Introduction
In post-genomic era, post- and co-translational modifications (P/C-TM) of proteins are known as more essential elements for protein function activation.[1] Recent research has revealed that these modifications regulate a wide range of biological processes, including several diseases such as cancer, Alzheimer’s disease, and Parkinson’s disease.[2-5] Among these protein modifications, glycosylation is one of the most abundant modifications in eukaryotic cells. Glycoproteins are found on the cell surface and in cellular fluids, and these proteins are modified with serine/threonine-linked O-glycans or asparagine-linked N-glycans.6 In case of N-glycans, the oligosaccharides are generally attached to the nitrogen of asparagine side chain which is present as a part of consensus sequences; Asn-Xaa-Ser/Thr (Xaa: any amino acid except for Pro), whereas these oligosaccharides show considerable heterogeneity in their structure.7 Structures of N-glycans are mainly classified with three types including complex type, high-mannose type, and hybrid type saccharide (Fig1-1).
Such diverse glycosylation patterns increase the diversity of the corresponding glycoproteins’ structure and as a result, the functions of glycoproteins become more complex. However, the structural heterogeneity of glycosylation makes the functional analysis of individual glycosylation pattern very difficult. In order to have homogeneous glycoprotiens, chemical methods to synthesize glycoproteins with chemically uniform glycan structures have been developed.
The total chemical synthesis of proteins requires solid phase peptide synthesis to obtain several peptides, and subsequent ligations to assemble these peptides sequentially and afford desired full-length protein. Among these novel ligation methods, native chemical ligation (NCL) which enables the chemo-selective coupling between unprotected peptides were established and widely used. The development of NCL by Dawson and coworkers greatly propelled the chemical synthesis of (glycol)proteins.8-10 The reaction is performed between two fragments: a peptide thioester and an N-terminal cysteinyl peptide fragment, to afford a natural amide bond. (Fig1-2)
