of Orf13 (S. refuineus) as a heme-dependent peroxidase catalyzing the ortho-hydroxylation of L-tyrosine to L-DOPA. The putative tyrosine hydroxylase coded by orf13 of the anthramycin biosynthesis gene cluster has been expressed and purified. Heme b has been identified as the required cofactor for catalysis, and maximal L-tyrosine conversion to L-DOPA is observed in the presence of hydrogen peroxide. Preincubation of L-tyrosine with Orf13 prior to the addition of hydrogen peroxide is required for L-DOPA production. However, the enzyme becomes inactivated by hydrogen peroxide during catalysis. Steady-state kinetic analysis of L-tyrosine hydroxylation revealed similar catalytic efficiency for both L-tyrosine and hydrogen peroxide. Spectroscopic data from a reduced-CO(g) UV-vis spectrum of Orf13 and electron paramagnetic resonance of ferric heme Orf13 are consistent with heme peroxidases that have a histidyl-ligated heme iron. Contrary to the classical heme peroxidase oxidation reaction with hydrogen peroxide that produces coupled aromatic products such as o,o'-dityrosine, Orf13 is novel in its ability to catalyze aromatic amino acid hydroxylation with hydrogen peroxide, in the substrate addition order and for its substrate specificity for L-tyrosine. Peroxygenase activity of Orf13 for the ortho-hydroxylation of L-tyrosine to L-DOPA by a molecular oxygen dependent pathway in the presence of dihydroxyfumaric acid is also observed. This reaction behavior is consistent with peroxygenase activity reported with horseradish peroxidase for the hydroxylation of phenol. Overall, the putative function of Orf13 as a tyrosine hydroxylase has been confirmed and establishes the first bacterial class of tyrosine hydroxylases.

Kinetic, crystallographic, and mechanistic characterization of TomN: elucidation of a function for a 4-oxalocrotonate tautomerase homologue in the tomaymycin biosynthetic pathway.

The biosynthesis of the C ring of the antitumor antibiotic agent, tomaymycin, is proposed to proceed through five enzyme-catalyzed steps from l-tyrosine. The genes encoding these enzymes have recently been cloned and their functions tentatively assigned, but there is limited biochemical evidence supporting the assignments of the last three steps. One enzyme, TomN, shows 58% pairwise sequence similarity with 4-oxalocrotonate tautomerase (4-OT), an enzyme found in a catabolic pathway for aromatic hydrocarbons. The TomN sequence includes three amino acids (Pro-1, Arg-11, and Arg-39) that have been identified as critical catalytic residues in 4-OT. However, the proposed substrate for TomN is very different from that processed by 4-OT. To establish the function and mechanism of TomN and its relationship with 4-OT, we conducted kinetic, mutagenic, and structural studies. The kinetic parameters for TomN, and four alanine mutants, P1A, R11A, R39A, and R61A, were determined using 2-hydroxymuconate, the substrate for 4-OT. The TomN-catalyzed reaction using this substrate compares favorably to that of 4-OT. In addition, the kinetic parameters for the P1A, R11A, and R39A mutants of TomN parallel the trends observed for the corresponding 4-OT mutants, implicating an analogous mechanism. A high-resolution crystal structure (1.4 Å) of TomN shows that the overall structure and the active site region are highly similar to those of 4-OT with a root-mean-square deviation of 0.81 Å. Moreover, key active site residues are positionally conserved. The combined results suggest that the tentative assignment for TomN and the proposed sequence of events in the biosynthetic pathway leading to the formation of the C ring of tomaymycin might not be correct. An alternative pathway that awaits biochemical confirmation is proposed.

An ancestral glutamine-dependent NAD(+) synthetase revealed by poor kinetic synergism.

NAD(+) synthetase catalyzes the formation of NAD(+) from ATP, nicotinic acid adenine dinucleotide and ammonia. Glutamine-dependent NAD(+) synthetase obtains ammonia through the hydrolysis of glutamine to glutamate, which takes place in the glutaminase domain. The ammonia is subsequently transported to the synthetase domain through an interdomain ammonia tunnel. NAD(+) synthetase from the thermophilic bacteria Thermotoga maritima was cloned and expressed. Steady-state kinetics and stoichiometric analysis of product formation revealed an enzyme that is significantly inefficient in the synchronization of the two active sites resulting in wasteful hydrolysis of glutamine and that is not specific for glutamine over ammonia. Phylogenetic analysis of glutamine-dependent NAD(+) synthetases identifies three main groups remotely related. The T. maritima NAD(+) synthetase's group is proposed to represent the ancestral group based on the phylogenetic analysis and on the kinetic characterizations. The phylogenetic results nicely correlate also with the degree of catalytic efficiency measured for M. tuberculosis, S. cerevisiae and T. maritima NAD(+) synthetases. Furthermore, the data here reported in combination with structural data available for glutamine-dependent NAD(+) synthetase lays the foundation for further investigation on the mechanism of active site coupling in these enzymes.

