A convenient synthesis of the C-1-phosphonate analogue of UDP-GlcNAc and its evaluation as an inhibitor of O-linked GlcNAc transferase (OGT)

On a sugar high: Role of O-GlcNAcylation in cancer

Recent advances in the understanding of the molecular mechanisms underlying cancer progression have led to the development of novel therapeutic targeting strategies. Aberrant glycosylation patterns and their implication in cancer have gained increasing attention as potential targets due to the critical role of glycosylation in regulating tumor-specific pathways that contribute to cancer cell survival, proliferation, and progression. A special type of glycosylation that has been gaining momentum in cancer research is the modification of nuclear, cytoplasmic, and mitochondrial proteins, termed O-GlcNAcylation. This protein modification is catalyzed by an enzyme called O-GlcNAc transferase (OGT), which uses the final product of the Hexosamine Biosynthetic Pathway (HBP) to connect altered nutrient availability to changes in cellular signaling that contribute to multiple aspects of tumor progression. Both O-GlcNAc and its enzyme OGT are highly elevated in cancer and fulfill the crucial role in regulating many hallmarks of cancer. In this review, we present and discuss the latest findings elucidating the involvement of OGT and O-GlcNAc in cancer.

Tunable Approach to C-Linked Analogs of Glycosamines

The synthesis of (1R)-2-amino-2-deoxy-β-l-gulopyranosyl benzene and the α and β forms of 2-amino-2-deoxy-l-idopyranosyl benzene derivatives was accomplished through stereospecific addition of tributylstannyllithium to readily available (S_R)- or (S_S)-N-tert-butanesulfinyl-arabinofuranosylamine building blocks, followed by stereoretentive Pd-catalyzed Migita-Kosugi-Stille cross-coupling, stereoselective reduction, and an activation-cyclization strategy. Application of this methodology paves the way to new three-dimensional chemical space and preparation of unknown (non-natural) and complex 2-amino-2-deoxy sugars of biological interest.

Recent Advances in the Synthesis and Biological Applications of Peptidoglycan Fragments

The biosynthesis, breakdown, and modification of peptidoglycan (PG) play vital roles in both bacterial viability and in the response of human physiology to bacterial infection. Studies on PG biochemistry are hampered by the fact that PG is an inhomogeneous insoluble macromolecule. Chemical synthesis is therefore an important means to obtain PG fragments that may serve as enzyme substrates and elicitors of the human immune response. This review outlines the recent advances in the synthesis and biochemical studies of PG fragments, PG biosynthetic intermediates (such as Park's nucleotides and PG lipids), and PG breakdown products (such as muramyl dipeptides and anhydro-muramic acid-containing fragments). A rich variety of synthetic approaches has been applied to preparing such compounds since carbohydrate, peptide, and phospholipid chemical methodologies must all be applied.

Synthetic Route to Glycosyl β-1C-(phosphino)-phosphonates as Unprecedented Stable Glycosyl Diphosphate Analogs and Their Preliminary Biological Evaluation

The synthesis of glycosyl-β-1C-(phosphino)-phosphonates is a challenge since it has not yet been described. In this paper, we report an innovative synthetic method for their preparation from Glc-, Man-, and GlcNAc- lactone derivatives. The proposed original strategy involves the addition of the corresponding δ-hexonolactones onto the dianion of (methylphosphino) phosphonate as a key step, followed by dehydration and stereoselective addition of dihydrogen on the resulting double bond. Final deprotection provides the new glycosyl diphosphate analogs in 35%, 36%, and 10% yield over 6 steps from the corresponding δ-hexonolactones. The synthetized compounds were evaluated as inhibitors of phosphatase and diphosphatase activities and found to have complex concentration-dependent activatory and inhibitory properties on alkaline phosphatase. The synthetized tools should be useful to study other enzymes such as transferases.

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.

