Exploring fluorinated heptose phosphate analogues as inhibitors of HldA and HldE, key enzymes in the biosynthesis of lipopolysaccharide

The growing threat of bacterial resistance to antibiotics has led to the rise of anti-virulence strategies as a promising approach. These strategies aim to disarm bacterial pathogens and improve their clearance by the host immune system. Lipopolysaccharide, a key virulence factor in Gram-negative bacteria, has been identified as a potential target for anti-virulence agents. In this study, we focus on inhibiting HldA and HldE, bacterial enzymes from the heptose biosynthesis pathway, which plays a key role in lipopolysaccharide biosynthesis. We present the synthesis of two fluorinated non-hydrolysable heptose phosphate analogues. Additionally, the inhibitory activity of a family of eight heptose phosphate analogues against HldA and HldE was assessed. This evaluation revealed inhibitors with affinities in the low μM range, with the most potent compound showing inhibition constant values of 15.4 μM for HldA and 16.9 μM for HldE. The requirement for a phosphate group at the C-7 position was deemed essential for inhibitory activity, while the presence of a hydroxy anomeric group was found to be beneficial, a phenomenon rationalized through computational modeling. Additionally, the introduction of a single fluorine atom α to the phosphonate moiety conferred a slight advantage for inhibition. These findings suggest that mimicking the structure of d-glycero-β-d-manno-heptose 1,7-bisphosphate, the product of the phosphorylation step in heptose biosynthesis, could be a promising strategy to disrupt this biosynthetic pathway. In terms of the in vivo effects, these heptose phosphate analogues neither demonstrated significant LPS-disrupting effects nor exhibited growth inhibitory activity on their own. Additionally, they did not alter the susceptibility of bacteria to hydrophobic antibiotics. The highly charged nature of these molecules may hinder their ability to penetrate the bacterial cell wall. To overcome this limitation, alternative strategies such as incorporating protecting groups that facilitate their entry and can subsequently be cleaved within the bacterial cytoplasm could be explored.

Far-reaching effects of tyrosine64 phosphorylation on Ras revealed with BeF3- complexes

Tyrosine phosphorylation on Ras by Src kinase is known to uncouple Ras from upstream regulation and downstream communication. However, the mechanisms by which phosphorylation modulates these interactions have not been detailed. Here, the major mono-phosphorylation level on tyrosine64 is quantified by 31P NMR and mutagenesis. Crystal structures of unphosphorylated and tyrosine64-phosphorylated Ras in complex with a BeF3- ground state analogue reveal "closed" Ras conformations very different from those of the "open" conformations previously observed for non-hydrolysable GTP analogue structures of Ras. They deliver new mechanistic and conformational insights into intrinsic GTP hydrolysis. Phosphorylation of tyrosine64 delivers conformational changes distant from the active site, showing why phosphorylated Ras has reduced affinity to its downstream effector Raf. 19F NMR provides evidence for changes in the intrinsic GTPase and nucleotide exchange rate and identifies the concurrent presence of a major "closed" conformation alongside a minor yet functionally important "open" conformation at the ground state of Ras. This study expands the application of metal fluoride complexes in revealing major and minor conformational changes of dynamic and modified Ras proteins.

Synthesis of a novel fluorinated phosphonyl C-glycoside, (3-deoxy-3-fluoro-β-d-glucopyranosyl)methylphosphonate, a potential inhibitor of β-phosphoglucomutase

β-phosphoglucomutase (βPGM) catalyzes the conversion of β-glucose 1-phosphate (βG1P) to glucose-6-phosphate (G6P), a universal source of cellular energy, in a two-step process. Transition state analogue (TSA) complexes formed from substrate analogues and a metal fluoride (MgF3- and AlF4-) enable analysis of each of these enzymatic steps independently. Novel substrate analogues incorporating fluorine offer opportunities to interrogate the enzyme mechanism using 19F NMR spectroscopy. Herein, the synthesis of a novel fluorinated phosphonyl C-glycoside (3-deoxy-3-fluoro-β-d-glucopyranosyl)methylphosphonate (1), in 12 steps (0.85 % overall yield) is disclosed. A four-stage synthetic strategy was employed, involving: 1) fluorine addition to the monosaccharide, 2) selective anomeric deprotection, 3) phosphonylation of the anomeric centre, and 4) global deprotection. Analysis of βPGM and 1 will be reported in due course.

