Design, synthesis and evaluation of transition-state analogue inhibitors of Escherichia coli gamma-glutamylcysteine synthetase

PKLR promotes colorectal cancer liver colonization through induction of glutathione synthesis

Colorectal cancer metastasis to the liver is a major cause of cancer-related death; however, the genes and pathways that govern this metastatic colonization event remain poorly characterized. Here, using a large-scale in vivo RNAi screen, we identified liver and red blood cell pyruvate kinase (PKLR) as a driver of metastatic liver colonization. PKLR expression was increased in liver metastases as well as in primary colorectal tumors of patients with metastatic disease. Evaluation of a murine liver colonization model revealed that PKLR promotes cell survival in the tumor core during conditions of high cell density and oxygen deprivation by increasing glutathione, the primary endogenous antioxidant. PKLR negatively regulated the glycolytic activity of PKM2, the major pyruvate kinase isoenzyme known to regulate cellular glutathione levels. Glutathione is critical for metastasis, and we determined that the rate-limiting enzyme of glutathione synthesis, GCLC, becomes overexpressed in patient liver metastases, promotes cell survival under hypoxic and cell-dense conditions, and mediates metastatic liver colonization. RNAi-mediated inhibition of glutathione synthesis impaired survival of multiple colon cancer cell lines, and pharmacological targeting of this metabolic pathway reduced colonization in a primary patient-derived xenograft model. Our findings highlight the impact of metabolic reprogramming within the niche as metastases progress and suggest clinical potential for targeting this pathway in colorectal cancer.

Continuous-flow thermolysis for the preparation of vinylglycine derivatives

Syn sulfoxide elimination was carried out under continuous-flow conditions in a mesofluidic thermolysis reactor. The design of the reactor enabled accurate control of reaction time and conditions, affording a convenient scale-independent procedure for the production of N,C-protected vinylglycine derivatives. Thermolysis at 270 °C under 1000 psi of pressure in superheated toluene enabled typical daily outputs ranging from 11 to 46 g per day with excellent selectivities and ee (>97%). The various competitive reaction pathways were studied and rationalized according to a computational study.

A sulfoximine-based inhibitor of human asparagine synthetase kills L-asparaginase-resistant leukemia cells

An adenylated sulfoximine transition-state analogue 1, which inhibits human asparagine synthetase (hASNS) with nanomolar potency, has been reported to suppress the proliferation of an l-asparagine amidohydrolase (ASNase)-resistant MOLT-4 leukemia cell line (MOLT-4R) when l-asparagine is depleted in the medium. We now report the synthesis and biological activity of two new sulfoximine analogues of 1 that have been studied as part of systematic efforts to identify compounds with improved cell permeability and/or metabolic stability. One of these new analogues, an amino sulfoximine 5 having no net charge at cellular pH, is a better hASNS inhibitor (K(I)(∗)=8 nM) than 1 and suppresses proliferation of MOLT-4R cells at 10-fold lower concentration (IC(50)=0.1mM). More importantly, and in contrast to the lead compound 1, the presence of sulfoximine 5 at concentrations above 0.25 mM causes the death of MOLT-4R cells even when ASNase is absent in the culture medium. The amino sulfoximine 5 exhibits different dose-response behavior when incubated with an ASNase-sensitive MOLT-4 cell line (MOLT-4S), supporting the hypothesis that sulfoximine 5 exerts its effect by inhibiting hASNS in the cell. Our work provides further evidence for the idea that hASNS represents a chemotherapeutic target for the treatment of leukemia, and perhaps other cancers, including those of the prostate.

Structural basis for the inhibition of Mycobacterium tuberculosis glutamine synthetase by novel ATP-competitive inhibitors

Glutamine synthetase (GS, EC 6.3.1.2; also known as gamma-glutamyl:ammonia ligase) catalyzes the ATP-dependent condensation of glutamate and ammonia to form glutamine. The enzyme has essential roles in different tissues and species, which have led to its consideration as a drug or an herbicide target. In this article, we describe studies aimed at the discovery of new antimicrobial agents targeting Mycobacterium tuberculosis, the causative pathogen of tuberculosis. A number of distinct classes of GS inhibitors with an IC(50) of micromolar value or better were identified via high-throughput screening. A commercially available purine analogue similar to one of the clusters identified (the diketopurines), 1-[(3,4-dichlorophenyl)methyl]-3,7-dimethyl-8-morpholin-4-yl-purine-2,6-dione, was also shown to inhibit the enzyme, with a measured IC(50) of 2.5+/-0.4 microM. Two X-ray structures are presented: one is a complex of the enzyme with the purine analogue alone (2.55-A resolution), and the other includes the compound together with methionine sulfoximine phosphate, magnesium and phosphate (2.2-A resolution). The former represents a relaxed, inactive conformation of the enzyme, while the latter is a taut, active one. These structures show that the compound binds at the same position in the nucleotide site, regardless of the conformational state. The ATP-binding site of the human enzyme differs substantially, explaining why it has an approximately 60-fold lower affinity for this compound than the bacterial GS. As part of this work, we devised a new synthetic procedure for generating l-(SR)-methionine sulfoximine phosphate from l-(SR)-methionine sulfoximine, which will facilitate future investigations of novel GS inhibitors.