Demystifying O-GlcNAcylation: hints from peptide substrates

O-GlcNAcylation, analogous to phosphorylation, is an essential post-translational modification of proteins at Ser/Thr residues with a single β-N-acetylglucosamine moiety. This dynamic protein modification regulates many fundamental cellular processes and its deregulation has been linked to chronic diseases such as cancer, diabetes and neurodegenerative disorders. Reversible attachment and removal of O-GlcNAc is governed only by O-GlcNAc transferase and O-GlcNAcase, respectively. Peptide substrates, derived from natural O-GlcNAcylation targets, function in the catalytic cores of these two enzymes by maintaining interactions between enzyme and substrate, which makes them ideal models for the study of O-GlcNAcylation and deglycosylation. These peptides provide valuable tools for a deeper understanding of O-GlcNAc processing enzymes. By taking advantage of peptide chemistry, recent progress in the study of activity and regulatory mechanisms of these two enzymes has advanced our understanding of their fundamental specificities as well as their potential as therapeutic targets. Hence, this review summarizes the recent achievements on this modification studied at the peptide level, focusing on enzyme activity, enzyme specificity, direct function, site-specific antibodies and peptide substrate-inspired inhibitors.

Synthesis and Preliminary Evaluation of Biological Activity of Glycoconjugates Analogues of Acyclic Uridine Derivatives

Herein we present the methodology for obtaining glycosyltransferase inhibitors, analogues of natural enzyme substrates of donor-type: UDP-glucose and UDP-galactose. The synthesis concerned glycoconjugates, nucleoside analogues containing an acyclic ribose mimetic linked to a uracil moiety in their structure. The biological activity of the synthesised compounds was determined on the basis of their ability to inhibit the model enzyme action of β-1,4-galactosyltransferase from bovine milk. The obtained results allowed to expand and supplement the existing library of synthetic compounds that are able to regulate the biological activity of enzymes from the GT class.

Novel Uridine Glycoconjugates, Derivatives of 4-Aminophenyl 1-Thioglycosides, as Potential Antiviral Compounds

A novel series of uridine glycoconjugates, derivatives of 4-aminophenyl 1-thioglycosides, was designed and synthesized. All compounds were evaluated in vitro for their antiviral activity against hepatitis C virus (HCV) and classical swine fever virus (CSFV), two important human and animal viral pathogens for which new or improved therapeutic options are needed. The antiviral activity of all synthesized compounds was confirmed using pseudo-plaque reduction assays in which a significant arrest of CSFV and HCV growth was observed in the presence of these compounds. Two of the synthesized compounds, 9 and 12, displayed a significant inhibitory effect on HCV and CSFV propagation with IC₅₀ values of 4.9 and 13.5 µM for HCV and 4.2 and 4 µM for CSFV, respectively, with low cytotoxicity. Using various infection and replication models, we have shown that both compounds were able to significantly reduce viral genome replication by up to 90% with IC₅₀ values in the low micromolar range. A structure activity analysis of the synthesized compounds showed that the high antiviral activity was attributed to the hydrophobicity of glycoconjugates and the introduction of elements capable to coordinate metal ions into the spacer connecting the sugar and uridine moiety, which can be useful in the development of new antiviral compounds in the future.

Structural basis of the molecular ruler mechanism of a bacterial glycosyltransferase

The membrane-associated, processive and retaining glycosyltransferase PglH from Campylobacter jejuni is part of the biosynthetic pathway of the lipid-linked oligosaccharide (LLO) that serves as the glycan donor in bacterial protein N-glycosylation. Using an unknown counting mechanism, PglH catalyzes the transfer of exactly three α1,4 N-acetylgalactosamine (GalNAc) units to the growing LLO precursor, GalNAc-α1,4-GalNAc-α1,3-Bac-α1-PP-undecaprenyl. Here, we present crystal structures of PglH in three distinct states, including a binary complex with UDP-GalNAc and two ternary complexes containing a chemo-enzymatically generated LLO analog and either UDP or synthetic, nonhydrolyzable UDP-CH₂-GalNAc. PglH contains an amphipathic helix ("ruler helix") that has a dual role of facilitating membrane attachment and glycan counting. The ruler helix contains three positively charged side chains that can bind the pyrophosphate group of the LLO substrate and thus limit the addition of GalNAc units to three. These results, combined with molecular dynamics simulations, provide the mechanism of glycan counting by PglH.

