Kinetic mechanism of Escherichia coli asparagine synthetase B

Cryo-EM and Molecular Dynamics Simulations Reveal Hidden Conformational Dynamics Controlling Ammonia Transport in Human Asparagine Synthetase

How motions in enzymes might be linked to catalytic function is of considerable general interest. Recent advances in X-ray crystallography and cryogenic electron microscopy offer the promise of elucidating functionally relevant motions in proteins that are not easily amenable to study by other biophysical methods. Here we use 3D variability analysis (3DVA) on cryo-EM maps for wild type (WT) human asparagine synthetase (ASNS) and the R142I ASNS variant to identify conformational changes in the Arg-142 side chain, which mediates the formation of a catalytically relevant intramolecular tunnel. Our 3DVA results for WT ASNS are consistent with independent molecular dynamics (MD) simulations on a model generated from the X-ray structure of human ASNS. Moreover, MD simulations of computational models for the ASNS/β-aspartyl-AMP/MgPP i and R142I/β-aspartyl-AMP/MgPP i ternary complexes, suggest that the structural integrity of the tunnel is impaired in the R142I variant when β-aspartyl-AMP is present in the synthetase active site. The kinetic properties of the R142I ASNS variant support the proposed function of Arg-142. These studies illustrate the power of cryo-EM to identify localized motions and dissect the conformational landscape of large proteins. When combined with MD simulations, 3DVA is a powerful approach to understanding how conformational dynamics might regulate function in multi-domain enzymes possessing multiple active sites.

Genome-based analysis of the type II PKS biosynthesis pathway of xanthones in Streptomyces caelestis and their antifungal activity

The ethyl acetate extract from the liquid fermentation of S. caelestis Aw99c exhibited high and broad antifungal activities against plant pathogenic fungi. Bioassay guide fractionation led to the discovery of two xanthones, citreamicin ε and θ. The draft genome sequence of S. caelestis Aw99c was analyzed by a similarity-based approach to elucidate the pathway for the citreamicins. A 48 kb citreamicin (cit) gene cluster with 51 open reading frames encoding type II polyketide synthases and unique polyketide tailoring enzymes was proposed based on the genome analysis and the chemical structure derivation. In vitro antifungal assay showed that citreamicin ε exhibited significant growth inhibition against the plant pathogenic fungi with MIC values ranging from 1.56 to 12.5 μM. The cellular structural change of M. grisea treated with citreamicin ε was detected by SEM and the result showed that citreamicin ε caused disruptive surface of the mycelia.

The ethyl acetate extract from the liquid fermentation of S. caelestis Aw99c exhibited high and broad antifungal activities against plant pathogenic fungi.

High-resolution crystal structure of human asparagine synthetase enables analysis of inhibitor binding and selectivity

Expression of human asparagine synthetase (ASNS) promotes metastatic progression and tumor cell invasiveness in colorectal and breast cancer, presumably by altering cellular levels of L-asparagine. Human ASNS is therefore emerging as a bona fide drug target for cancer therapy. Here we show that a slow-onset, tight binding inhibitor, which exhibits nanomolar affinity for human ASNS in vitro, exhibits excellent selectivity at 10 μM concentration in HCT-116 cell lysates with almost no off-target binding. The high-resolution (1.85 Å) crystal structure of human ASNS has enabled us to identify a cluster of negatively charged side chains in the synthetase domain that plays a key role in inhibitor binding. Comparing this structure with those of evolutionarily related AMP-forming enzymes provides insights into intermolecular interactions that give rise to the observed binding selectivity. Our findings demonstrate the feasibility of developing second generation human ASNS inhibitors as lead compounds for the discovery of drugs against metastasis.

Wen Zhu et al. report the crystal structure of human asparagine synthetase at a 1.85 Å resolution, enabling computational analysis of inhibitor binding. They also find new insights into the intermolecular interactions contributing to binding specificity of inhibitors.

