Synthesis of a Glycosylphosphatidylinositol Anchor Derived from Leishmania donovani That Can Be Functionalized by Cu-Catalyzed Azide-Alkyne Cycloadditions

Structures, functions, and syntheses of glycero-glycophospholipids

Biological membranes consist of integral and peripheral protein-associated lipid bilayers. Although constituent lipids vary among cells, membrane lipids are mainly classified as phospholipids, glycolipids, and sterols. Phospholipids are further divided into glycerophospholipids and sphingophospholipids, whereas glycolipids are further classified as glyceroglycolipids and sphingoglycolipids. Both glycerophospholipids and glyceroglycolipids contain diacylglycerol as the common backbone, but their head groups differ. Most glycerolipids have polar head groups containing phosphate esters or sugar moieties. However, trace components termed glycero-glycophospholipids, each possessing both a phosphate ester and a sugar moiety, exist in membranes. Recently, the unique biological activities of glycero-glycophospholipids have attracted considerable attention. In this review, we describe the structure, distribution, function, biosynthesis, and chemical synthetic approaches of representative glycero-glycophospholipids-phosphatidylglucoside (PtdGlc) and enterobacterial common antigen (ECA). In addition, we introduce our recent studies on the rare glycero-glyco"pyrophospho"lipid, membrane protein integrase (MPIase), which is involved in protein translocation across biomembranes.

Diversity-Oriented Synthesis of Glycosylphosphatidylinositol Probes Based on an Orthogonally Protected Pentasaccharide

Two glycosylphosphatidylinositol (GPI) derivatives having an alkynyl group at different positions were derived from the same orthogonally protected pentasaccharide that in turn was assembled by a convergent [3+2] glycosylation strategy. The resultant alkynylated GPIs are useful biological probes and are suitable for further modification by click reaction to obtain other GPI probes. The pentasaccharide is a versatile platform for the synthesis of various uniquely functionalized GPI probes.

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Virulence factors of Leishmania parasite: Their paramount importance in unraveling novel vaccine candidates and therapeutic targets

Leishmaniasis is a neglected tropical disease and remains a global concern for healthcare. It is caused by an opportunistic protozoan parasite belonging to the genus Leishmania and affects millions worldwide. This disease is mainly prevalent in tropical and subtropical regions and is associated with a high risk of public morbidity and mortality if left untreated. Transmission of this deadly disease is aggravated by the bite of female sand-fly vectors (Phlebotomus and Lutzomyia). With time, significant advancement in leishmaniasis-related research has been carried out to cope with the disease burden. Still, the Leishmania parasite has also co-evolved with its host and adapted successfully within the host's lethal milieu/environment. Thus, understanding and knowledge of various leishmanial virulence factors responsible for the parasitic infection are essential for exploring drug targets and vaccine candidates. The present review elucidates the importance of virulence factors in pathogenesis and summarizes the major leishmanial virulence molecules contributing to the parasitic infection during host-pathogen interaction. Furthermore, we have also elaborated on the potential contribution of leishmanial virulence proteins in developing vaccine candidates and exploring novel therapeutics against this parasitic disease. We aim to represent a clearer picture of parasite pathogenesis within the human host that can further aid in unraveling new strategies to fight against the deadly infection of leishmaniasis.

Rescue of Glycosylphosphatidylinositol-Anchored Protein Biosynthesis Using Synthetic Glycosylphosphatidylinositol Oligosaccharides