Molecular basis of lipid-linked oligosaccharide recognition and processing by bacterial oligosaccharyltransferase

Oligosaccharyltransferase (OST) is a membrane-integral enzyme that catalyzes the transfer of glycans from lipid-linked oligosaccharides (LLOs) onto asparagine side chains, the first step in protein N-glycosylation. Here, we report the X-ray structure of a single-subunit OST, PglB from Campylobacter lari, trapped in an intermediate state bound to an acceptor peptide and a synthetic LLO analog. The structure reveals the role of the external loop EL5, present in all OST enzymes, in substrate recognition. Whereas the N-terminal half of EL5 binds LLO, the C-terminal half interacts with the acceptor peptide. The glycan moiety of LLO must thread under EL5 to access the active site. Reducing EL5 mobility decreases the catalytic rate of OST when full-size heptasaccharide LLO is provided, but not for a monosaccharide-containing LLO analog. Our results define the chemistry of a ternary complex state, assign functional roles to conserved OST motifs, and provide opportunities for glycoengineering by rational design of PglB.

Novel thioglycosyl analogs of glycosyltransferase substrates as antiviral compounds against classical swine fever virus and hepatitis C virus

Hepatitis C virus (HCV) and classical swine fever virus (CSFV) are important pathogens for which new therapeutic approaches are in high demand. Herein, we report the synthesis of newly designed thioglycosyl analogs of glycosyltransferase substrates which were evaluated using cell-based assays for cytotoxicity and antiviral activity against both viruses. The antiviral activity of synthesized compounds against CSFV and HCV was confirmed using pseudo-plaque reduction assays where a significant arrest of viral growth was observed in the presence of selected compounds. We showed that compounds 13 and 14 exerted the most significant inhibitory effect on in vitro CSFV and HCV infections in the series. Glycoconjugates 13 and 14 not only inhibited both viral propagation with IC₅₀ values in low micromolar range, but efficiently suppressed the production of viral proteins in a dose-dependent manner. In addition, studies using in vitro HCV infection and replication models have shown that both compounds are able to significantly reduce viral genomic replication. We demonstrated that compounds 13 and 14 showed a strong inhibition, up to 90% of replication which inscribe them in the promising alternative approach for the development of new anti-CSFV and anti-HCV drugs.

Glycomimetics Targeting Glycosyltransferases: Synthetic, Computational and Structural Studies of Less-Polar Conjugates

The Leloir donors are nucleotide sugars essential for a variety of glycosyltransferases (GTs) involved in the transfer of a carbohydrate to an acceptor substrate, typically a protein or an oligosaccharide. A series of less-polar nucleotide sugar analogues derived from uridine have been prepared by replacing one phosphate unit with an alkyl chain. The methodology is based on the radical hydrophosphonylation of alkenes, which allows coupling of allyl glycosyl compounds with a phosphate unit suitable for conjugation to uridine. Two of these compounds, the GalNAc and galactose derivatives, were further tested on a model GT, such as GalNAc-T2 (an important GT widely distributed in human tissues), to probe that both compounds bound in the medium-high micromolar range. The crystal structure of GalNAc-T2 with the galactose derivative traps the enzyme in an inactive form; this suggests that compounds only containing the β-phosphate could be efficient ligands for the enzyme. Computational studies with GalNAc-T2 corroborate these findings and provide further insights into the mechanism of the catalytic cycle of this family of enzymes.