Knockdown of asparagine synthetase A renders Trypanosoma brucei auxotrophic to asparagine

Asparagine synthetase (AS) catalyzes the ATP-dependent conversion of aspartate into asparagine using ammonia or glutamine as nitrogen source. There are two distinct types of AS, asparagine synthetase A (AS-A), known as strictly ammonia-dependent, and asparagine synthetase B (AS-B), which can use either ammonia or glutamine. The absence of AS-A in humans, and its presence in trypanosomes, suggested AS-A as a potential drug target that deserved further investigation. We report the presence of functional AS-A in Trypanosoma cruzi (TcAS-A) and Trypanosoma brucei (TbAS-A): the purified enzymes convert L-aspartate into L-asparagine in the presence of ATP, ammonia and Mg²⁺. TcAS-A and TbAS-A use preferentially ammonia as a nitrogen donor, but surprisingly, can also use glutamine, a characteristic so far never described for any AS-A. TbAS-A knockdown by RNAi didn't affect in vitro growth of bloodstream forms of the parasite. However, growth was significantly impaired when TbAS-A knockdown parasites were cultured in medium with reduced levels of asparagine. As expected, mice infections with induced and non-induced T. brucei RNAi clones were similar to those from wild-type parasites. However, when induced T. brucei RNAi clones were injected in mice undergoing asparaginase treatment, which depletes blood asparagine, the mice exhibited lower parasitemia and a prolonged survival in comparison to similarly-treated mice infected with control parasites. Our results show that TbAS-A can be important under in vivo conditions when asparagine is limiting, but is unlikely to be suitable as a drug target.

The amino acid asparagine is important not only for protein biosynthesis, but also for nitrogen homeostasis. Asparagine synthetase catalyzes the synthesis of this amino acid. There are two forms of asparagine synthetase, A and B. The presence of type A in trypanosomes, and its absence in humans, makes this protein a potential drug target. Trypanosomes are responsible for serious parasitic diseases that rely on limited drug therapeutic options for control. In our study we present a functional characterization of trypanosomes asparagine synthetase A. We describe that Trypanosoma brucei and Trypanosoma cruzi type A enzymes are able to use either ammonia or glutamine as a nitrogen donor, within the conversion of aspartate into asparagine. Furthermore, we show that asparagine synthetase A knockdown renders Trypanosoma brucei auxotrophic to asparagine. Overall, this study demonstrates that interfering with asparagine metabolism represents a way to control parasite growth and infectivity.

Elucidation of Piericidin A1 biosynthetic locus revealed a thioesterase-dependent mechanism of α-pyridone ring formation

Piericidins are a class of α-pyridone antibiotics that inhibit mitochondrial respiratory chain and exhibit antimicrobial, antifungal, and antitumor activities. Sequential analysis of Streptomyces piomogeues var. Hangzhouwanensis genome revealed six modular polyketide synthases, an amidotransferase, two methyltransferases, and a monooxygenase for piericidin A1 production. Gene functional analysis and deletion results provide overview of the biosynthesis pathway. Furthermore, in vitro characterization of the terminal polyketide synthase module with the thioesterase domain using β-ketoacyl substrates was performed. That revealed a pathway where the α-pyridone ring formation is dependent on hydrolysis of the product β, δ-diketo carboxylic acid by the C-terminal thioesterase followed by amidation and cyclization. These findings set the stage to investigate unusual enzymatic mechanisms in α-pyridone antibiotics biosynthesis, provide a foundation for genome mining of α-pyridone antibiotics, and produce analogs by molecular engineering.

A conserved glutamate controls the commitment to acyl-adenylate formation in asparagine synthetase