Synthesis of P,N-heterocycles from omega-amino-H-phosphinates: conformationally restricted alpha-amino acid analogs

P,N-heterocycles (3-hydroxy-1,3-azaphospholane and 3-hydroxy-1,3-azaphosphorinane-3-oxide) are synthesized in moderate yield from readily available omega-amino-H-phosphinates and aldehydes or ketones via an intramolecular Kabachnik-Fields reaction. The products are conformationally restricted phosphinic analogs of alpha-amino acids. The multigram-scale syntheses of the H2N(CH2)(n)PO2H2 phosphinic precursors (n = 1, 2, 3) and some derivatives are also described.

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.

Alkylation of H-phosphinate esters under basic conditions

An efficient and general procedure was developed for the direct alkylation of H-phosphinate esters with LHMDS at low temperature. The simplicity of the reaction allows the use of various H-phosphinate esters and takes place with a wide range of electrophiles. The approach can be employed to access some GABA analogues or precursors to GABA analogues. The isolated yields are moderate to good. This is the first report of an alkylation with a secondary iodide or a primary chloride.

Structure-based identification of novel human gamma-glutamylcysteine synthetase inhibitors

Glutathione depletion represents a potentially important strategy to sensitize tumors to cytotoxic drugs. l-Buthionine-(R,S)-sulfoximine (l-BSO) has been studied in both preclinical and early clinical trials, but limitation on its access has led to a search for alternatives. Using a 3D molecular model of human gamma-glutamylcysteine synthetase (gamma-GCS(H)), the major subunit of the rate-limiting GSH synthetic enzyme, we virtually screened the National Cancer Institute chemical database to identify compounds that could bind to and potentially inhibit gamma-GCS(H). We identified 51 test chemicals, all with structures very distinct from l-BSO. We subjected these compounds to biological assays measuring gamma-GCS(H) inhibition and glutathione (GSH) depletion. Among 10 novel gamma -GCS inhibitors identified, 4 compounds depleted glutathione in cells, and 2 with related structures sensitized tumor cells to melphalan treatment. This work validates the use of model-based database mining and identified inhibitors of gamma-GCS(H) with novel chemical structures.

Enzyme inhibitors as chemical tools to study enzyme catalysis: rational design, synthesis, and applications

Carefully designed molecules that are intimately related to the reaction mechanism of enzymes are often highly selective and potent inhibitors that serve as extremely useful chemical probes for understanding the reaction mechanism and structure of enzymes. This article describes the design, synthesis, and applications of specific inhibitors of two mechanistically distinct groups of enzymes, ATP-dependent amide ligases and Ser- and Thr-hydrolases. Our strategy is based on the premise that stable analogues of the transition state (transition-state analogues) are highly potent inhibitors that serve as good mechanistic probes, and that a key structure of a good inhibitor of one enzyme is also utilized for the inhibitors of other enzymes that share the same chemistry in their catalyzed reactions, irrespective of the degree of structural similarity and evolutionary link between the enzymes. According to these principles, we designed and synthesized a series of phosphinate- and sulfoximine-based transition-state analogue inhibitors of glutathione synthetase, gamma-glutamylcysteine synthetase and asparagine synthetase. For the second group of enzymes, we synthesized a gamma-monofluorophosphono glutamate analogue for mechanism-based affinity labeling of gamma-glutamyltranspeptidase and fluorescent phosphonic acid esters for the active-site titration of lipase. These inhibitors were used successfully as ligands for detailed kinetic analyses, X-ray crystallography, and mass analysis of the enzymes to identify the key amino acid residues responsible for catalysis and substrate recognition in the transition state.