Biosynthesis of sibiromycin, a potent antitumor antibiotic.

Pyrrolobenzodiazepines, a class of natural products produced by actinomycetes, are sequence selective DNA alkylating compounds with significant antitumor properties. Among the pyrrolo[1,4]benzodiazepines (PBDs) sibiromycin, one of two identified glycosylated PBDs, displays the highest affinity for DNA and the most potent antitumor properties. Despite the promising antitumor properties clinical trials of sibiromycin were precluded by the cardiotoxicity effect in animals attributed to the presence of the C-9 hydroxyl group. As a first step toward the development of sibiromycin analogs, we have cloned and localized the sibiromycin gene cluster to a 32.7-kb contiguous DNA region. Cluster boundaries tentatively assigned by comparative genomics were verified by gene replacement experiments. The sibiromycin gene cluster consisting of 26 open reading frames reveals a "modular" strategy in which the synthesis of the anthranilic and dihydropyrrole moieties is completed before assembly by the nonribosomal peptide synthetase enzymes. In addition, the gene cluster identified includes open reading frames encoding enzymes involved in sibirosamine biosynthesis, as well as regulatory and resistance proteins. Gene replacement and chemical complementation studies are reported to support the proposed biosynthetic pathway.

Cloning and characterization of the biosynthetic gene cluster for tomaymycin, an SJG-136 monomeric analog.

Tomaymycin produced by Streptomyces achromogenes is a naturally produced pyrrolobenzodiazepine (PBD). The biosynthetic gene cluster for tomaymycin was identified and sequenced. The gene cluster analysis reveals a novel biosynthetic pathway for the anthranilate moiety of PBDs. Gene replacement and chemical complementation studies were used to confirm the proposed biosynthetic pathway.

Regulation of active site coupling in glutamine-dependent NAD(+) synthetase.

NAD(+) is an essential metabolite both as a cofactor in energy metabolism and redox homeostasis and as a regulator of cellular processes. In contrast to humans, Mycobacterium tuberculosis NAD(+) biosynthesis is absolutely dependent on the activity of a multifunctional glutamine-dependent NAD(+) synthetase, which catalyzes the ATP-dependent formation of NAD(+) at the synthetase domain using ammonia derived from L-glutamine in the glutaminase domain. Here we report the kinetics and structural characterization of M. tuberculosis NAD(+) synthetase. The kinetics data strongly suggest tightly coupled regulation of the catalytic activities. The structure, the first of a glutamine-dependent NAD(+) synthetase, reveals a homooctameric subunit organization suggesting a tight dependence of catalysis on the quaternary structure, a 40-A intersubunit ammonia tunnel and structural elements that may be involved in the transfer of information between catalytic sites.

Rate-limiting steps and role of active site Lys443 in the mechanism of carbapenam synthetase.

Carbapenam synthetase (hereafter named CPS) catalyzes the formation of the beta-lactam ring in the biosynthetic pathway to (5R)-carbapen-2-em-3-carboxylate, the simplest of the carbapenem antibiotics. Kinetic studies showed remarkable tolerance to substrate stereochemistry in the turnover rate but did not distinguish between chemistry and a nonchemical step such as product release or conformational change as being rate-determining. Also, X-ray structural studies and modest sequence homology to beta-lactam synthetase, an enzyme that catalyzes the formation of a monocyclic beta-lactam ring in a similar ATP/Mg2+-dependent reaction, implicate K443 as an essential residue for substrate binding and intermediate stabilization. In these experiments, we use pH-rate profiles, deuterium solvent isotope effects, and solvent viscosity measurements to examine the rate-limiting step in this complex overall process of substrate adenylation and intramolecular ring formation. Mutagenesis and chemical rescue demonstrate that K443 is the general acid visible in the pH-rate profile of the wild-type CPS-catalyzed reaction. On the basis of these results, we propose a mechanism in which the rate-limiting step is beta-lactam ring formation coupled to a protein conformational change and underscore the role of K443 throughout the reaction.

Carboxymethylproline synthase from Pectobacterium carotorova: a multifaceted member of the crotonase superfamily.

