Top-Down Chemoenzymatic Approach to a High-Mannose-Type Glycan Library: Synthesis of a Common Precursor and Its Enzymatic Trimming

Chemical Synthesis of Sialyl N-Glycans and Analysis of Their Recognition by Neuraminidase

The chemical synthesis of a fully sialylated tetraantennary N-glycan has been achieved for the first time by using the diacetyl strategy, in which NHAc is protected as NAc₂ to improve reactivity by preventing intermolecular hydrogen bonds. Another key was the glycosylation to the branched mannose in an ether solvent, which promoted the desired glycosylation by stabilizing the oxocarbenium ion intermediate. Furthermore, high α-selectivity of these glycosylation reactions was realized by utilizing remote participation. Two asymmetrically deuterium labeled sialyl N-glycans were also synthesized by the same strategy. The synthesized N-glycans were used to probe the molecular basis of H1N1 neuraminidase recognition. The asymmetrically deuterated N-glycans revealed a difference in the recognition of sialic acid on each branch. Meanwhile, the tetraantennary N-glycan was used to evaluate the effects of multivalency and steric hinderance by forming branching structures.

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Convergent Synthesis of a Bisecting N-Acetylglucosamine (GlcNAc)-Containing N-Glycan

The chemical synthesis of a bisecting N-acetylglucosamine (GlcNAc)-containing N-glycan was achieved by a convergent synthetic route through [4+2] and [6+2] glycosylations. This synthetic route reduced the number of reaction steps, although the key glycosylations were challenging in terms of yields and selectivities owing to steric hindrance at the glycosylation site and a lack of neighboring group participation. The yields of these glycosylations were enhanced by stabilizing the oxocarbenium ion intermediate through ether coordination. Glycosyl donor protecting groups were explored in an effort to realize perfect α selectivity by manipulating remote participation. The simultaneous glycosylations of a tetrasaccharide with two disaccharides was investigated to efficiently construct a bisecting GlcNAc-containing N-glycan.

Endo-α-Mannosidase-Catalyzed Transglycosylation

In order for facilitating the synthesis of oligosaccharides, transglycosylation reactions mediated by glycoside hydrolases have been studied in various contexts. In this study, we examined the transglycosylating activity of a Golgi endo-α-mannosidase. We prepared various glycosyl donors and acceptors, and recombinant human Golgi endo-α-mannosidase and its various mutants were expressed. The enzyme was able to mediate transglycosylation from α-glycosyl-fluorides. Systematic screening of various point mutants revealed that the E407D mutant had excellent transglycosylation activity and extremely low hydrolytic activity. Substrate specificity analysis revealed that minimum motif required for glycosyl acceptor is Manα1- 2Man. The synthetic utility of the enzyme was demonstrated by generation of a high-mannose-type undecasaccharide (Glc₁ Man₉ GlcNAc₂ ).