Synthesis of multiantennary complex type N-glycans by use of modular building blocks

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Streamlining the chemoenzymatic synthesis of complex N-glycans by a stop and go strategy

Contemporary chemoenzymatic approaches can provide highly complex multi-antennary N-linked glycans. These procedures are, however, very demanding and typically involve as many as 100 chemical steps to prepare advanced intermediates that can be diversified by glycosyltransferases in a branch selective manner to give asymmetrical structures commonly found in Nature. Only highly specialized laboratories can perform such syntheses, which greatly hampers progress in glycoscience. Here we describe a biomimetic approach in which a readily available bi-antennary glycopeptide can be converted in 10 or fewer chemical and enzymatic steps into multi-antennary N-glycans that at each arm can be uniquely extended by glycosyltransferases to give access to highly complex asymmetrically branched N-glycans. A key feature of our approach is the installation of additional branching points using recombinant MGAT4 and MGAT5 in combination with unnatural sugar donors. At an appropriate point in the enzymatic synthesis, the unnatural monosaccharides can be converted into their natural counterpart allowing each arm to be elaborated into a unique appendage.

Synthesis of Modular Building Blocks using Glycosyl Phosphate Donors for the Construction of Asymmetric N-Glycans

Glycosyl phosphates are known as versatile donors for the synthesis of complex oligosaccharides both chemically and enzymatically. Herein, we report the stereoselective construction of modular building blocks for the synthesis of N-glycan using glycosyl phosphates as donors. We have synthesized four trisaccharide building blocks with orthogonal protecting groups, namely, Manβ2GlcNAc(OAc)3β6GlcNAc (9), Manβ2GlcNAc-β6GlcNAc(OAc)3 (15), Manβ2GlcNAc(OAc)3β4GlcNAc (18) and Manβ2GlcNAcβ4GlcNAc(OAc) (22) for further selective elongation using glycosyltransferases. The glycosylation reaction using glycosyl phosphate was found to be high yielding with shorter reaction time. Initially, The phthalimide protected glucosamine donor was exploited to ensure the formation of β-glycosidic linkage and later converted to the N-acetyl group before the enzymatic synthesis. The selective deprotection of O-benzyl group was performed prior to enzymatic synthesis to avoid its negative interference.

Chemoenzymatic Synthesis of Asymmetrical Multi-Antennary N-Glycans to Dissect Glycan-Mediated Interactions between Human Sperm and Oocytes

Complex N-glycans of glycoproteins of the zona pellucida (ZP) of human oocytes have been implicated in the binding of spermatozoa. The termini of these unusual bi-, tri-, and tetra-antennary N-glycans consist of the tetrasaccharide sialyl-Lewis^x (SLe^x ), which was previously identified as the minimal epitope for sperm binding. We describe here the chemoenzymatic synthesis of highly complex triantennary N-glycans derived from ZP carrying SLe^x moieties at the C-2 and C-2' arm and a sialyl-Lewis^x -Lewis^x (SLe^x -Le^x ) residue at the C-6 antenna and two closely related analogues. The compounds were examined for their ability to inhibit the interaction of human sperm to ZP. It was found that the SLe^x -Le^x moiety is critical for inhibitory activity, whereas the other SLe^x moieties exerted minimal effect. Further studies with SLe^x -Le^x and SLe^x showed that the extended structure is the more potent inhibitor. In addition, trivalent SLe^x -Le^x and SLe^x were prepared which showed greater inhibitory activity compared to their monovalent counterparts. Our studies show that although SLe^x can inhibit the binding of spermatozoa, presenting this epitope in the context of a complex N-glycan results in a loss of inhibitory potential, and in this context only SLe^x -Le^x can make productive interactions. It is not the multivalent display of SLe^x on a multi-antennary glycan but the presentation of multiple SLe^x -Le^x on the various glycosylation sites of ZP that accounts for high avidity binding.

Chemoenzymatic Approach for the Preparation of Asymmetric Bi-, Tri-, and Tetra-Antennary N-Glycans from a Common Precursor

Progress in glycoscience is hampered by a lack of well-defined complex oligosaccharide standards that are needed to fabricate the next generation of microarrays, to develop analytical protocols to determine exact structures of isolated glycans, and to elucidate pathways of glycan biosynthesis. We describe here a chemoenzymatic methodology that makes it possible, for the first time, to prepare any bi-, tri-, and tetra-antennary asymmetric N-glycan from a single precursor. It is based on the chemical synthesis of a tetra-antennary glycan that has N-acetylglucosamine (GlcNAc), N-acetyllactosamine (LacNAc), and unnatural Galα(1,4)-GlcNAc and Manβ(1,4)-GlcNAc appendages. Mammalian glycosyltransferases recognize only the terminal LacNAc moiety as a substrate, and thus this structure can be uniquely extended. Next, the β-GlcNAc terminating antenna can be converted into LacNAc by galactosylation and can then be enzymatically modified into a complex structure. The unnatural α-Gal and β-Man terminating antennae can sequentially be decaged by an appropriate glycosidase to liberate a terminal β-GlcNAc moiety, which can be converted into LacNAc and then elaborated by a panel of glycosyltransferases. Asymmetric bi- and triantennary glycans could be obtained by removal of a terminal β-GlcNAc moiety by treatment with β-N-acetylglucosaminidase and selective extension of the other arms. The power of the methodology is demonstrated by the preparation of an asymmetric tetra-antennary N-glycan found in human breast carcinoma tissue, which represents the most complex N-glycan ever synthesized. Multistage mass spectrometry of the two isomeric triantennary glycans uncovered unique fragment ions that will facilitate identification of exact structures of glycans in biological samples.