The simplest carbapenem antibiotic, (5R)-carbapen-2-em-3-carboxylic acid, is biosynthesized from primary metabolites in Pectobacterium carotorova by the action of three enzymes, carboxymethylproline synthase (hereafter named CarB), carbapenam synthetase, and carbapenem synthase. CarB, a member of the crotonase superfamily, catalyzes the formation of (2S,5S)-5-carboxymethylproline from malonyl-CoA and l-pyrroline-5-carboxylate. In this study we show that, in addition, CarB catalyzes the independent decarboxylation of malonyl-CoA and methylmalonyl-CoA and the hydrolysis of CoA esters such as acetyl-CoA and propionyl-CoA. The steady-state rate constants for these reactions are reported. We have identified the intermediates in the CarB reactions with l-pyrroline-5-carboxylate and malonyl-CoA or methylmalonyl-CoA as the CoA esters of (2S,5S)-5-carboxymethylproline and (2S,5S)-6-methyl-5-carboxymethylproline, respectively. The data provided indicate that these intermediates partition between completing turnover and dissociating from the enzyme. On the basis of the steady-state rate constants measured for the CarB-catalyzed hydrolysis of synthetic (2S,5S)-5-carboxymethylprolyl-CoA and for the CarB reaction with malonyl-CoA and l-pyrroline-5-carboxylate, we have calculated the rate constants for each step of these reactions. The results identify CarB as a particularly interesting member of the crotonase superfamily that combines in one net reaction three activities of this superfamily, decarboxylation, C-C bond formation, and CoA ester hydrolysis.

Crystal structure of carbapenam synthetase (CarA).

Carbapenam synthetase (CarA) is an ATP/Mg2+-dependent enzyme that catalyzes formation of the beta-lactam ring in (5R)-carbapenem-3-carboxylic acid biosynthesis. CarA is homologous to beta-lactam synthetase (beta-LS), which is involved in clavulanic acid biosynthesis. The catalytic cycles of CarA and beta-LS mediate substrate adenylation followed by beta-lactamization via a tetrahedral intermediate or transition state. Another member of this family of ATP/Mg2+-dependent enzymes, asparagine synthetase (AS-B), catalyzes intermolecular, rather than intramolecular, amide bond formation in asparagine biosynthesis. The crystal structures of apo-CarA and CarA complexed with the substrate (2S,5S)-5-carboxymethylproline (CMPr), ATP analog alpha,beta-methyleneadenosine 5'-triphosphate (AMP-CPP), and a single Mg2+ ion have been determined. CarA forms a tetramer. Each monomer resembles beta-LS and AS-B in overall fold, but key differences are observed. The N-terminal domain lacks the glutaminase active site found in AS-B, and an extended loop region not observed in beta-LS or AS-B is present. Comparison of the C-terminal synthetase active site to that in beta-LS reveals that the ATP binding site is highly conserved. By contrast, variations in the substrate binding pocket reflect the different substrates of the two enzymes. The Mg2+ coordination is also different. Several key residues in the active site are conserved between CarA and beta-LS, supporting proposed roles in beta-lactam formation. These data provide further insight into the structures of this class of enzymes and suggest that CarA might be a versatile target for protein engineering experiments aimed at developing improved production methods and new carbapenem antibiotics.

Inhibition and alternate substrate studies on the mechanism of carbapenam synthetase from Erwinia carotovora.

The Erwinia carotorova carA, carB, and carC gene products are essential for the biosynthesis of (5R)-carbapen-2-em-3-carboxylic acid, the simplest carbapenem beta-lactam antibiotic. CarA (hereafter named carbapenam synthetase) has been proposed to catalyze formation of (3S,5S)-carbapenam-3-carboxylic acid from (2S,5S)-5-carboxymethyl proline based on characterization of the products of fermentation experiments in Escherichia coli cells transformed with pET24a/carB and pET24a/carAB, and on sequence homology to beta-lactam synthetase, an enzyme that catalyzes formation of a monocyclic beta-lactam ring with concomitant ATP hydrolysis. In this study, we have purified recombinant carbapenam synthetase and shown in vitro that it catalyzes the ATP-dependent formation of (3S,5S)-carbapenam-3-carboxylic acid from (2S,5S)-5-carboxymethyl proline. The kinetic mechanism is Bi-Ter where ATP is the first substrate to bind followed by (2S,5S)-5-carboxymethyl proline and PPi is the last product released based on initial velocity, product and dead-end inhibition studies. The reactions catalyzed by carbapenam synthetase with different diastereomers of the natural substrate and with alternate alpha-amino diacid substrates were studied by HPLC, ESI mass spectrometry, and steady-state kinetic analysis. On the basis of these results, we have proposed a role for each moiety of (2S,5S)-5-carboxymethyl proline for binding to the active site of carbapenam synthetase. Coupled enzyme assays of AMP and pyrophosphate release in the reactions catalyzed by carbapenam synthetase with adipic and glutaric acid, which lack the alpha-amino group, in the presence and absence of hydroxylamine support the formation of an acyladenylate intermediate in the catalytic cycle.