Synthesis of 1-C-Glycoside-Linked Lipid II Analogues Toward Bacterial Transglycosylase Inhibition

Preparation of Lipid II analogues containing an enzymatically uncleavable 1-C-glycoside linkage between the disaccharide moiety and the pyrophosphate- or pyrophosphonate-lipid moiety is described. The synthesis of a common 1-C-vinyl disaccharide intermediate has been developed that allows easy preparation of both an elongated sugar-phosphate bond and a sugar-phosphonate moiety, which are coupled with the polyprenyl phosphate to give the desired molecules. Inhibition studies show how a subtle structural modification results in dramatically different potency toward bacterial transglycosylase (TGase), and the results identify Lipid II-C-O-PP (IC50 =25 μM) as a potential TGase inhibitor.

Chemical tools to explore nutrient-driven O-GlcNAc cycling

Posttranslational modifications (PTM) including glycosylation, phosphorylation, acetylation, methylation and ubiquitination dynamically alter the proteome. The evolutionarily conserved enzymes O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase are responsible for the addition and removal, respectively, of the nutrient-sensitive PTM of protein serine and threonine residues with O-GlcNAc. Indeed, the O-GlcNAc modification acts at every step in the "central dogma" of molecular biology and alters signaling pathways leading to amplified or blunted biological responses. The cellular roles of OGT and the dynamic PTM O-GlcNAc have been clarified with recently developed chemical tools including high-throughput assays, structural and mechanistic studies and potent enzyme inhibitors. These evolving chemical tools complement genetic and biochemical approaches for exposing the underlying biological information conferred by O-GlcNAc cycling.

Stereoselective synthesis of fluorinated aminoglycosyl phosphonates

We report the highly stereoselective addition of lithiated difluorophosphonates to nitroglycals, providing synthetic access to biologically relevant fluorinated aminoglycosyl phosphonates.

Design of glycosyltransferase inhibitors: pyridine as a pyrophosphate surrogate

A series of ten glycosyltransferase inhibitors has been designed and synthesized by using pyridine as a pyrophosphate surrogate. The series was prepared by conjugation of carbohydrate, pyridine, and nucleoside building blocks by using a combination of glycosylation, the Staudinger-Vilarrasa amide-bond formation, and azide-alkyne click chemistry. The compounds were evaluated as inhibitors of five metal-dependent galactosyltransferases. Crystallographic analyses of three inhibitors complexed in the active site of one of the enzymes confirmed that the pyridine moiety chelates the Mn(2+) ion causing a slight displacement (2 Å) from its original position. The carbohydrate head group occupies a different position than in the natural uridine diphosphate (UDP)-Gal substrate with little interaction with the enzyme.

The importance of hydrogen bonding and aromatic stacking to the affinity and efficacy of cannabinoid receptor CB2 antagonist, 5-(4-chloro-3-methylphenyl)-1-[(4-methylphenyl)methyl]-N-[(1S,2S,4R)-1,3,3-trimethylbicyclo[2.2.1]hept-2-yl]-1H-pyrazole-3-carboxamide (SR144528)

Despite the therapeutic promise of the subnanomolar affinity cannabinoid CB2 antagonist, 5-(4-chloro-3-methylphenyl)-1-[(4-methylphenyl)methyl]-N-[(1S,2S,4R)-1,3,3-trimethylbicyclo[2.2.1]hept-2-yl]-1H-pyrazole-3-carboxamide (SR144528, 1), little is known about its binding site interactions and no primary interaction site for 1 at CB2 has been identified. We report here the results of Glide docking studies in our cannabinoid CB2 inactive state model that were then tested via compound synthesis, binding, and functional assays. Our results show that the amide functional group of 1 is critical to its CB2 affinity and efficacy and that aromatic stacking interactions in the TMH5/6 aromatic cluster of CB2 are also important. Molecular modifications that increased the positive electrostatic potential in the region between the fenchyl and aromatic rings led to more efficacious compounds. This result is consistent with the EC-3 loop negatively charged amino acid, D275 (identified via Glide docking studies) acting as the primary interaction site for 1 and its analogues.

