Recent advances in glycotechnology for glycoconjugate synthesis using microbial endoglycosidases

The first report of enzymatic transglycosylation catalyzed by family GH84 N-acetyl-β-d-glucosaminidase using a sugar oxazoline derivative as a glycosyl donor

O-Glycosylated N-acetyl-β-d-glucosamine-selective N-acetyl-β-d-glucosaminidase (O-GlcNAcase), belonging to glycoside hydrolase family 84 (GH84), is known as a retaining glycosidase with the possibility of enzymatic transglycosylation. However, no enzymatic transglycosylation catalyzed by GH84 O-GlcNAcase has been reported. Here, enzymatic transglycosylation catalyzed by GH84 O-GlcNAcase was first reported. The enzymatic transglycosylation catalyzed by the GH84 O-GlcNAcase from Bacteroides thetaiotaomicron (BtGH84 O-GlcNAcase) was attained using 1,2-oxazoline derivative of N-acetyl-d-glucosamine (GlcNAc oxazoline) as a glycosyl donor substrate. The β-linked N-acetyl-d-glucosamine (GlcNAc) derivative was enzymatically synthesized using N-(2-hydroxyethyl)acrylamide as an acceptor substrate. Interestingly, the β1,6-linked disaccharide derivative of GlcNAc was also obtained in the case of using the GlcNAc derivative with a triazole-linked acrylamide group as an acceptor substrate. Additionally, a one-pot chemo-enzymatic transglycosylation starting from unprotected GlcNAc through GlcNAc oxazoline successfully showed through the combination with the direct synthesis of GlcNAc oxazoline in water and the enzymatic transglycosylation.

Recent advances in glycan synthesis

Carbohydrates play important roles in life science, but their synthesis is always hampered by their complicated chemical structures. Scientists have never stopped trying to solve the problem of glycan synthesis from various aspects. Here a brief overview of recent progress in glycan synthesis, including chemical approaches, chemoenzymatic approaches, and automated synthesis, will be discussed, focusing on the efficiency of new glycosylation methods, the stereoselectivity of coupled products, and their applications in the assembly of complex glycan chains.

Strategies for chemoenzymatic synthesis of carbohydrates

Carbohydrates are structurally complex but functionally important biomolecules. Therefore, they have been challenging but attractive synthetic targets. While substantial progress has been made on advancing chemical glycosylation methods, incorporating enzymes into carbohydrate synthetic schemes has become increasingly practical as more carbohydrate biosynthetic and metabolic enzymes as well as their mutants with synthetic application are identified and expressed for preparative and large-scale synthesis. Chemoenzymatic strategies that integrate the flexibility of chemical derivatization with enzyme-catalyzed reactions have been extremely powerful. Briefly summarized here are our experiences on developing one-pot multienzyme (OPME) systems and representative chemoenzymatic strategies from others using glycosyltransferase-catalyzed reactions for synthesizing diverse structures of oligosaccharides, polysaccharides, and glycoconjugates. These strategies allow the synthesis of complex carbohydrates including those containing naturally occurring carbohydrate postglycosylational modifications (PGMs) and non-natural functional groups. By combining these srategies with facile purification schemes, synthetic access to the diverse space of carbohydrate structures can be automated and will not be limited to specialists.

Specificity of Donor Structures for endo-β-N-Acetylglucosaminidase-Catalyzed Transglycosylation Reactions

To demonstrate the structural specificity of the glycosyl donor for the transglycosylation reaction by using endo-β-N-acetylglucosaminidase from Mucor hiemalis (endo-M), a series of tetrasaccharide oxazoline derivatives was synthesized. These derivatives correspond to the core structure of an asparagine-linked glycoprotein glycan with a β-mannose unit of a non-natural-type monosaccharide, including β-glucose, β-galactose, and β-talose in place of the β-mannose moiety. The transglycosylation activity of wildtype (WT) endo-M and two mutants, N175Q and N175A, was examined by using these tetrasaccharide donors with p-nitrophenyl N-acetylglucosaminide (GlcNAc-pNp). The essential configuration of the hydroxy group for the transglycosylation reaction was determined. On the basis of these results, the transglycosylation reaction was investigated by using chemically modified donors, and transglycosylated products were successfully obtained.

Occurrence of free sialyl oligosaccharides related to N-glycans (sialyl free N-glycans) in animal sera

Free oligosaccharides that are structurally related to N-glycans [free N-glycans (FNGs)] are widely distributed in the cytosol of animal cells. The diverse molecular mechanisms responsible for the formation of these FNGs have been well clarified. In this study we demonstrate the wide occurrence of sialylated FNGs in sera of various animals. The features of these extracellular FNGs are quite distinct from the cytosolic FNGs, as they are Gn2-type glycans, bearing an N,N'-diacetylchitobiose unit at their reducing termini, while the cytosolic FNGs are predominantly Gn1-type, with a single GlcNAc at their reducing termini. The major structures observed varied from species to species, and the structures of the FNGs appear to be correlated with the major sialyl N-glycans on serum glycoproteins, suggesting that the serum FNGs are produced by hepatocytes. Interestingly, glycan-profiles of the FNGs indicated that they are altered in a developmental stage-dependent manner. Sialyl FNGs in the sera may not only be of biological relevance, in that they might reflect the functionality of the liver, but also can be attractive sources for obtaining uniform sialyl FNGs in the chemoenzymatic synthesis of glycoproteins.

Facile preparation of the N-acetyl-glucosaminylated asparagine derivative with TFA-sensitive protecting groups useful for solid-phase glycopeptide synthesis

In this study, a novel N-acetyl-glucosaminylated asparagine derivative was developed. This derivative carried TFA-sensitive protecting groups and was derived from commercially available compounds only in three steps. It was applicable to the ordinary 9-fluorenylmethoxycarbonyl (Fmoc)-based solid-phase peptide synthesis (SPPS) method, and the protecting groups on the carbohydrate moiety could be removed by a single step of TFA cocktail treatment generally used for the final deprotection step in Fmoc-SPPS.