The attachment of proteins to the cell membrane using a glycosylphosphatidylinositol (GPI) anchor is a ubiquitous process in eukaryotic cells. Deficiencies in the biosynthesis of GPIs and the concomitant production of GPI-anchored proteins lead to a series of rare and complicated disorders associated with inherited GPI deficiencies (IGDs) in humans. Currently, there is no treatment for patients suffering from IGDs. Here, we report the design, synthesis, and use of GPI fragments to rescue the biosynthesis of GPI-anchored proteins (GPI-APs) caused by mutation in genes involved in the assembly of GPI-glycolipids in cells. We demonstrated that the synthetic fragments GlcNAc-PI (1), Man-GlcN-PI (5), and GlcN-PI with two (3) and three lipid chains (4) rescue the deletion of the GPI biosynthesis in cells devoid of the PIGA, PIGL, and PIGW genes in vitro. The compounds allowed for concentration-dependent recovery of GPI biosynthesis and were highly active on the cytoplasmic face of the endoplasmic reticulum membrane. These synthetic molecules are leads for the development of treatments for IGDs and tools to study GPI-AP biosynthesis.

Convergent Chemoenzymatic Strategy to Deliver a Diversity of Shigella flexneri Serotype-Specific O-Antigen Segments from a Unique Lightly Protected Tetrasaccharide Core

Progress in glycoscience is strongly dependent on the availability of broadly diverse tailor-made, well-defined, and often complex oligosaccharides. Herein, going beyond natural resources and aiming to circumvent chemical boundaries in glycochemistry, we tackle the development of an in vitro chemoenzymatic strategy holding great potential to answer the need for molecular diversity characterizing microbial cell-surface carbohydrates. The concept is exemplified in the context of Shigella flexneri, a major cause of diarrhoeal disease. Aiming at a broad serotype coverage S. flexneri glycoconjugate vaccine, a non-natural lightly protected tetrasaccharide was designed for compatibility with (i) serotype-specific glucosylations and O-acetylations defining S. flexneri O-antigens, (ii) recognition by suitable α-transglucosylases, and (iii) programmed oligomerization following enzymatic α-d-glucosylation. The tetrasaccharide core was chemically synthesized from two crystalline monosaccharide precursors. Six α-transglucosylases found in the glycoside hydrolase family 70 were shown to transfer glucosyl residues on the non-natural acceptor. The successful proof of concept is achieved for a pentasaccharide featuring the glucosylation pattern from the S. flexneri type IV O-antigen. It demonstrates the potential of appropriately planned chemoenzymatic pathways involving non-natural acceptors and low-cost donor/transglucosylase systems to achieve the demanding regioselective α-d-glucosylation of large substrates, paving the way to microbial oligosaccharides of vaccinal interest.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

A practical and scalable synthesis of KRN7000 using glycosyl iodide as the glycosyl donor

KRN7000 is particularly useful because it is a powerful and specific CD1d agonist and has prompted intense interest in the context of immunology in the past 25 years. Its limited commercial availability and high price has led to the publication of many different syntheses. However, almost all of them focused on the methodology development rather than a scalable synthesis. Herein, we have described a practical and scalable procedure for the synthesis of KRN7000 basing on the glycosyl iodide method. This procedure involves total of eight steps to obtain the highly pure product KNR7000 on gram scale from the commercially available starting materials (d-galactose and the phytosphingosine) with only three column chromatographic purifications.

Advances in the Chemical Synthesis of Carbohydrates and Glycoconjugates

Carbohydrates are functional and structural biomolecules with structures ranging from monosaccharides to polysaccharides. They are naturally found as pure glycans or attached to lipids and proteins forming glycoconjugates. The biosynthesis of carbohydrates is not genetically controlled. The regulation takes place by the expression of enzymes that transfer and hydrolyze the glycan units, leading to glycocojugates having complex mixtures of glycan structures. Chemical synthesis emerged as the best strategy to obtain defined glycan and glycoconjugates and overcome the challenging purification processes. Here, we review the recent advances in the synthesis of oligosaccharides using manual and automated methods. The chapter covers the methods for the preparation of building blocks and control of stereoselectivity and regioselectivity during glycosylations. Finally, it also presents the strategies to obtain natural and non-natural glycoconjugates with lipids and proteins.