Glutathione Synthesis in Streptococcus agalactiae

Gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GS), distinct enzymes that together account for glutathione (GSH) synthesis, have been isolated and characterized from several Gram-negative prokaryotes and from numerous eukaryotes including mammals, amphibians, plants, yeast, and protozoa. Glutathione synthesis is relatively uncommon among the Gram-positive bacteria, and, to date, neither the genes nor the proteins involved have been identified. In the present report, we show that crude extracts of Streptococcus agalactiae catalyze the gamma-GCS and GS reactions and can synthesize GSH from its constituent amino acids. The putative gene for S. agalactiae gamma-GCS was identified and cloned, and the corresponding protein was expressed and purified. Surprisingly, it was found that the isolated enzyme catalyzes both the ATP-dependent synthesis of L-gamma-glutamyl-L-cysteine from L-glutamate and L-cysteine and the ATP-dependent synthesis of GSH from L-gamma-glutamyl-L-cysteine and glycine. This novel bifunctional enzyme, referred to as gamma-GCS-GS, has been characterized in terms of catalytic activity, substrate specificity, and inhibition by GSH, cystamine, and transition state analog sulfoximines. The N-terminal 518 amino acids of gamma-GCS-GS (total M(r) 85,000) show 32% identity and 43% similarity with E. coli gamma-GCS (M(r) 58,000), but the C-terminal putative GS domain (remaining 202 amino acids) of gamma-GCS-GS shows no significant homology with known GS sequences. The C terminus (360 amino acids) is, however, homologous to D-Ala, D-Ala ligase (24% identity; 38% similarity), an enzyme having the same protein fold as known GS proteins. These results are discussed in terms of the evolution of GSH synthesis and the possible occurrence of a similar bifunctional GSH synthesis enzyme in other bacterial species.

Crystal structure of gamma-glutamylcysteine synthetase: insights into the mechanism of catalysis by a key enzyme for glutathione homeostasis

Gamma-glutamylcysteine synthetase (gammaGCS), a rate-limiting enzyme in glutathione biosynthesis, plays a central role in glutathione homeostasis and is a target for development of potential therapeutic agents against parasites and cancer. We have determined the crystal structures of Escherichia coli gammaGCS unliganded and complexed with a sulfoximine-based transition-state analog inhibitor at resolutions of 2.5 and 2.1 A, respectively. In the crystal structure of the complex, the bound inhibitor is phosphorylated at the sulfoximido nitrogen and is coordinated to three Mg2+ ions. The cysteine-binding site was identified; it is formed inductively at the transition state. In the unliganded structure, an open space exists around the representative cysteine-binding site and is probably responsible for the competitive binding of glutathione. Upon inhibitor binding, the side chains of Tyr-241 and Tyr-300 turn, forming a hydrogen-bonding triad with the carboxyl group of the inhibitor's cysteine moiety, allowing this moiety to fit tightly into the cysteine-binding site with concomitant accommodation of its side chain into a shallow pocket. This movement is caused by a conformational change of a switch loop (residues 240-249). Based on this crystal structure, the cysteine-binding sites of mammalian and parasitic gammaGCSs were predicted by multiple sequence alignment, although no significant sequence identity exists between the E. coli gammaGCS and its eukaryotic homologues. The identification of this cysteine-binding site provides important information for the rational design of novel gammaGCS inhibitors.

Facile synthesis of alpha-substituted acrylate esters

Treatment of 5-monosubstituted Meldrum's acids with dimethylmethyleneimmonium iodide (Eschenmoser's iodide salt) in methanol gives alpha-substituted acrylate methyl esters in good yields. Easy access to 5-monosubstituted Meldrum's acids allowed us to synthesize a wide variety of alpha-substituted acrylate methyl esters. The reaction conditions are mild and tolerate many functional groups commonly used in organic synthesis; thus, this new method has potential as an alternative to conventional preparative methods for alpha-substituted acrylate esters.

Highly selective addition of chiral, sulfonimidoyl substituted bis(allyl)titanium complexes to N-sulfonyl alpha-imino esters: asymmetric synthesis of gamma,delta-unsaturated alpha-amino acids bearing a chiral, electron-withdrawing nucleofuge at the delta-position