Highly Efficient Synthesis of Multiantennary Bisected N-glycans Based on Imidates

The occurrence of N-glycans with a bisecting GlcNAc modification on glycoproteins has many implications in developmental and immune biology. However, these particular N-glycans are difficult to obtain either from nature or through synthesis. We have developed a flexible and general method for synthesizing bisected N-glycans of the complex type by employing modular TFAc-protected donors for all antennae. The TFAc-protected N-glycans are suitable for the late introduction of a bisecting GlcNAc. This integrated strategy permits for the first time the use of a single approach for multiantennary N-glycans as well as their bisected derivatives via imidates, with unprecedented yields even in a one-pot double glycosylation. With this new method, rare N-glycans of the bisected type can be obtained readily, thereby providing defined tools to decipher the biological roles of bisecting GlcNAc modifications.

Semisynthesis of Intact Complex-Type Triantennary Oligosaccharides from a Biantennary Oligosaccharide Isolated from a Natural Source by Selective Chemical and Enzymatic Glycosylation

Attachment of oligosaccharides to proteins is a major post-translational modification. Chemical syntheses of oligosaccharides have contributed to clarifying the functions of these oligosaccharides. However, syntheses of oligosaccharide-linked proteins are still challenging because of their inherent complicated structures, including diverse di- to tetra-antennary forms. We report a highly efficient strategy to access the representative two types of triantennary oligosaccharides through only 9- or 10-step chemical conversions from a biantennary oligosaccharide, which can be isolated in exceptionally homogeneous form from egg yolk. Four benzylidene acetals were successfully introduced to the terminal two galactosides and two core mannosides of the biantennary asialononasaccharide bearing 24 hydroxy groups, followed by protection of the remaining hydroxy groups with acetyl groups. Selective removal of one of the benzylidene acetals gave two types of suitably protected glycosyl acceptors. Glycosylation toward the individual acceptors with protected Gal-β-1,4-GlcN thioglycoside and subsequent deprotection steps successfully yielded two types of complex-type triantennary oligosaccharides.

Efficient Chemoenzymatic Synthesis of an N-glycan Isomer Library

Quantification, characterization and biofunctional studies of N-glycans on proteins remain challenging tasks due to complexity, diversity and low abundance of these glycans. The availability of structurally defined N-glycans (especially isomers) libraries is essential to help on solving these tasks. We reported herein an efficient chemoenzymatic strategy, namely Core Synthesis/Enzymatic Extension (CSEE), for rapid production of diverse N-glycans. Starting with 5 chemically prepared building blocks, 8 N-glycan core structures containing one or two terminal N-acetyl-D-glucosamine (GlcNAc) residue(s) were chemically synthesized via consistent use of oligosaccharyl thioethers as glycosylation donors in the convergent fragment coupling strategy. Each of these core structures was then extended to 5 to 15 N-glycan sequences by enzymatic reactions catalyzed by 4 robust glycosyltransferases. Success in synthesizing N-glycans with Neu5Gc and core-fucosylation further expanded the ability of enzymatic extension. High performance liquid chromatography with an amide column enabled rapid and efficient purification (>98% purity) of N-glycans in milligram scales. A total of 73 N-glycans (63 isomers) were successfully prepared and characterized by MS² and NMR. The CSEE strategy provides a practical approach for "mass production" of structurally defined N-glycans, which are important standards and probes for Glycoscience.

Efficient convergent synthesis of bi-, tri-, and tetra-antennary complex type N-glycans and their HIV-1 antigenicity

The structural diversity of glycoproteins often comes from post-translational glycosylation with heterogeneous N-glycans. Understanding the complexity of glycans related to various biochemical processes demands a well-defined synthetic sugar library. We report herein a unified convergent strategy for the rapid production of bi-, tri-, and tetra-antennary complex type N-glycans with and without terminal N-acetylneuraminic acid residues connected via the α-2,6 or α-2,3 linkages. Moreover, using sialyltransferases to install sialic acid can minimize synthetic steps through the use of shared intermediates to simplify the complicated procedures associated with conventional sialic acid chemistry. Furthermore, these synthetic complex oligosaccharides were compiled to create a glycan array for the profiling of HIV-1 broadly neutralizing antibodies PG9 and PG16 that were isolated from HIV infected donors. From the study of antibody PG16, we identified potential natural and unnatural glycan ligands, which may facilitate the design of carbohydrate-based immunogens and hasten the HIV vaccine development.

A general strategy for the chemoenzymatic synthesis of asymmetrically branched N-glycans

A systematic, efficient means of producing diverse libraries of asymmetrically branched N-glycans is needed to investigate the specificities and biology of glycan-binding proteins. To that end, we describe a core pentasaccharide that at potential branching positions is modified by orthogonal protecting groups to allow selective attachment of specific saccharide moieties by chemical glycosylation. The appendages were selected so that the antenna of the resulting deprotected compounds could be selectively extended by glycosyltransferases to give libraries of asymmetrical multi-antennary glycans. The power of the methodology was demonstrated by the preparation of a series of complex oligosaccharides that were printed as microarrays and screened for binding to lectins and influenza-virus hemagglutinins, which showed that recognition is modulated by presentation of minimal epitopes in the context of complex N-glycans.