Chemoselective ligation applied to the synthesis of a biantennary N-linked glycoform of CD52

One-Pot Glycosylation Strategy Assisted by Ion Mobility-Mass Spectrometry Analysis toward the Synthesis of N-Linked Oligosaccharides

N-Glycans are major constituents of several cellular glycoproteins. One-pot strategies for the synthesis of N-glycans are crucial for the rapid generation of pure samples to determine their biological functions. Herein, we describe a double one-pot strategy for the synthesis of N-glycans assisted by an IM-MS analysis approach for rapid screening of optimized glycosylation reaction conditions. This research includes triflate-mediated direct β-mannosylation and tandem glycosylation in a one-pot strategy for the synthesis of the challenging N-linked trisaccharide core β-5. Furthermore, a one-pot sequential glycosylation of the N-linked trisaccharide core 7 furnishes diverse high-mannose type N-glycans with excellent stereo- and regioselectivities. In particular, ion mobility-mass spectrometry-based quantitative analysis is applied to identify the stereo- and regioselective outcomes of the crude reaction mixtures to develop a highly efficient one-pot protocol.

Zinc(II) Iodide-Directed β-Mannosylation: Reaction Selectivity, Mode, and Application

A direct, efficient, and versatile glycosylation methodology promises the systematic synthesis of oligosaccharides and glycoconjugates in a streamlined fashion like the synthesis of medium to long-chain nucleotides and peptides. The development of a generally applicable approach for the construction of 1,2-cis-glycosidic bond with controlled stereoselectivity remains a major challenge, especially for the synthesis of β-mannosides. Here, we report a direct mannosylation strategy mediated by ZnI₂, a mild Lewis acid, for the highly stereoselective construction of 1,2-cis-β linkages employing easily accessible 4,6-O-tethered mannosyl trichloroacetimidate donors. The versatility and effectiveness of this strategy were demonstrated with successful β-mannosylation of a wide variety of alcohol acceptors, including complex natural products, amino acids, and glycosides. Through iteratively performing ZnI₂-mediated mannosylation with the chitobiosyl azide acceptor followed by site-selective deprotection of the mannosylation product, the novel methodology enables the modular synthesis of the key intermediate trisaccharide with Man-β-(1 → 4)-GlcNAc-β-(1 → 4)-GlcNAc linkage for N-glycan synthesis. Theoretical investigations with density functional theory calculations delved into the mechanistic details of this β-selective mannosylation and elucidated two zinc cations' essential roles as the activating agent of the donor and the principal mediator of the cis-directing intermolecular interaction.

Chemical Synthesis of Homogeneous Human E-Cadherin N-Linked Glycopeptides: Stereoselective Convergent Glycosylation and Chemoselective Solid-Phase Aspartylation

We report herein an efficient chemical synthesis of homogeneous human E-cadherin N-linked glycopeptides consisting of a heptapeptide sequence adjacent to the Asn-633 N-glycosylation site with representative N-glycan structures, including a conserved trisaccharide, a core-fucosylated tetrasaccharide, and a complex-type biantennary octasaccharide. The key steps are a chemoselective on-resin aspartylation using a pseudoproline-containing peptide and stereoselective glycosylation using glycosyl fluororide as a donor. This synthetic strategy demonstrates potential utility in accessing a wide range of homogeneous N-linked glycopeptides for the examination of their biological function.

Absolute Quantitation of Glycoforms of Two Human IgG Subclasses Using Synthetic Fc Peptides and Glycopeptides