Synthesis of monophosphoryl lipid A using 2-naphtylmethyl ethers as permanent protecting groups

Lipid A, which is a conserved component of lipopolysaccharides of gram-negative bacteria, has attracted considerable interest for the development of immuno-adjuvants. Most approaches for lipid A synthesis rely on the use of benzyl ethers as permanent protecting groups. Due to the amphiphilic character of lipid A, these compounds aggregate during the hydrogenation step to remove benzyl ethers, resulting in a sluggish reaction and by-product formation. To address this problem, we have developed a synthetic approach based on the use of 2-naphtylmethyl ether (Nap) ethers as permanent protecting group for hydroxyls. At the end of a synthetic sequence, multiple of these protecting groups can readily be removed by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Di-allyl N,N-diisopropylphosphoramidite was employed to install the phosphate ester and the resulting allyl esters were cleaved using palladium tetrakistriphenylphosphine. The synthetic strategy allows late stage introduction of different fatty acids at the amines of the target compound, which is facilitated by Troc and Fmoc as orthogonal amino-protecting groups.

Leishmania lipophosphoglycan components: A potent target for synthetic neoglycoproteins as a vaccine candidate for leishmaniasis

Leishmania is an obligate intracellular pathogen that invades phagocytic host cells. Due to its high morbidity and mortality rates, leishmaniasis attracts significant attention. The disease, which is caused by Leishmania parasites, is distributed worldwide, particularly among developing communities, and causes fatal complications if not treated expediently. Unfortunately, the existing treatments are not preventive and do not impede Leishmania infection. Many drugs available for leishmaniasis are becoming less effective due to emerging resistance in some Leishmania species. Other drugs have drawbacks such as low cost-effectiveness, toxicity, and side effects. The World Health Organization (WHO) considers leishmaniasis to be a major public health problem and suggests that the best prevention is to develop a vaccine for this dangerous disease. In this review, we focus on the unique components of lipophosphoglycan (LPG), a component of the Leishmania cell wall, particularly [Galp(1 → 4)-β-[Manp-(1 → 2)-α-Manp-(1 → 2)-α]-Manp] in the cryptic tetrasaccharide cap, and on synthetic approaches as a potent candidate for a leishmaniasis vaccine.

Investigation of the remote acyl group participation in glycosylation from conformational perspectives by using trichloroacetimidate as the acetyl surrogate

The remote acyl group participation in glycosylation was studied by using trichloroacetimidate as the acetyl surrogate. The bridging participation intermediates were systematically trapped, and DFT calculations were applied to explain the results.

Synthetic Phosphodiester-Linked 4-Amino-4-deoxy-l-arabinose Derivatives Demonstrate that ArnT is an Inverting Aminoarabinosyl Transferase

4‐Amino‐4‐deoxy‐l‐arabinopyranose (Ara4N) residues have been linked to antibiotic resistance due to reduction of the negative charge in the lipid A and core regions of the bacterial lipopolysaccharide (LPS). To study the enzymatic transfer of Ara4N onto lipid A, which is catalysed by the ArnT transferase, we chemically synthesised a series of anomeric phosphodiester‐linked lipid Ara4N derivatives containing linear aliphatic chains as well as E‐ and Z‐configured monoterpene units. Coupling reactions were based on sugar‐derived H‐phosphonates, followed by oxidation and global deprotection. The enzymatic Ara4N transfer was performed in vitro with crude membranes from a deep‐rough mutant from Escherichia coli as acceptor. Product formation was detected by TLC and LC‐ESI‐QTOF mass spectrometry. Out of seven analogues tested, only the α‐neryl derivative was accepted by the Burkholderia cenocepacia ArnT protein, leading to substitution of the Kdo₂‐lipid A acceptor and thus affording evidence that ArnT is an inverting glycosyl transferase that requires the Z‐configured double bond next to the anomeric phosphate moiety. This approach provides an easily accessible donor substrate for biochemical studies relating to modifications of bacterial LPS that modulate antibiotic resistance and immune recognition.