Enzymatic characterization of recombinant enzymes of O-GlcNAc cycling

The dynamic addition of O-GlcNAc to target proteins is now recognized as a major signaling paradigm impacting phosphorylation, protein turnover, gene expression, and other posttranslational modifications influencing epigenetics. Here we describe the production of and methods for assay of the recombinant enzymes of O-GlcNAc cycling: O-linked GlcNAc Transferase (OGT) and O-GlcNAcase (OGA).

A neutral diphosphate mimic crosslinks the active site of human O-GlcNAc transferase

Glycosyltransferases (Gtfs) catalyze the formation of a diverse array of glycoconjugates. Small molecule inhibitors to manipulate Gtf activity in cells have long been sought as tools to understand Gtf function. Success has been limited due to challenges in designing inhibitors that mimic the negatively-charged diphosphate substrates. Here we report the mechanism of action of a small molecule that inhibits O-GlcNAc transferase (OGT), an essential human enzyme that modulates cell signaling pathways by catalyzing a unique intracellular post translational modification, β-O-GlcNAcylation. The molecule contains a five heteroatom dicarbamate core that functions as a neutral diphosphate mimic. One dicarbamate carbonyl reacts with an essential active site lysine that anchors the diphosphate of the nucleotide-sugar substrate. The lysine adduct reacts again with a nearby cysteine to crosslink the OGT active site. While this unprecedented mechanism reflects the unique architecture of the OGT active site, related dicarbamate scaffolds may inhibit other enzymes that bind diphosphate containing substrates.

Synthesis and biological evaluation of potential new inhibitors of the bacterial transferase MraY with a β-ketophosphonate structure

Stable analogs of bacterial transferase MraY substrate or product with a pyrophosphate surrogate in their structure are described. β-ketophosphonates were designed as pyrophosphate bioisosteres and were investigated as UDP-GlcNAc mimics. The developed strategy allows introduction of structural diversity at a late stage of the synthesis. The biological activity of the synthesized compounds was evaluated on the MraY enzyme.

The first C-glycosidic analogue of a novel galactosyltransferase inhibitor

Structural analogues and mimics of the natural sugar-nucleotide UDP-galactose (UDP-Gal) are sought after as chemical tools for glycobiology and drug discovery. We have recently developed a novel class of galactosyltransferase (GalT) inhibitors derived from UDP-Gal, bearing an additional substituent at the 5-position of the uracil base. Herein we report the first C-glycosidic derivative of this new class of GalT inhibitors. We describe a practical convergent synthesis of the new UDP-C-Gal derivative, including a systematic study into the use of radical chemistry for the preparation of galactosyl ethylphosphonate, a key synthetic intermediate. The new inhibitor showed activity against a bacterial UDP-Gal 4'-epimerase at micromolar concentrations. This is the first example of a base-modified UDP-sugar as an inhibitor of a UDP-sugar-dependent enzyme which is not a glycosyltransferase, and these results may therefore have implications for the design of inhibitors of these enzymes in the future.

Chemoenzymatic synthesis, inhibition studies, and X-ray crystallographic analysis of the phosphono analog of UDP-Galp as an inhibitor and mechanistic probe for UDP-galactopyranose mutase