Inhibitor docking studies have implicated a conserved glutamate residue (Glu-348) as a general base in the synthetase active site of the enzyme asparagine synthetase B from Escherichia coli (AS-B). We now report steady-state kinetic, isotope transfer, and positional isotope exchange experiments for a series of site-directed AS-B mutants in which Glu-348 is substituted by conservative amino acid replacements. We find that formation of the β-aspartyl-AMP intermediate, and therefore the eventual production of asparagine, is dependent on the presence of a carboxylate side chain at this position in the synthetase active site. In addition, Glu-348 may also play a role in mediating the conformational changes needed to (i) coordinate, albeit weakly, the glutaminase and synthetase activities of the enzyme and (ii) establish the structural integrity of the intramolecular tunnel along which ammonia is translocated. The importance of Glu-348 in mediating acyl-adenylate formation contrasts with the functional role of the cognate residues in β-lactam synthetase (BLS) and carbapenem synthetase (CPS) (Tyr-348 and Tyr-345, respectively), which both likely evolved from asparagine synthetase. Given the similarity of the chemistry catalyzed by AS-B, BLS, and CPS, our work highlights the difficulty of predicting the functional outcome of single site mutations on enzymes that catalyze almost identical chemical transformations.

A conserved tyrosyl-glutamyl catalytic dyad in evolutionarily linked enzymes: carbapenam synthetase and beta-lactam synthetase

Beta-lactam-synthesizing enzymes carbapenam synthetase (CPS) and beta-lactam synthetase (beta-LS) are evolutionarily linked to a common ancestor, asparagine synthetase B (AS-B). These three relatives catalyze substrate acyl-adenylation and nucleophilic acyl substitution by either an external (AS-B) or internal (CPS, beta-LS) nitrogen source. Unlike AS-B, crystal structures of CPS and beta-LS revealed a putative Tyr-Glu dyad (CPS, Y345/E380; beta-LS, Y348/E382) proposed to deprotonate the respective internal nucleophile. CPS and beta-LS site-directed mutagenesis (Y345/8A, Y345/8F, E380/2D, E380/2Q, E380A) resulted in the reduction of their catalytic efficiency, with Y345A, E380A, and E382Q producing undetectable amounts of beta-lactam product. However, [(32)P]PP(i)-ATP exchange assays demonstrated Y345A and E380A undergo the first half-reaction, with the remaining active mutants showing decreased forward commitment to beta-lactam cyclization. pH-rate profiles of CPS and beta-LS supported the importance of a Tyr-Glu dyad in beta-lactam formation and suggested its reverse protonation in beta-LS. The kinetics of CPS double-site mutants reinforced the synergism of Tyr-Glu in catalysis. Furthermore, significant solvent isotope effects on k(cat) ((D)k(cat)) for Y345F (1.9) and Y348F (1.7) maintained the assignment of Y345/8 in proton transfer. A proton inventory on Y348F determined its (D)(k(cat)/K(m)) = 0.2 to arise from multiple reactant-state fractionation factors, presumably from water molecule(s) replacing the missing Tyr hydroxyl. The role of a CPS and beta-LS Tyr-Glu catalytic dyad was solidified by a significant decrease in mutant k(cat) viscosity dependence with respect to the wild-type enzymes. The evolutionary relation and potential for engineered biosynthesis were demonstrated by beta-LS acting as a carbapenam synthetase.

Dissection of the stepwise mechanism to beta-lactam formation and elucidation of a rate-determining conformational change in beta-lactam synthetase

Clavulanic acid is a widely used beta-lactamase inhibitor whose key beta-lactam core is formed by beta-lactam synthetase. beta-Lactam synthetase exhibits a Bi-Ter mechanism consisting of two chemical steps, acyl-adenylation followed by beta-lactam formation. 32PPi-ATP exchange assays showed the first irreversible step of catalysis is acyl-adenylation. From a small, normal solvent isotope effect (1.38 +/- 0.04), it was concluded that beta-lactam synthesis contributes at least partially to kcat. Site-specific mutation of Lys-443 identified this residue as the ionizable group at pKa approximately 8.1 apparent in the pH-kcat profile that stabilizes the beta-lactam-forming step. Viscosity studies demonstrated that a protein conformational change was also partially rate-limiting on kcat attenuating the observed solvent isotope effect on beta-lactam formation. Adherence to Kramers' theory gave a slope of 1.66 +/- 0.08 from a plot of log(o kcat/kcat) versus log(eta/eta(o)) consistent with opening of a structured loop visible in x-ray data preceding product release. Internal "friction" within the enzyme contributes to a slope of > 1 in this analysis. Correspondingly, earlier in the catalytic cycle ordering of a mobile active site loop upon substrate binding was manifested by an inverse solvent isotope effect (0.67 +/- 0.15) on kcat/Km. The increased second-order rate constant in heavy water was expected from ordering of this loop over the active site imposing torsional strain. Finally, an Eyring plot displayed a large enthalpic change accompanying loop movement (DeltaH approximately 20 kcal/mol) comparable to the chemical barrier of beta-lactam formation.