Synthesis of fluorinated and nonfluorinated sugar alkenylphosphonates via highly stereoselective Horner-Wadsworth-Emmons olefination

New fluorinated and nonfluorinated sugar alkenylphosphonates were obtained. In all cases 1,2;5,6-di-O-isopropylidene-α-d-glucofuranose was used as the starting material. The synthesis of alkenylphosphonates was based on Horner-Wadsworth-Emmons olefination. The process led to products with E-stereochemistry exclusively or predominately.

Synthesis and application of phosphorylated saccharides in researching carbohydrate-based drugs

Phosphorylated saccharides are valuable targets in glycochemistry and glycobiology, which play an important role in various physiological and pathological processes. The current research on phosphorylated saccharides primarily focuses on small molecule inhibitors, glycoconjugate vaccines and novel anti-tumour targeted drug carrier materials. It can maximise the pharmacological effects and reduce the toxicity risk caused by nonspecific off-target reactions of drug molecules. However, the number and types of natural phosphorylated saccharides are limited, and the complexity and heterogeneity of their structures after extraction and separation seriously restrict their applications in pharmaceutical development. The increasing demands for the research on these molecules have extensively promoted the development of carbohydrate synthesis. Numerous innovative synthetic methodologies have been reported regarding the continuous expansion of the potential building blocks, catalysts, and phosphorylation reagents. This review summarizes the latest methods for enzymatic and chemical synthesis of phosphorylated saccharides, emphasizing their breakthroughs in yield, reactivity, regioselectivity, and application scope. Additionally, the anti-bacterial, anti-tumour, immunoregulatory and other biological activities of some phosphorylated saccharides and their applications were also reviewed. Their structure-activity relationship and mechanism of action were discussed and the key phosphorylation characteristics, sites and extents responsible for observed biological activities were emphasised. This paper will provide a reference for the application of phosphorylated saccharide in the research of carbohydrate-based drugs in the future.

Fluorinated carbohydrates as chemical probes for molecular recognition studies. Current status and perspectives

This review provides an extensive summary of the effects of carbohydrate fluorination with regard to changes in physical, chemical and biological properties with respect to regular saccharides. The specific structural, conformational, stability, reactivity and interaction features of fluorinated sugars are described, as well as their applications as probes and in chemical biology.

Synthesis of C-glycosyl phosphonate derivatives of 4-amino-4-deoxy-α-ʟ-arabinose

The incorporation of basic substituents into the structurally conserved domains of cell wall lipopolysaccharides has been identified as a major mechanism contributing to antimicrobial resistance of Gram-negative pathogenic bacteria. Inhibition of the corresponding enzymatic steps, specifically the transfer of 4-amino-4-deoxy-ʟ-arabinose, would thus restore the activity of cationic antimicrobial peptides and several antimicrobial drugs. C-glycosidically-linked phospholipid derivatives of 4-amino-4-deoxy-ʟ-arabinose have been prepared as hydrolytically stable and chain-shortened analogues of the native undecaprenyl donor. The C-phosphonate unit was installed via a Wittig reaction of benzyl-protected 1,5-arabinonic acid lactone with the lithium salt of dimethyl methylphosphonate followed by an elimination step of the resulting hemiketal, leading to the corresponding exo- and endo-glycal derivatives. The ensuing selective monodemethylation and hydrogenolysis of the benzyl groups and reduction of the 4-azido group gave the α-ʟ-anomeric arabino- and ribo-configured methyl phosphonate esters. In addition, the monomethyl phosphonate glycal intermediates were converted into n-octyl derivatives followed by subsequent selective removal of the methyl phosphonate ester group and hydrogenation to give the octylphosphono derivatives. These intermediates will be of value for their future conversion into transition state analogues as well as for the introduction of various lipid extensions at the anomeric phosphonate moiety.