Selective addition of the chiral, sulfonimidoyl substituted bis(allyl)titanium complexes 5a-d, which are configurationally labile in regard to the Calpha-atoms, to N-toluenesulfonyl (Ts)-, N-2-trimethylsilylethanesulfonyl (SES)-, and N-tert-butylsulfonyl (Bus) alpha-imino ester (9a-c) in the presence of Ti(OiPr)(4) and ClTi(OiPr)(3) afforded with high regio- and diastereoselectivities in good yields the (syn, E)-configured beta-alkyl-gamma,delta-unsaturated alpha-amino acid derivatives 2a-g, which carry a chiral, electron-withdrawing nucleofuge at the delta-position and a cyclohexyl, an isopropyl, a phenyl, and a methyl group at the beta-position. Addition of the cyclic bis(allyl)titanium complex 14 to N-Bus alpha-imino ester 9c afforded with similar high regio- and diastereoselectivities the (E)- and (Z)-configured amino acid derivatives (E)-8 and (Z)-8. Reaction of complexes 5a-d with alpha-imino esters 9a-c in the presence of Ti(OiPr)(4) occurs stepwise to give first the mono(allyl)titanium complexes containing 2a-g as ligands, which react in the presence of ClTi(OiPr)(3) with a second molecule of 9a-c with formation of two molecules of 2a-g. Formation of (S,R,E)-configured homoallylic amines 2a-g entails Si,Re,E processes of alpha-imino esters 9a-c with the (R,R)-configured bis(allyl)titanium complexes (R,R)-5a-d and (R)-configured mono(allyl)titanium complexes (R)-17a-d, both of which are most likely in rapid equilibrium with their (S,S)-diastereomers and (S)-diastereomers, respectively. Interestingly, in the reaction of 5a-d with aldehydes, the (S,S)-configured complexes (S,S)-5a-d are the ones which react faster. Reaction of the N-titanated amino acid derivatives Ti-2a and Ti-2b with N-Ts alpha-imino ester 9a led to the highly diastereoselective formation of imidazolidinones 15a and 15b, respectively. Cleavage of the sulfonamide group of the N-Bus amino acid derivative 2d with CF(3)SO(3)H gave quantitatively the sulfonimidoyl functionalized amino acid H-2d. A Ni-catalyzed cross-coupling reaction of the amino acid derivative 2e with ZnPh(2) led to a substitution of the sulfonimidoyl group by a phenyl group and furnished the enantiomerically pure protected alpha-amino acid Bus-1. Two new N-sulfonyl alpha-imino esters, the SES and the Bus alpha-imino esters 9b and 9c, respectively, have been synthesized from the corresponding sulfonamides by the Kresze method in medium to good yields. The N-SES alpha-imino ester 9b and the N-Bus alpha-imino ester 9c should find many synthetic applications, in particular, in cases where the N-Ts alpha-imino ester 9a had been used before.

Triethylborane-initiated room temperature radical addition of hypophosphites to olefins: synthesis of monosubstituted phosphinic acids and esters

A novel and practical approach to monosubstituted phosphinic acid (alkylphosphonous acid) derivatives from hypophosphite salts or esters is described. Phosphorus-centered radical formation is initiated with Et(3)B/O(2), and the reaction is conveniently conducted at room temperature in an open flask. In contrast to previously reported conditions for the radical reaction of hypophosphorous acid and sodium hypophosphite (peroxide initiators, acid catalysis, heat), the method proceeds under neutral conditions and therefore tolerates a wide range of functional groups. Previously inaccessible phosphinic acids can be prepared in a single step from cheap starting materials. Excellent selectivity is observed for monoaddition, and symmetrical dialkyl phosphinates do not form in significant amounts. Monosubstituted phosphinic acids are usually obtained in better than 90% purity by a simple extractive workup; however, isolated yields are diminished if the substituent is polar. Because radicals derived from hypophosphites are electrophilic, the reaction is limited to the use of electron-rich olefins. The reaction conditions can also be employed in the room temperature radical reduction of alkyl halides and provide an exceptionally mild and environmentally friendly alternative to the use of tributyltin hydride. The remarkable mild nature of the reaction conditions allows for the radical reaction of sensitive alkyl hypophosphites to occur, in which case, a catalytic amount of Et(3)B suffices to deliver alkyl phosphinate esters in reasonable yield.

Orthosilicate-mediated esterification of monosubstituted phosphinic acids

Monosubstituted phosphinic acids are esterified with orthosilicates in excellent yields. Phosphinylidene-containing acids react selectively under these conditions, while disubstituted phosphinic acids and phosphonic acids remain unchanged. One-pot procedures are also described for the preparation of phosphinate esters from an alcohol. This novel method provides a convenient and general alternative to more commonly employed conditions such as diazomethane or carbodiimide.

Biologic and pharmacologic regulation of mammalian glutathione synthesis

Glutathione (L-gamma-glutamyl-L-cysteinylglycine, GSH) is synthesized from its constituent amino acids by the sequential action of gamma-glutamylcysteine synthetase (gamma-GCS) and GSH synthetase. The intracellular GSH concentration, typically 1-8 mM, reflects a dynamic balance between the rate of GSH synthesis and the combined rate of GSH consumption within the cell and loss through efflux. The gamma-GCS reaction is rate limiting for GSH synthesis, and regulation of gamma-GCS expression and activity is critical for GSH homeostasis. Transcription of the gamma-GCS subunit genes is controlled by a variety of factors through mechanisms that are not yet fully elucidated. Glutathione synthesis is also modulated by the availability of gamma-GCS substrates, primarily L-cysteine, by feedback inhibition of gamma-GCS by GSH, and by covalent inhibition of gamma-GCS by phosphorylation or nitrosation. Because GSH plays a critical role in cellular defenses against electrophiles, oxidative stress and nitrosating species, pharmacologic manipulation of GSH synthesis has received much attention. Administration of L-cysteine precursors and other strategies allow GSH levels to be maintained under conditions that would otherwise result in GSH depletion and cytotoxicity. Conversely, inhibitors of gamma-GCS have been used to deplete GSH as a strategy for increasing the sensitivity of tumors and parasites to certain therapeutic interventions.