Partial randomization of the four sequential amidation reactions catalyzed by cobyric acid synthetase with a single point mutation

Cobyric acid synthetase (CbiP) from Salmonella typhimurium catalyzes the glutamine and ATP-dependent amidation of carboxylates b, d, e, and g within adenosyl cobyrinic acid a,c-diamide. After each round of catalysis the partially amidated intermediates are released into solution and the four carboxylates are amidated in the sequential order of e, d, b, and g for the wild type enzyme. In the presence of [gamma-18O4]-ATP and adenosyl cobyrinic a,c-diamide the enzyme will catalyze the positional isotope exchange of the betagamma-bridge oxygen with the two beta-nonbridge oxygens. These results support the proposal that ATP is used to activate the carboxylate groups via the formation of a phosphorylated intermediate. CbiP catalyzes the hydrolysis of glutamine in the absence of ATP or adenosyl cobyrinic acid a,c-diamide, but the rate of glutamine hydrolysis is enhanced by a factor of 60 in the presence of these two substrates together. This result suggests that the formation of the phosphorylated intermediate is coupled to the activation of the site utilized for the hydrolysis of glutamine. However, the rate of glutamine hydrolysis is approximately 2.5 times the rate of ADP formation, indicating that the two active sites are partially uncoupled from one another and that some of the ammonia from glutamine hydrolysis leaks into the bulk solution. The mutation of D146 to either alanine or asparagine results in a protein that is able to catalyze the formation of cobyric acid. However, the strict amidation order observed with the wild type CbiP is partially randomized with carboxylate b being amidated last. With the D146N mutant, the predominant pathway occurs in the sequence d, e, g, and b. It is proposed that this residue enforces the amidation order in the wild type enzyme via charge-charge repulsion between the side chain carboxylate and the carboxylates of the substrate.

An inhibitor of human asparagine synthetase suppresses proliferation of an L-asparaginase-resistant leukemia cell line

Drug resistance in lymphoblastic and myeloblastic leukemia cells is poorly understood, with several lines of evidence suggesting that resistance can be correlated with upregulation of human asparagine synthetase (hASNS) expression, although this hypothesis is controversial. New tools are needed to investigate this clinically important question, including potent hASNS inhibitors. In vitro experiments show an adenylated sulfoximine to be a slow-onset, tight-binding inhibitor of hASNS with nanomolar affinity. This binding affinity represents a 10-fold improvement over that reported for the only other well-characterized hASNS inhibitor. The adenylated sulfoximine has a cytostatic effect on L-asparaginase-resistant MOLT-4 cells cultured in the presence of L-asparaginase, an enzyme that depletes L-asparagine in the growth medium. These observations represent direct evidence that potent hASNS inhibitors may prove to be effective agents for the clinical treatment of acute lymphoblastic leukemia.

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.

Engineered biosynthesis of a novel amidated polyketide, using the malonamyl-specific initiation module from the oxytetracycline polyketide synthase

Tetracyclines are aromatic polyketides biosynthesized by bacterial type II polyketide synthases (PKSs). Understanding the biochemistry of tetracycline PKSs is an important step toward the rational and combinatorial manipulation of tetracycline biosynthesis. To this end, we have sequenced the gene cluster of oxytetracycline (oxy and otc genes) PKS genes from Streptomyces rimosus. Sequence analysis revealed a total of 21 genes between the otrA and otrB resistance genes. We hypothesized that an amidotransferase, OxyD, synthesizes the malonamate starter unit that is a universal building block for tetracycline compounds. In vivo reconstitution using strain CH999 revealed that the minimal PKS and OxyD are necessary and sufficient for the biosynthesis of amidated polyketides. A novel alkaloid (WJ35, or compound 2) was synthesized as the major product when the oxy-encoded minimal PKS, the C-9 ketoreductase (OxyJ), and OxyD were coexpressed in CH999. WJ35 is an isoquinolone compound derived from an amidated decaketide backbone and cyclized with novel regioselectivity. The expression of OxyD with a heterologous minimal PKS did not afford similarly amidated polyketides, suggesting that the oxy-encoded minimal PKS possesses novel starter unit specificity.