Rapid Entry into Biologically Relevant α,α-Difluoroalkylphosphonates Bearing Allyl Protection-Deblocking under Ru(II)/(IV)-Catalysis

A convenient synthetic route to α,α-difluoroalkylphosphonates is described. Structurally diverse aldehydes are condensed with LiF2CP(O)(OCH2CH═CH2)2. The resultant alcohols are captured as the pentafluorophenyl thionocarbonates and efficiently deoxygenated with HSnBu3, BEt3, and O2, and then smoothly deblocked with CpRu(IV)(π-allyl)quinoline-2-carboxylate (1-2 mol %) in methanol as an allyl cation scavenger. These mild deprotection conditions provide access to free α,α-difluoroalkylphosphonates in nearly quantitative yield. This methodology is used to rapidly construct new bis-α,α-difluoroalkyl phosphonate inhibitors of PTPIB (protein phosphotyrosine phosphatase-1B).

Synthesis, Derivatization, and Structural Analysis of Phosphorylated Mono-, Di-, and Trifluorinated d-Gluco-heptuloses by Glucokinase: Tunable Phosphoglucomutase Inhibition

Glucokinase phosphorylated a series of C-1 fluorinated α-d-gluco-heptuloses. These phosphorylated products were discovered to be inhibitors of α-phosphomannomutase/phosphoglucomutase (αPMM/PGM) and β-phosphoglucomutase (βPGM). Inhibition potency with both mutases inversely correlated to the degree of fluorination. Structural analysis with αPMM demonstrated the inhibitor binding to the active site, with the phosphate in the phosphate binding site and the anomeric hydroxyl directed to the catalytic site.

Pyrrolidine and oxazolidine ring transformations in proline and serine derivatives of α-hydroxyphosphonates induced by deoxyfluorinating reagents

Transformations of α-hydroxyphosphonates derived from proline or serine by treatment with different deoxyfluorinating reagents (DAST, Deoxofluor, PyFluor) are reported. Depending on the applied reagent, as well as the protecting group used (N-Cbz, N-Boc, N-Bn) different types of products are observed. The reaction of N-Cbz or N-Boc prolinols with DAST or Deoxofluor due to aziridinium intermediate participation gave fluorinated amino phosphonates such as piperidine and pyrrolidine derivatives and/or oxazolidine-2-ones. Similarly, the analogous reaction of N-Cbz or N-Boc protected serinol yielded oxazolidine-2-ones or its fluorinated analogues. As the second type of product formed by DAST-induced reaction of serine derivatives, aziridines were obtained. Only in the case of deoxyfluorination of N-benzyl prolinols were both diastereoisomers of β-fluoropiperidine-α-phosphonates formed, while the reaction of protected N-benzyl serinols gave fluorinated oxazolidines. Moreover, application of PyFluor gave sulfonate derivatives.

Diastereoselective reactions of hydroxyphosphonates from proline or serine with fluorinating agents yielding piperidine-, oxazolidine-, aziridine- or sulfonate phosphonates were reported.

The acidity of β-phosphoglucomutase monofluoromethylenephosphonate ligands probed by NMR spectroscopy and quantum mechanical methods

We recently described the binding of 1-β-phosphonomethylene-1-deoxy-d-glucopyranose, (S)-1-β-phosphonofluoromethylene-1-deoxy-D-glucopyranose (βG1CFSP), and (R)-1-β-phosphonofluoromethylene-1-deoxy-d-glucopyranose (βG1CFRP) to the enzyme β-phosphoglucomutase as transition state analogues of phosphoryl transfer through formation of stable MgF3− and AlF4− complexes (Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 12384). Herein, we describe in detail the five-fold difference in acidity (pKa2) for the (S)- and (R)-configured diastereomeric fluorophosphonates through a series of NMR spectroscopy experiments. The differences in acidity were corroborated using computational quantum mechanical calculations to determine structures of lowest energy conformers and provide insight into why the (S) isomer is substantially more acidic.

5'-β,γ-CHF-ATP diastereomers: synthesis and fluorine-mediated selective binding by c-Src protein kinase

The first preparation of the individual β,γ-CHF-ATP stereoisomers 12a and 12b is reported. Configurationally differing solely by the orientation of the C-F fluorine, 12a and 12b have discrete (31)P (202 MHz, pH 10.9, ΔδPα 6 Hz, ΔδPβ 4 Hz) and (19)F NMR (470 MHz, pH 9.8, ΔδF 25 Hz) spectral signatures and exhibit a 6-fold difference in IC50 values for c-Src kinase, attributed to a unique interaction of the (S)-fluorine of bound 12b with R388 in the active site.