UDP (uridine diphosphate) galactopyranose mutase (UGM) is involved in the cell wall biosynthesis of many pathogenic microorganisms. UGM catalyzes the reversible conversion of UDP-α-D-galactopyranose into UDP-α-D-galactofuranose, with the latter being the precursor of galactofuranose (Galf) residues in cell walls. Glycoconjugates of Galf are essential components in the cell wall of various pathogenic bacteria, including Mycobacterium tuberculosis, the causative agent of tuberculosis. The absence of Galf in humans and its bacterial requirement make UGM a potential target for developing novel antibacterial agents. In this article, we report the synthesis, inhibitory activity, and X-ray crystallographic studies of UDP-phosphono-galactopyranose, a nonhydrolyzable C-glycosidic phosphonate. This is the first report on the synthesis of a phosphonate analog of UDP-α-D-galactopyranose by a chemoenzymatic phosphoryl coupling method. The phosphonate was evaluated against three bacterial UGMs and showed only moderate inhibition. We determined the crystal structure of the phosphonate analog bound to Deinococcus radiodurans UGM at 2.6 Å resolution. The phosphonate analog is bound in a novel conformation not observed in UGM-substrate complex structures or in other enzyme-sugar nucleotide phosphonate complexes. This complex structure provides a structural basis for the observed micromolar inhibition towards UGM. Steric clashes, loss of electrostatic stabilization between an active-site arginine (Arg305) and the phosphonate analog, and a 180° flip of the hexose moiety account for the differences in the binding orientations of the isosteric phosphonate analog and the physiological substrate. This provides new insight into the ability of a sugar-nucleotide-binding enzyme to orient a substrate analog in an unexpected geometry and should be taken into consideration in designing such enzyme inhibitors.

Substrate and product analogues as human O-GlcNAc transferase inhibitors

Protein glycosylation on serine/threonine residues with N-acetylglucosamine (O-GlcNAc) is a dynamic, inducible and abundant post-translational modification. It is thought to regulate many cellular processes and there are examples of interplay between O-GlcNAc and protein phosphorylation. In metazoa, a single, highly conserved and essential gene encodes the O-GlcNAc transferase (OGT) that transfers GlcNAc onto substrate proteins using UDP-GlcNAc as the sugar donor. Specific inhibitors of human OGT would be useful tools to probe the role of this post-translational modification in regulating processes in the living cell. Here, we describe the synthesis of novel UDP-GlcNAc/UDP analogues and evaluate their inhibitory properties and structural binding modes in vitro alongside alloxan, a previously reported weak OGT inhibitor. While the novel analogues are not active on living cells, they inhibit the enzyme in the micromolar range and together with the structural data provide useful templates for further optimisation.

Synthesis and evaluation of non-hydrolyzable D-mannose 6-phosphate surrogates reveal 6-deoxy-6-dicarboxymethyl-D-mannose as a new strong inhibitor of phosphomannose isomerases

Non-hydrolyzable d-mannose 6-phosphate analogues in which the phosphate group was replaced by a phosphonomethyl, a dicarboxymethyl, or a carboxymethyl group were synthesized and kinetically evaluated as substrate analogues acting as potential inhibitors of type I phosphomannose isomerases (PMIs) from Saccharomyces cerevisiae and Escherichia coli. While 6-deoxy-6-phosphonomethyl-d-mannose and 6-deoxy-6-carboxymethyl-D-mannose did not inhibit the enzymes significantly, 6-deoxy-6-dicarboxymethyl-D-mannose appeared as a new strong competitive inhibitor of both S. cerevisiae and E. coli PMIs with K(m)/K(i) ratios of 28 and 8, respectively. We thus report the first malonate-based inhibitor of an aldose-ketose isomerase to date. Phosphonomethyl mimics of the 1,2-cis-enediolate high-energy intermediate postulated for the isomerization reaction catalyzed by PMIs were also synthesized but behave as poor inhibitors of PMIs. A polarizable molecular mechanics (SIBFA) study was performed on the complexes of d-mannose 6-phosphate and two of its analogues with PMI from Candida albicans, an enzyme involved in yeast infection homologous to S. cerevisiae and E. coli PMIs. It shows that effective binding to the catalytic site occurs with retention of the Zn(II)-bound water molecule. Thus the binding of the hydroxyl group on C1 of the ligand to Zn(II) should be water-mediated. The kinetic study reported here also suggests the dianionic character of the phosphate surrogate as a likely essential parameter for strong binding of the inhibitor to the enzyme active site.