Immunoglobulins, such as immunoglobulin G (IgG), are of prime importance in the immune system. Polyclonal human IgG comprises four subclasses, of which IgG1 and IgG2 are the most abundant in healthy individuals. In an effort to develop an absolute MALDI-ToF-MS quantitative method for these subclasses and their Fc N-glycoforms, (glyco)peptides were synthesized using a solid-phase approach and used as internal standards. Tryptic digest glycopeptides from monoclonal IgG1 and IgG2 samples were first quantified using EEQYN(GlcNAc)STYR and EEQFN(GlcNAc)STFR standards, respectively. For IgG1, a similar glycopeptide where tyrosine (Y) was isotopically labelled was used to quantify monoclonal IgG1 that had been treated with the enzyme Endo-F2, i.e., yielding tryptic glycopeptide EEQYN(GlcNAc)STYR. The next step was to quantify single subclasses within polyclonal human IgG samples. Although ion abundances in the MALDI spectra often showed higher signals for IgG2 than IgG1, depending on the spotting solvent used, determination of amounts using the newly developed quantitative method allowed to obtain accurate concentrations where IgG1 species were predominant. It was observed that simultaneous analysis of IgG1 and IgG2 yielded non-quantitative results and that more success was obtained when subclasses were quantified one by one. More experiments served to assess the respective extraction and ionization efficiencies of EEQYNSTYR/EEQFNSTFR and EEQYN(GlcNAc)STYR/EEQFN(GlcNAc)STFR mixtures under different solvent and concentration conditions. Graphical Abstract ᅟ.

Synthesis of Two Tetrasaccharide Pentenyl Glycosides Related to the Pectic Rhamnogalacturonan I Polysaccharide

The synthesis of two protected tetrasaccharide pentenyl glycosides with diarabinan and digalactan branching related to the pectic polysaccharide rhamnogalacturonan I is reported. The strategy relies on the coupling of N-phenyl trifluoroacetimidate disaccharide donors to a common rhamnosyl acceptor. The resulting trisaccharide thioglycosides were finally coupled to an n-pentenyl galactoside acceptor to access the two protected branched tetrasaccharides.

Complete tetraglycosylation of a calix[4]arene by a chemo-enzymatic approach

It was demonstrated that a calixarene can be a substrate for glycosyltransferases and thanks to an exhaustive glycosylation a multivalent tetralactosaminyl calix[4]arene was obtained.

Chemical Methods for Encoding and Decoding of Posttranslational Modifications

A large array of posttranslational modifications can dramatically change the properties of proteins and influence different aspects of their biological function such as enzymatic activity, binding interactions, and proteostasis. Despite the significant knowledge that has been gained about the function of posttranslational modifications using traditional biological techniques, the analysis of the site-specific effects of a particular modification, the identification of the full complement of modified proteins in the proteome, and the detection of new types of modifications remains challenging. Over the years, chemical methods have contributed significantly in both of these areas of research. This review highlights several posttranslational modifications where chemistry-based approaches have made significant contributions to our ability to both prepare homogeneously modified proteins and identify and characterize particular modifications in complex biological settings. As the number and chemical diversity of documented posttranslational modifications continues to rise, we believe that chemical strategies will be essential to advance the field in years to come.

Stereoselective Construction of β-Mannopyranosides by Anomeric O-Alkylation: Synthesis of the Trisaccharide Core of N-linked Glycans

A new and efficient approach for direct and stereoselective synthesis of β-mannopyranosides by anomeric O-alkylation has been developed. This anomeric O-alkylation of mannopyranose-derived lactols is proposed to occur under synergistic control of a kinetic anomeric effect and metal chelation. The presence of a conformationally flexible C6 oxygen atom in the sugar-derived lactol donors is required for this anomeric O-alkylation to be efficient, probably because of its chelation with cesium ion. In contrast, the presence of a C2 oxygen atom plays a minor role. This glycosylation method has been successfully utilized for the synthesis of the trisaccharide core of complex N-linked glycans.

Chemical synthesis of asparagine-linked archaeal N-glycan from Methanothermus fervidus

Several N-linked glycoproteins have been identified in archaea and there is growing evidence that the N-glycan is involved in survival and functioning of archaea in extreme conditions. Chemical synthesis of the archaeal N-glycans represents a crucial step towards understanding the putative function of protein glycosylation in archaea. Herein the first total synthesis of the archaeal L-asparagine linked hexasaccharide from Methanothermus fervidus is reported using a highly convergent [3+3] glycosylation approach in high overall yields. The synthesis relies on efficient preparation of regioselectively protected thioglycoside building blocks for orthogonal glycosylations and late stage N-aspartylation.