Mechanistic evaluation of a nucleoside tetraphosphate with a thymidylyltransferase

Pyrimidine polyphosphates were first detected in cells 5 decades ago; however, their biological significance remains only partially resolved. Such nucleoside polyphosphates are believed to be produced nonspecifically by promiscuous enzymes. Herein, synthetically prepared deoxythymidine 5'-tetraphosphate (p4dT) was evaluated with a thymidylyltransferase, Cps2L. We have identified p4dT as a substrate for Cps2L and evaluated the reaction pathway by analysis of products using high-performance liquid chromatography, liquid chromatography and tandem mass spectrometry, and 31P nuclear magnetic resonance spectroscopy. Product analysis confirmed production of dTDP-Glc and triphosphate (P3) and showed no trace of dTTP-Glc and PPi, which could arise from alternative pathways for the reaction mechanism.

Polyphosphate-containing bisubstrate analogues as inhibitors of a bacterial cell wall thymidylyltransferase

The first synthesis and evaluation of bisubstrate analogues with a thymidylyltransferase is reported. WaterLOGSY NMR and kinetic analyses provide insight into bisubstrate analogue binding.

Selectfluor and NFSI exo-glycal fluorination strategies applied to the enhancement of the binding affinity of galactofuranosyltransferase GlfT2 inhibitors

Two complementary methods for the synthesis of fluorinated exo-glycals have been developed, for which previously no general reaction had been available. First, a Selectfluor-mediated fluorination was optimized after detailed analysis of all the reaction parameters. A dramatic effect of molecular sieves on the course of the reaction was observed. The reaction was generalized with a set of biologically relevant furanosides and pyranosides. A second direct approach involving carbanionic chemistry and the use of N-fluorobenzenesulfonimide (NFSI) was performed and this method gave better diastereoselectivities. Assignment of the Z/E configuration of all the fluorinated exo-glycals was achieved based on the results of HOESY experiments. Furthermore, fluorinated exo-glycal analogues of UDP-galactofuranose were prepared and assayed against GlfT2, which is a key enzyme involved in the cell-wall biosynthesis of major pathogens. The fluorinated exo-glycals proved to be potent inhibitors as compared with a series of C-glycosidic analogues of UDP-Galf, thus demonstrating the double beneficial effect of the exocyclic enol ether functionality and the fluorine atom.

α-Fluorophosphonates reveal how a phosphomutase conserves transition state conformation over hexose recognition in its two-step reaction

β-Phosphoglucomutase (βPGM) catalyzes isomerization of β-D-glucose 1-phosphate (βG1P) into D-glucose 6-phosphate (G6P) via sequential phosphoryl transfer steps using a β-D-glucose 1,6-bisphosphate (βG16BP) intermediate. Synthetic fluoromethylenephosphonate and methylenephosphonate analogs of βG1P deliver novel step 1 transition state analog (TSA) complexes for βPGM, incorporating trifluoromagnesate and tetrafluoroaluminate surrogates of the phosphoryl group. Within an invariant protein conformation, the β-D-glucopyranose ring in the βG1P TSA complexes (step 1) is flipped over and shifted relative to the G6P TSA complexes (step 2). Its equatorial hydroxyl groups are hydrogen-bonded directly to the enzyme rather than indirectly via water molecules as in step 2. The (C)O-P bond orientation for binding the phosphate in the inert phosphate site differs by ∼ 30° between steps 1 and 2. By contrast, the orientations for the axial O-Mg-O alignment for the TSA of the phosphoryl group in the catalytic site differ by only ∼ 5°, and the atoms representing the five phosphorus-bonded oxygens in the two transition states (TSs) are virtually superimposable. The conformation of βG16BP in step 1 does not fit into the same invariant active site for step 2 by simple positional interchange of the phosphates: the TS alignment is achieved by conformational change of the hexose rather than the protein.

