N-Glycosylation with synthetic undecaprenyl pyrophosphate-linked oligosaccharide to oligopeptides by PglB oligosaccharyltransferase from Campylobacter jejuni

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Structural Basis of Protein Asn-Glycosylation by Oligosaccharyltransferases

Glycosylation of asparagine residues is a ubiquitous protein modification. This N-glycosylation is essential in Eukaryotes, but principally nonessential in Prokaryotes (Archaea and Eubacteria), although it facilitates their survival and pathogenicity. In many reviews, Archaea have received far less attention than Eubacteria, but this review will cover the N-glycosylation in the three domains of life. The oligosaccharide chain is preassembled on a lipid-phospho carrier to form a donor substrate, lipid-linked oligosaccharide (LLO). The en bloc transfer of an oligosaccharide from LLO to selected Asn residues in the Asn-X-Ser/Thr (X≠Pro) sequons in a polypeptide chain is catalyzed by a membrane-bound enzyme, oligosaccharyltransferase (OST). Over the last 10 years, the three-dimensional structures of the catalytic subunits of the Stt3/AglB/PglB proteins, with an acceptor peptide and a donor LLO, have been determined by X-ray crystallography, and recently the complex structures with other subunits have been determined by cryo-electron microscopy . Structural comparisons within the same species and across the different domains of life yielded a unified view of the structures and functions of OSTs. A catalytic structure in the TM region accounts for the amide bond twisting, which increases the reactivity of the side-chain nitrogen atom of the acceptor Asn residue in the sequon. The Ser/Thr-binding pocket in the C-terminal domain explains the requirement for hydroxy amino acid residues in the sequon. As expected, the two functional structures are formed by the involvement of short amino acid motifs conserved across the three domains of life.

Glycosylation, an effective synthetic strategy to improve the bioavailability of therapeutic peptides

Glycosylation of peptides is a promising strategy for modulating the physicochemical properties of peptide drugs and for improving their absorption through biological membranes. This review highlights various methods for the synthesis of glycoconjugates and recent progress in the development of glycosylated peptide therapeutics. Furthermore, the impacts of glycosylation in overcoming the existing barriers that restrict oral and brain delivery of peptides are described herein.