Structural analyses of enzymes involved in the O-GlcNAc modification

In order to study the O-GlcNAc modification in vivo, it is evident that a range of specific small molecule inhibitors would be a valuable asset. One strategy for the design of such compounds would be to utilise 3-D structural information in tandem with knowledge of catalytic mechanism. The last few years has seen major breakthroughs in our understanding of the 3-D structure of the enzymes involved in the O-GlcNAc modification notably from the study of the tetratricopeptide repeat (TPR) domain of the human O-GlcNAc transferase, of the bacterial homologs of the O-GlcNAc hydrolase and more latterly bacterial homologs of the O-GlcNAc transferase itself. Of particular note are the bacterial O-GlcNAc hydrolase homologs that provide near identical active centres to the human enzyme. These have informed the design and/or subsequent analysis of inhibitors of this enzyme which have found great use in the chemical dissection of the O-GlcNAc in vivo, as described by Macauley and Vocadlo elsewhere in this issue.

Molecular recognition in the P2Y(14) receptor: Probing the structurally permissive terminal sugar moiety of uridine-5'-diphosphoglucose

The P2Y(14) receptor, a nucleotide signaling protein, is activated by uridine-5'-diphosphoglucose 1 and other uracil nucleotides. We have determined that the glucose moiety of 1 is the most structurally permissive region for designing analogues of this P2Y(14) agonist. For example, the carboxylate group of uridine-5'-diphosphoglucuronic acid proved to be suitable for flexible substitution by chain extension through an amide linkage. Functionalized congeners containing terminal 2-acylaminoethylamides prepared by this strategy retained P2Y(14) activity, and molecular modeling predicted close proximity of this chain to the second extracellular loop of the receptor. In addition, replacement of glucose with other sugars did not diminish P2Y(14) potency. For example, the [5'']ribose derivative had an EC(50) of 0.24muM. Selective monofluorination of the glucose moiety indicated a role for the 2''- and 6''-hydroxyl groups of 1 in receptor recognition. The beta-glucoside was twofold less potent than the native alpha-isomer, but methylene replacement of the 1''-oxygen abolished activity. Replacement of the ribose ring system with cyclopentyl or rigid bicyclo[3.1.0]hexane groups abolished activity. Uridine-5'-diphosphoglucose also activates the P2Y(2) receptor, but the 2-thio analogue and several of the potent modified-glucose analogues were P2Y(14)-selective.

Chemoenzymatic syntheses of carbasugar analogues of nucleoside diphosphate sugars: UDP-carba-Gal, UDP-carba-GlcNAc, UDP-carba-Glc, and GDP-carba-Man

Chemoenzymatic syntheses of several NDP-carba-sugars have been successfully carried out, and these essential cofactor analogues are expected to be selective inhibitors of glycosyltransferase enzymes.

Structural insights into mechanism and specificity of O-GlcNAc transferase

Post-translational modification of protein serines/threonines with N-acetylglucosamine (O-GlcNAc) is dynamic, inducible and abundant, regulating many cellular processes by interfering with protein phosphorylation. O-GlcNAcylation is regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase, both encoded by single, essential, genes in metazoan genomes. It is not understood how OGT recognises its sugar nucleotide donor and performs O-GlcNAc transfer onto proteins/peptides, and how the enzyme recognises specific cellular protein substrates. Here, we show, by X-ray crystallography and mutagenesis, that OGT adopts the (metal-independent) GT-B fold and binds a UDP-GlcNAc analogue at the bottom of a highly conserved putative peptide-binding groove, covered by a mobile loop. Strikingly, the tetratricopeptide repeats (TPRs) tightly interact with the active site to form a continuous 120 Å putative interaction surface, whereas the previously predicted phosphatidylinositide-binding site locates to the opposite end of the catalytic domain. On the basis of the structure, we identify truncation/point mutants of the TPRs that have differential effects on activity towards proteins/peptides, giving first insights into how OGT may recognise its substrates.