Building biologics by chemical synthesis: practical preparation of di- and triantennary N-linked glycoconjugates

A unified strategy for the syntheses of bi- and triantennary fully sialylated N-glycans is described. The synthesis capitalizes on a global glycosylation strategy that delivers the desired undeca- and tetradecasaccharide in excellent yields. Finally, conjugation of the glycan to PSMA oligopeptide is described.

An advance in the chemical synthesis of homogeneous N-linked glycopolypeptides by convergent aspartylation

We describe a useful advance in glycopeptide synthesis. We have developed a one-flask aspartylation/deprotection method, wherein long peptide fragments, bearing proximal pseudoproline functionality are merged with complex glycan domains. Following aspartylation, acidmediated global deprotection reveals the elaborated glycopeptide. The temporary pseudoproline functionality serves to suppress formation of aspartimide side products during solid phase peptide synthesis and aspartylation.

Unusual transglycosylation activity of Flavobacterium meningosepticum endoglycosidases enables convergent chemoenzymatic synthesis of core fucosylated complex N-glycopeptides

Structurally well defined, homogeneous glycopeptides and glycoproteins are indispensable tools for functional glycomics studies. By screening of various endo-β-N-acetylglucosaminidases through the use of appropriate synthetic donor and acceptor substrates, we have found that the Flavobacterium meningosepticum endo-β-N-acetyl-glucosaminidases (GH family 18), including Endo-F2 and Endo-F3, were able to glycosylate α-1,6-fucosylated GlcNAc derivative to provide natural, core-fucosylated complex-type N-glycopeptides. The Endo-F2 and Endo-F3 were efficient for transferring both sialylated and asia-lylated glycans and were highly specific for an α-1,6-fucosylated GlcNAc-peptide as acceptor for transglycosylation, showing only marginal activity with non-fucosylated GlcNAc-peptides. In contrast, we found that the commonly used endoglycosidases such as Endo-A and Endo-M, which belong to GH family 85, were unable to take α-1,6-fucosyl-GlcNAc derivative as acceptors for transglycosylation. The novel activity of Endo-F2 and Endo-F3 was successfully applied for a highly convergent chemoenzymatic synthesis of a full-sized CD52 glycopeptide antigen carrying both terminal sialic acid and core fucose. This is the first report on endoglycosidases that are able to glycosylate α-1,6-fucosylated GlcNAc derivatives to form natural core-fucosylated glycopeptides.

Expeditious chemoenzymatic synthesis of CD52 glycopeptide antigens

CD52 is a glycosylphosphatidylinositol (GPI)-anchored glycopeptide antigen found on sperm cells and human lymphocytes. Recent structural studies indicate that sperm-associated CD52 antigen carries both a complex type N-glycan and an O-glycan on the polypeptide backbone. To facilitate functional and immunological studies of distinct CD52 glycoforms, we report in this paper the first chemoenzymatic synthesis of homogeneous CD52 glycoforms carrying both N- and O-glycans. The synthetic strategy consists of two key steps: monosaccharide primers GlcNAc and GalNAc were first installed at the pre-determined N- and O-glycosylation sites by a facile solid-phase peptide synthesis, and then the N- and O-glycans were extended by respective enzymatic glycosylations. It was found that the endoglycosidase-catalyzed transglycosylation allowed efficient attachment of an intact N-glycan in a single step at the N-glycosylation site, while the recombinant human T-synthase could independently extend the O-linked GalNAc to form the core 1 O-glycan. This chemoenzymatic approach is highly convergent and permits easy construction of various homogeneous CD52 glycoforms from a common polypeptide precursor. In addition, the introduction of a latent thiol group in the form of protected cysteamine at the C-terminus of the CD52 glycoforms will enable site-specific conjugation to a carrier protein to provide immunogens for generating CD52 glycoform-specific antibodies for functional studies.