Asparagine synthetase chemotherapy

Modern clinical treatments of childhood acute lymphoblastic leukemia (ALL) employ enzyme-based methods for depletion of blood asparagine in combination with standard chemotherapeutic agents. Significant side effects can arise in these protocols and, in many cases, patients develop drug-resistant forms of the disease that may be correlated with up-regulation of the enzyme glutamine-dependent asparagine synthetase (ASNS). Though the precise molecular mechanisms that result in the appearance of drug resistance are the subject of active study, potent ASNS inhibitors may have clinical utility in treating asparaginase-resistant forms of childhood ALL. This review provides an overview of recent developments in our understanding of (a) the structure and catalytic mechanism of ASNS, and (b) the role that ASNS may play in the onset of drug-resistant childhood ALL. In addition, the first successful, mechanism-based efforts to prepare and characterize nanomolar ASNS inhibitors are discussed, together with the implications of these studies for future efforts to develop useful drugs.

The enzymology of clavam and carbapenem biosynthesis

The enzyme-catalysed reactions involved in formation of the bicyclic clavam and carbapenem nuclei, including beta-amino acid and beta-lactam formation, are discussed and compared with those involved in penicillin and cephalosporin biosynthesis. The common role of unusual oxidation reactions in the biosynthetic pathways and the lack of synthetic reagents available to effect them are highlighted.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Kinetic mechanism of asparagine synthetase from Vibrio cholerae

Asparagine synthetase B (AsnB) catalyzes the formation of asparagine in an ATP-dependent reaction using glutamine or ammonia as a nitrogen source. To obtain a better understanding of the catalytic mechanism of this enzyme, we report the cloning, expression, and kinetic analysis of the glutamine- and ammonia-dependent activities of AsnB from Vibrio cholerae. Initial velocity, product inhibition, and dead-end inhibition studies were utilized in the construction of a model for the kinetic mechanism of the ammonia- and glutamine-dependent activities. The reaction sequence begins with the ordered addition of ATP and aspartate. Pyrophosphate is released, followed by the addition of ammonia and the release of asparagine and AMP. Glutamine is simultaneously hydrolyzed at a second site and the ammonia intermediate diffuses through an interdomain protein tunnel from the site of production to the site of utilization. The data were also consistent with the dead-end binding of asparagine to the glutamine binding site and PP(i) with free enzyme. The rate of hydrolysis of glutamine is largely independent of the activation of aspartate and thus the reaction rates at the two active sites are essentially uncoupled from one another.

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.

Synthesis and characterization of an N-acylsulfonamide inhibitor of human asparagine synthetase

[structure: see text] The synthesis of N-acylsulfonamide 6, which is an analogue of beta-aspartyl-AMP, is described. This compound appears to be the first and only potent inhibitor of human asparagine synthetase that has been described to date. The N-acylsulfonamide 6 exhibits slow-onset inhibition kinetics, with a K(i) of 728 nM. Preparation and characterization of two additional N-acylsulfonamide analogues has also demonstrated the importance of hydrogen-bonding interactions in the recognition of the AS inhibitor with the enzyme. These observations provide the basis for the discovery of new compounds with application in the treatment of drug-resistant leukemia.