On the observation of discrete fluorine NMR spectra for uridine 5'-β,γ-fluoromethylenetriphosphate diastereomers at basic pH

Jakeman et al. recently reported the inability to distinguish the diastereomers of uridine 5'-β,γ-fluoromethylenetriphosphate (β,γ-CHF-UTP, 1) by (19)F NMR under conditions we previously prescribed for the resolution of the corresponding β,γ-CHF-dGTP spectra, stating further that 1 decomposed under these basic conditions. Here we show that the (19)F NMR spectra of 1 (~1:1 diastereomer mixture prepared by coupling of UMP-morpholidate with fluoromethylenebis(phosphonic acid)) in D2O at pH 10 are indeed readily distinguishable. 1 in this solution was stable for 24 h at rt.

Phosphate analogues in the dissection of mechanism

Phosphoryl group transfer is central to genetic replication, cellular signalling and many metabolic processes. Understanding the mechanisms of phosphorylation and phosphate ester and anhydride cleavage is key to efforts towards biotechnological and biomedical exploitation of phosphate-handling enzymes. Analogues of phosphate esters and anhydrides are indispensable tools, alongside protein mutagenesis and computational methods, for the dissection of phosphoryl transfer mechanisms. Hydrolysable and non-hydrolysable phosphate analogues have provided insight into the nature and sites of phosphoryl transfer processes. Kinetic isotope effects and crystallography using transition state analogues have painted more detailed pictures of transition states and how enzymes work to stabilise them.

The effect of bisphosphonate acidity on the activity of a thymidylyltransferase

Thymidylyltransferases (thymidine diphospho pyrophosphorylases) are nucleotidylyltransferases that play key roles in the biosynthesis of carbohydrate components within bacterial cell walls and in the biosynthesis of glycosylated natural products. They catalyze the formation of sugar nucleotides concomitant with the release of pyrophosphate. Protein engineering of thymidylyltransferases has been an approach for the production of a variety of non-physiological sugar nucleotides. In this work, we have explored chemical approaches towards modifying the activity of the thymidylyltransferase (Cps2L) cloned from S. pneumoniae, through the use of chemically synthesized 'activated' nucleoside triphosphates with enhanced leaving groups, or by switching the metal ion co-factor specificity. Within a series of phosphonate-containing nucleoside triphosphate analogues, thymidylyltransferase activity is enhanced based on the acidity of the leaving group and a Brønsted-type analysis indicated that leaving group departure is rate limiting. We have also determined IC50 values for a series of bisphosphonates as inhibitors of thymidylyltransferases. No correlation between the acidity of the inhibitors (pKa) and the magnitude of enzyme inhibition was found.

Synthesis and evaluation of l-rhamnose 1C-phosphonates as nucleotidylyltransferase inhibitors

We report the synthesis of a series of phosphonates and ketosephosphonates possessing an L-rhamnose scaffold with varying degrees of fluorination. These compounds were evaluated as potential inhibitors of α-D-glucose 1-phosphate thymidylyltransferase (Cps2L), the first enzyme in Streptococcus pneumoniae L-rhamnose biosynthesis, and a novel antibiotic target. Enzyme-substrate and enzyme-inhibitor binding experiments were performed using water-ligand observed binding via gradient spectroscopy (WaterLOGSY) NMR for known sugar nucleotide substrates and selected phosphonate analogues. IC50 values were measured and Ki values were calculated for inhibitors. New insights were gained into the binding promiscuity of enzymes within the prokaryotic L-rhamnose biosynthetic pathway (Cps2L, RmlB-D) and into the mechanism of inhibition for the most potent inhibitor in the series, L-rhamnose 1C-phosphonate.

Thiophosphate and thiophosphonate analogues of glucose-1-phosphate: synthesis and enzymatic activity with a thymidylyltransferase

Synthetic methods were investigated for the preparation of O and S-glucosyl thiophosphates and glucosyl 1C-thiophosphonate. Four protected glucosyl thiophosphate compounds were synthesized and characterized as precursors to glucose 1-thiophosphate. The effect of various reaction conditions and the nature of the carbohydrate and thiophosphate protecting groups and how they impact both the yields and α/β diastereoselectivity of the glucosyl thiophosphate products were explored. A novel isomerization from an O-linked to S-linked glucosyl thiophosphate was observed. α-D-Glucose-1C-thiophosphonate was synthesized and evaluated as a substrate for the thymidylyltransferase, Cps2L. Tandem mass spectrometric analysis determined the position of sulfur in the sugar nucleotide product.