Metal-catalyzed 1,2-shift of diverse migrating groups in allenyl systems as a new paradigm toward densely functionalized heterocycles

A general, mild, and efficient 1,2-migration/cycloisomerization methodology toward multisubstituted 3-thio-, seleno-, halo-, aryl-, and alkyl-furans and pyrroles, as well as fused heterocycles, valuable building blocks for synthetic chemistry, has been developed. Moreover, regiodivergent conditions have been identified for C-4 bromo- and thio-substituted allenones and alkynones for the assembly of regioisomeric 2-hetero substituted furans selectively. It was demonstrated that, depending on reaction conditions, ambident substrates can be selectively transformed into furan products, as well as undergo selective 6-exo-dig or Nazarov cyclizations. Our mechanistic investigations have revealed that the transformation proceeds via allenylcarbonyl or allenylimine intermediates followed by 1,2-group migration to the allenyl sp carbon during cycloisomerization. It was found that 1,2-migration of chalcogens and halogens predominantly proceeds via formation of irenium intermediates. Analogous intermediate can also be proposed for 1,2-aryl shift. Furthermore, it was shown that the cycloisomerization cascade can be catalyzed by Brønsted acids, albeit less efficiently, and commonly observed reactivity of Lewis acid catalysts cannot be attributed to the eventual formation of proton. Undoubtedly, thermally induced or Lewis acid-catalyzed transformations proceed via intramolecular Michael addition or activation of the enone moiety pathways, whereas certain carbophilic metals trigger carbenoid/oxonium type pathway. However, a facile cycloisomerization in the presence of cationic complexes, as well as observed migratory aptitude in the cycloisomerization of unsymmetrically disubstituted aryl- and alkylallenes, strongly supports electrophilic nature for this transformation. Full mechanistic details, as well as the scope of this transformation, are discussed.

Carbohydrate diversity: synthesis of glycoconjugates and complex carbohydrates

The fundamental role of glycoconjugates in many biological processes is now well appreciated and has intensified the development of innovative and improved synthetic strategies. All areas of synthetic methodology have seen major advances and many complex, highly branched carbohydrates and glycoproteins have been prepared using solution- and/or solid-phase approaches. The development of an automated oligosaccharide synthesizer provides rapid access to biologically relevant compounds. These chemical approaches help to produce sufficient quantities of defined oligosaccharides for biological studies. Synthetic chemistry also supports an improved understanding of glycobiology and will eventually result in the discovery of new therapeutics.

Chemoenzymatic synthesis of CD52 glycoproteins carrying native N-glycans

A facile synthesis of homogeneous CD52 glycoproteins carrying native N-glycans was achieved using an endolycosidase-catalyzed oligosaccharide transfer as the key step. The synthesis consists of two steps: the solid phase synthesis of GlcNAc-CD52 and the transfer of a high-mannose type or complex type N-glycan from Man(9)GlcNAc(2) Asn or a sialglycopeptide to the GlcNAc-CD52, under the catalysis of the endo-beta-N-acetylglucosaminidases from Arthrobacter (Endo-A) and Mucor hiemalis (Endo-M), respectively.

Glycopeptide and glycoprotein synthesis involving unprotected carbohydrate building blocks

This review summarizes the chemical and chemoenzymatic synthesis of glycopeptides and glycoproteins using unprotected carbohydrates as key intermediates. The synthetic methods covered herein include the convergent synthesis of glycopeptides by chemoselective ligation of peptides and free glycans, solution- and solid-phase synthesis of glycopeptides by sequential peptide elongation with unprotected glycosyl amino acids or short glycopeptides as building blocks, and the synthesis of glycopeptides by enzymatic and/or chemical elongation of the free glycans. The use of unprotected carbohydrates in these syntheses can circumvent the final-stage carbohydrate deprotection, lead to highly convergent synthetic designs, and more significantly, take advantage of the commercially available free glycans isolated from nature, which could considerably facilitate the synthesis of complex glycopeptides and glycoproteins.

Novel forms of chemical protein diversity — in nature and in the laboratory

Novel chemical variants of proteins have been found in nature, including potent 'microprotein' natural products and folded protein molecules that contain a cyclic polypeptide chain. Researchers have used chemical synthesis and genetic methods to make these proteins and more: protein catenanes, neoglycoproteins, and artificial protein molecules with novel architectures or made from novel building blocks. De novo design has taken a big step forward with the accurate design and construction of proteins with complex molecular structure. A variety of non-coded amino acids and other building blocks has been used to make increasingly sophisticated protein molecular devices for use as biosensors and for the study of signal transduction inside living cells.