Revisiting the steady state kinetic mechanism of glutamine-dependent asparagine synthetase from Escherichia coli

Escherichia coli asparagine synthetase B (AS-B) catalyzes the formation of asparagine from aspartate in an ATP-dependent reaction for which glutamine is the in vivo nitrogen source. In an effort to reconcile several different kinetic models that have been proposed for glutamine-dependent asparagine synthetases, we have used numerical methods to investigate the kinetic mechanism of AS-B. Our simulations demonstrate that literature proposals cannot reproduce the glutamine dependence of the glutamate/asparagine stoichiometry observed for AS-B, and we have therefore developed a new kinetic model that describes the behavior of AS-B more completely. The key difference between this new model and the literature proposals is the inclusion of an E.ATP.Asp.Gln quaternary complex that can either proceed to form asparagine or release ammonia through nonproductive glutamine hydrolysis. The implication of this model is that the two active sites in AS-B become coordinated only after formation of a beta-aspartyl-AMP intermediate in the synthetase site of the enzyme. The coupling of glutaminase and synthetase activities in AS is therefore different from that observed in all other well-characterized glutamine-dependent amidotransferases.

A concise synthesis of beta-asparaginyladenylate

Stable analogues of acyladenylate intermediates, such as N-acylphosphoramidates, are useful probes of tRNA aminoacylation and enzyme mechanism, and have potential application as enzyme inhibitors. We now report a concise, "one-pot" synthesis of beta-asparaginyladenylate using a novel coupling protocol that yields the target N-acylphosphoramidate in three reactions from readily available precursors. This simple synthetic procedure may represent a general approach for the preparation of functionalized N-acylphosphoramidates from amides that do not undergo coupling under the conditions of existing literature protocols.

Kinetic characterization of human glutamine-fructose-6-phosphate amidotransferase I: potent feedback inhibition by glucosamine 6-phosphate

Glutamine-fructose-6-phosphate amidotransferase (GFAT) catalyzes the first committed step in the pathway for biosynthesis of hexosamines in mammals. A member of the N-terminal nucleophile class of amidotransferases, GFAT transfers the amino group from the L-glutamine amide to D-fructose 6-phosphate, producing glutamic acid and glucosamine 6-phosphate. The kinetic constants reported previously for mammalian GFAT implicate a relatively low affinity for the acceptor substrate, fructose 6-phosphate (Fru-6-P, K(m) 0.2-1 mm). Utilizing a new sensitive assay that measures the production of glucosamine 6-phosphate (GlcN-6-P), purified recombinant human GFAT1 (hGFAT1) exhibited a K(m) for Fru-6-P of 7 microm, and was highly sensitive to product inhibition by GlcN-6-P. In a second assay method that measures the stimulation of glutaminase activity, a K(d) of 2 microm was measured for Fru-6-P binding to hGFAT1. Further, we report that the product, GlcN-6-P, is a potent competitive inhibitor for the Fru-6-P site, with a K(i) measured of 6 microm. Unlike other members of the amidotransferase family, where glutamate production is loosely coupled to amide transfer, we have demonstrated that hGFAT1 production of glutamate and GlcN-6-P are strictly coupled in the absence of inhibitors. Similar to other amidotransferases, competitive inhibitors that bind at the synthase site may inhibit the synthase activity without inhibiting the glutaminase activity at the hydrolase domain. GlcN-6-P, for example, inhibited the transfer reaction while fully activating the glutaminase activity at the hydrolase domain. Inhibition of hGFAT1 by the end product of the pathway, UDP-GlcNAc, was competitive with a K(i) of 4 microm. These data suggest that hGFAT1 is fully active at physiological levels of Fru-6-P and may be regulated by its product GlcN-6-P in addition to the pathway end product, UDP-GlcNAc.

Synthesis of L-4,4-difluoroglutamic acid via nucleophilic addition to a chiral aldehyde

Fluorine-containing derivatives of amino acids are assuming increasing importance as probes of biological function and enzyme mechanism. We now report a new, flexible route to enantiomerically pure L-4,4-difluoroglutamic acid that exploits the addition of difluorinated nucleophiles to configurationally stable alpha-aminoaldehydes. Conversion of the difluorinated adducts to L-4,4-difluoroglutamic acid can be accomplished in three steps by Barton-McCombie dehydroxylation and acid hydrolysis.